Welding Parameters

Engineering Encyclopedia Saudi Aramco DeskTop Standards

Welding Parameters

Note: The source of the technical material in this volume is the Professional Engineering Development Program (PEDP) of Engineering Services. Warning: The material contained in this document was developed for Saudi Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

Chapter : Welding File Reference: COE11403

For additional information on this subject, contact A.A. Omar

Engineering Encyclopedia

Welding Welding Parameters

Contents

Pages

INTRODUCTION................................................................................................................ 1 COMMONLY USED BASE METALS IN THE OIL AND GAS INDUSTRY..................... 2 Base Metal Definition ............................................................................................... 2 Base Metal Types and Properties............................................................................... 2 Carbon Steels ................................................................................................ 3 Low Alloy Steels ........................................................................................... 3 Stainless Steels .............................................................................................. 4 Iron Carbon Reactions............................................................................................... 5 Fusion Zone................................................................................................... 5 Heat-Affected Zone (HAZ)............................................................................ 7 Base Metal..................................................................................................... 7 Melting Properties ......................................................................................... 7 Solidification Properties ................................................................................ 9 Thermal Cycles ............................................................................................11 Phase Transformations..................................................................................16 WELD JOINT DESIGNS AND SYMBOLS........................................................................18 Types of Weld Joints................................................................................................18 Fillet Weld Joint ...........................................................................................18 Butt Weld Joint ............................................................................................20 Partial Penetration Weld Joint.......................................................................23 Combination.................................................................................................24 Stud Weld Joint ............................................................................................25 AWS Weld Symbols ................................................................................................26 Elements of a Weld Symbol..........................................................................26 Fillet Weld Symbol.......................................................................................28 Butt Weld Symbol ........................................................................................31 Partial Penetration Weld Symbol ..................................................................33 Combination.................................................................................................34 Stud Weld Joint Symbol ...............................................................................36 Saudi Aramco DeskTop Standards



WELDING CONSUMABLES ............................................................................................37 Types of Welding Consumables ...............................................................................37 Coated Electrodes.........................................................................................37 Bare Rods.....................................................................................................39 Bare Wires....................................................................................................39 Flux Cored Electrodes ..................................................................................39 AWS Classifications ................................................................................................41 Coated Electrodes.........................................................................................41 Bare Rods and Wire......................................................................................46 Flux Cored Electrodes ..................................................................................46 Shielding Gasses and Fluxes.........................................................................48 Gases............................................................................................................48 Fluxes...........................................................................................................48 Storage and Handling Requirements.........................................................................50 Coated Electrodes.........................................................................................50 Bare Rods and Wire......................................................................................51 Flux Cored Electrodes ..................................................................................51 HEAT INPUT EFFECTS ....................................................................................................52 Parameters ...............................................................................................................52 Current .........................................................................................................52 Voltage.........................................................................................................53 Travel Speed.................................................................................................53 Effects of Heat Input................................................................................................53 HEAT TREATMENT EFFECTS ........................................................................................55 Preheat.....................................................................................................................55 Purpose ........................................................................................................55 Methods .......................................................................................................56 Determination...............................................................................................60 Postweld Heat Treatment .........................................................................................63 Purpose ........................................................................................................64 Methods .......................................................................................................64 Saudi Aramco DeskTop Standards



Requirements................................................................................................66 GLOSSARY........................................................................................................................71 WORK AID 1: HOW TO IDENTIFY THE MOST COMMONLY.....................................73 WORK AID 2: HOW TO IDENTIFY WELD JOINT DESIGNS AND SYMBOLS ...........75 WORK AID 3: HOW TO IDENTIFY WELDING CONSUMABLES ................................76 WORK AID 4: HOW TO DESCRIBE HEAT INPUT EFFECTS .......................................78 WORK AID 5: HOW TO DESCRIBE HEAT TREATMENT EFFECTS ...........................80 BIBLIOGRAPHY ...............................................................................................................83

Saudi Aramco DeskTop Standards



INTRODUCTION This Module provides information on several of the most important parameters that are associated with welding operations. The emphasis on base metals and heat affects of welding is to introduce the Participant to the metallurgical properties of welding. The information on weld joint designs, symbols, and welding consumables provides additional background on welding parameters and operations. This Module contains the following topics: •

Commonly Used Base Metals in the Oil and Gas Industry

•

Weld Joint Designs and Symbols

•

Identifying Welding Consumables

•

Heat Input Effects

•

Heat Treatment Effects


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COMMONLY USED BASE METALS IN THE OIL AND GAS INDUSTRY This section contains a discussion of the most commonly used base metals in the oil and gas industry and, specifically, in Saudi Aramco. The information in this section provides background on welding-related base metal considerations and includes the following topics: •

Base Metal Definition

•

Base Metal Types and Properties

•

Iron Carbon Reactions

Base Metal Definition A base metal is the metal or metals that are to be welded. More specifically, the term base metal refers to the portion of the weld joint that has not been affected by the welding thermal cycles. Welding joins two pieces of metal to provide a single piece with mechanical properties that are equivalent to the mechanical properties of the original pieces. However, the two pieces of base metal that are joined are not always the same material. In some cases, the two base metals have completely different chemical and mechanical properties. In other cases, the base metals are of different product forms such as a forging and a seamless pipe. Base Metal Types and Properties The most common types of base metals that are used in the oil and gas industry are broadly classified as follows: •

Carbon steels

•

Low alloy steels

•

Stainless steels

Each of these common types of base metal have numerous sub-classifications that are called alloys, types, or grades. The American Society for Testing and Materials (ASTM) and the American Iron and Steel Institute (AISI) have classified all types of base metals to help identify the huge number of base metals that are available to the oil and gas industry, as well as other industries. This classification system uniquely identifies the chemical composition, mechanical properties, and product form of the base metal. Each of the three common base metals that are used in Saudi Aramco will be discussed in greater detail in the sections that follow.


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Carbon Steels Carbon steels are alloys of iron and carbon in which the carbon content is less than 1 percent, the manganese content is less than 1.65 percent, and the copper and silicon content are each less than 0.60 percent. Normally, other alloy agents are only present in residual amounts. The properties and weldability of carbon steels mainly depend on the carbon content. Other elements have a limited effect on the properties and weldability of carbon steels. Increased carbon content in a carbon steel leads to increased hardness and strength. There are three types of carbon steel base metals as follows: •

Low-carbon

•

Medium-carbon

•

High-carbon

Low-carbon Steels –0.10 to 0.25% carbon ( c ), and 0.25 to 1.5 % magnesium (Mn). Lowcarbon steels are widely used for industrial fabrication and construction. These steels are easily welded with all of the gas and arc welding processes. Medium-carbon Steels –0.25 to 0.50% c, and 0.60 to 1.65% Mn. Medium-carbon steels are readily weldable if proper preheat (300°F to 500°F) and postweld heat treatment is applied to the weldment. These steels are easily welded with all of the gas and arc welding processes. High-carbon Steels –0.50 to 1.03% c, and 0.30 to 1.00% Mn. High-carbon steels are readily weldable if proper preheat (400°F to 600°F) and postweld heat treatment is applied to the weldment. These steels are easily welded with all gas and arc welding processes. Low Alloy Steels Low alloy steels are designed to provide a combination of higher strength, better corrosion resistance, or improved notch toughness compared to conventional carbon steels. In accordance with the American Iron and Steel Institute, steel is considered to be a low-alloy steel when any of the following conditions exist: •

The amount of manganese is greater than 1.65 percent.

•

The amount of silicon is greater than 0.60 percent.

•

The amount of copper is greater than 0.60 percent.

•

A definite minimum quantity of any of the following elements is specified or required in alloy steels: aluminum, boron, chromium up to 3.99 percent, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium.

•

Any other alloying agent is added to obtain a desired alloying effect.


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Low alloy steels are readily weldable if proper preheat and postweld heat treatment are applied to the weldment. Low alloy steels are easily welded with all of the arc welding processes. Stainless Steels Stainless steels iron-base alloys with excellent corrosion resistance. Stainless steels do not rust, and they strongly resist attack by a great many liquids, gases, and chemicals. All stainless steels contain iron as the main element and chromium in amounts that vary from about 11 percent to 30 percent. The chromium provides the corrosion resistance. A thin film of chromium-oxide forms on the surface of the metal when the metal is exposed to the oxygen in the atmosphere. This chromium-oxide film acts as a barrier to further oxidation. In general, stainless steels have a lower melting temperature and higher coefficient of thermal expansion than carbon steels. Stainless steels are divided into the five following groups: •

Austenitic

•

Chromium Martensitic

•

Chromium Ferritic

•

Duplex

•

Precipitation-hardened

Austenitic Stainless Steels – are the most commonly used welded stainless steel in Saudi Aramco facilities. Austenitic stainless steels provide excellent corrosion resistance and are not magnetic. Among stainless steel groups, austenitic stainless steels are the easiest to weld because preheat and postweld heat treatments are not required. Chromium Martensitic Stainless Steels – are magnetic steels that contain 12 to 14 percent chromium and up to 0.35 percent carbon. Low carbon chromium martensitic stainless steels are readily welded. Welding of the higher carbon alloys generally requires preheat and postweld heat treatment. Chromium Ferritic Stainless steels – are also magnetic and readily welded; however, the gas welding processes are not recommended. Duplex (ferritic-austenitic) Stainless Steel – combine the corrosion resistance properties of austenitic S. S. grade, especially stress corrosion cracking (SCC), and the mechanical properties of the ferritic stainless steel grades. However, welding duplex stainless steels requires careful control over the selection of welding wires/electrodes, heat input, and interpass temperature in order to ensure a weld joint with similar metallurgical, corrosion resistance, mechanical properties as that of the base metal.


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Precipitation-hardened Stainless Steels – can develop high strength with reasonably simple heat treatments; however, not all of the precipitation-hardened stainless steels are readily weldable. Precipitation-hardened stainless steels that are readily welded require no preheat or solution annealing heat treatment. Iron Carbon Reactions Although an in-depth review of the metallurgy of a weld is not practical in this introductory course, several key topics must be addressed. The heat of welding changes both the structure of the base metal and the weld metal itself. Some of these changes occur while welding; other changes occur after the metal has cooled. The following discussions will present information about the properties and metallurgical transformations of iron carbon reactions that occur while welding carbon steels. Fusion Zone Figure 1 shows a full penetration weld joint and a typical metallographic cross-section of a multipass welded joint with topical areas pointed out. The fusion zone in Figure 1 represents the area of base metal that was melted while welding. The boundaries of the fusion zone are between the original weld bevel surface and the fusion line. The actual fusion zone can only be determined through removal of a cross-section of the weld to examine the metallurgical structure of the base metal. The depth of the fusion zone depends on the amount of heat applied to the weld joint while welding. When more heat is applied to the weld joint while welding, the fusion zone will be wider. When less heat is applied to the weld joint while welding, the fusion zone is narrower. The heat applied to the weld joint is controlled by the welding voltage, current, and the electrode travel speed.


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Use Word 6.0c or later to

view Macintosh picture.

Figure 1. Full Penetration Weld Joint


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Heat-Affected Zone (HAZ) The heat-affected zone (HAZ) in Figure 1 shows the portion of the base metal that was not melted but whose mechanical properties or microstructure were altered by the heat of welding. The alteration of the microstructure can be increased grain size as illustrated in the metallographic inset of Figure 1. The boundaries of the HAZ are between the fusion line and some point in the base metal. When heat is applied to a weldment from the electrode, the heat also transfers into the adjacent base metal. As the heat travels through the base metal, the heat dissipates as it gets further from the weld. Even though the temperature may not be great enough to melt the base metal that is in the HAZ, the temperature is sufficient to alter the microstructure and physical properties of the base metal in the HAZ. Base Metal The base metal in Figure 1 shows the material to be welded; and the base metal is shown as plate material. The boundaries of the base metal include all of the material up to the HAZ. Although the base metal is heated while welding, the amount of heat is not sufficient to change the microstructure and physical properties of the base metal. However, the heat can distort the base metal, which could result in improper alignment of welded components. Melting Properties Metals are crystalline solids whose atoms are arranged into distinct structures. The most common crystalline structures that are found in metals are face centered cubic (FCC), body centered cubic (BCC), and hexagonal close packed (HCP). These structures are shown in Figure 2. When metal is in a liquid state (e.g., molten weld metal), the metal loses its crystallinity and has no distinct structure or orderly arrangement of atoms. The individual atoms move freely within the liquid. The mobility of the atoms allows the liquid metal to yield to the slightest pressure and to conform to the shape of the weld joint. As heat is applied to the metal during welding, the thermal energy increases the kinetic energy of the individual atoms. When the kinetic energy of the atoms increases to a certain level (the melting point temperature), the atoms overcome the bonding energy in the crystalline structure and the atoms can move freely.


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Figure 2. Common Crystal Structures in Metals


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Solidification Properties Crystalline solids are usually produced when a liquid metal solidifies. Figure 3 illustrates the solidification process of liquid weld metal on a solid base metal. This figure shows the initial crystal formation, continued solidification, and complete solidification. When molten weld metal starts to cool to its solidification temperature, solid particles begin to form small initial crystals, which are called dendrites. These small initial crystals are already arranged in the specific atomic structure that is characteristic of the particular metal. This dendritic growth is a result of the hotter, solid material growing into the cooler, liquid, weld metal and more readily dissipating the latent heat of solidification. Solidification proceeds by the growth of the dendrites into larger solid particles that are called solid grains. As the amount of solid particles increases, the amount of liquid weld metal decreases. As the grains grow, the individual grains ultimately meet. The junction at the individual grains is a random arrangement of the atoms, which is called the grain boundary. The overall arrangement of grains and grain boundaries in a metal makes up the unique microstructure of that metal.


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Figure 3. Process of Solidification


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Thermal Cycles The most important physical characteristic of a normal arc weld is the weld's thermal behavior, which is the way in which the temperatures change in the weld and in the heat-affected zone. Welding is a dynamic process that causes rapid temperature changes. These rapid temperature changes result in a complex temperature environment that produces a wide variety of heat treatments in a weld. Because the various heat treatments can change some of the properties of metals, engineers need to understand how temperatures change at and near a fusion weld. Engineers also need to understand how a metal behaves during and after welding. In a butt weld, the electrode moves along the weld joint and applies heat to a specific point. Because the base metal is cold when welding is initiated, heat continually flows into the base metal and away from the region that is heated by the welding arc. The rate of heat flow into the surrounding base metal is governed by many factors that include the physical properties of the base metal and the rate of applied heat that is produced by the welding arc. Figure 4 shows a plate groove weld in-process with the base metal, solidified weld metal, and molten weld metal identified. To see the effects of heat flow while welding, a thermal "picture" of the weld puddle and plate at any given instant must be examined.

Figure 4. In-Process Plate Groove Weld


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Figure 5 is a thermal picture of a weld puddle and plate that illustrates the effect of heat flow while welding and a graphic illustration of the isothermal lines in the plate for a specific direction of welding. Figure 5 also shows the temperature profiles in a mild steel plate at a given instant while welding. In the figure, the W represents molten weld metal; the shaded area is the metal that is in the mushy stage and is bounded by the liquidus temperature (2,795°F) and the solidus temperature (2,714°F). The numbers 1 through 5 are reference points at various locations from the centerline of the weld. As the welding arc moves, the isotherms (lines of constant temperature) move along with the welding arc and do not change. The temperatures, 400°F through 2550°F, in Figure 5 are arbitrary temperatures that are used to indicate the temperature differences of the isothermal lines. As the welding arc moves, a wave of temperature is created that moves along with the welding arc. The line W n-n' marks the location of the peak temperatures at any distance from the centerline of the weld at a given instant in time.


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Figure 5. Weld Temperature Profiles


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Figure 6 shows the thermal cycles that occur in the five reference locations (from Figure 5) on an arbitrary scale of temperature versus time (in seconds) to show relative temperature effects. TM represents the melting point of the metal that is being welded, and line n-n' marks the location of the peak temperatures at any distance from the centerline of the weld at a given instant in time. Figure 6 shows that each reference point is rapidly heated to an elevated temperature, dwells momentarily, and then rapidly cools. The shape of these curves will vary based on the travel speed during the welding process. The following are the general characteristics of thermal cycles: •

The peak temperature that is reached decreases as the distance from the centerline of the weld increases.

•

The time that is required to reach the peak temperature increases as the distance from the centerline of the weld increases.

•

The heating and cooling rates decrease as the distances from the centerline of the weld increases.

Figure 6. Thermal Cycles Saudi Aramco DeskTop Standards

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Phase Transformations Temperature differences cause the atoms of many metals to vary their crystallographic structure. For example, the crystalline structure of iron at temperatures below 1,670°F is body centered cubic (BCC); at temperatures between 1,670°F and 2,540°F, the crystalline structure of iron is face centered cubic (FCC); and at temperatures between 2,535°F and 2,795°F (the temperature at which iron melts), the crystalline structure of iron is again BBC. The change in crystalline structure is formally called a phase transformation. However, steel is primarily an alloy of iron and carbon. The presence of carbon alters the temperature at which freezing and other phase transformations occur. Iron-carbon alloys freeze over a range of temperatures. Different liquidus and solidus temperatures exist for each unique composition. As the carbon content of steel increases up to 4.3% carbon, the liquidus and solidus temperatures decrease. Phase changes and solidification are best shown through use of a phase diagram or an equilibrium diagram. A brief explanation of the iron-carbon phase diagram provides insight into the behavior of steels during welding thermal cycles and heat treatment. Figure 7 shows an ironcarbon alloy phase diagram with 0 to 5% carbon content. As previously mentioned, iron exhibits two different crystalline structures (BCC and FCC). Above 2,795°F, pure iron (0% carbon) is in a liquid state and no crystalline structure exists. Below 2,795°F, pure iron solidifies and has a BCC structure that is called "delta iron". As the temperature is further reduced below 2,540°F, a transformation occurs and the crystalline structure changes to an FCC structure that is called "gamma iron". As much as 2.1% carbon can be held in solution in gamma iron at a specific temperature, which establishes a dividing point on the phase diagram; the alloys of iron and carbon that contain less than 2.1% carbon are called steels, and the alloys that contain more than 2.1% carbon are referred to as cast irons. Below, 1670°F, the iron transforms back to the BCC structure that is called "alpha iron". To better understand the iron-carbon phase diagram, consider a steel with a composition of 0.25% carbon. This steel is indicated on Figure 7 by drawing a vertical line midway between the 0.0 and 0.5% carbon line. Above approximately 2,768°F, the 0.25% carbon steel is molten. As the temperature decreases, delta iron starts to form in the liquid. At just below 2,732°F, the delta iron transforms to austenite (a solid solution of carbon in gamma iron) and molten metal. At about 2,696°F, all of the liquid metal solidifies and the composition is austenite. At approximately 1,500°F, the austenite breaks down and forms a new phase at the grain boundaries. This new phase is almost pure iron or ferrite. Ferrite formation continues until a temperature of 1,340°F is reached.


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Figure 7. Iron-Carbon Alloy Phase Diagram


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WELD JOINT DESIGNS AND SYMBOLS The following section identifies several of the most common weld joints and associated weld symbols that are used at Saudi Aramco. Welds are made at the junction of at least two members. These weld junctions, which are called weld joints, are the location at which two or more members are joined. The placement of these members defines the weld joint design. The American Welding Society (AWS) has developed a set of standard weld symbols to represent all the different types of weld joint designs that join members together. The information in this section provides some background on several types of welds and weld symbols. This information includes the following topics: •

Types of Weld Joints

•

AWS Weld Symbols

Types of Weld Joints As was noted in Module COE 114.01, the five basic types of weld joints are butt, corner, tee, lap, and edge. In some instances, several types of weld joints may be used in combination to complete a weldment. The specific weld joints designs described in this Module include fillet, butt, partial penetration, and stud. Several illustrations of each type of weld joint will be presented in the following sections. Fillet Weld Joint A fillet weld joint is a joint between two members that are at right angles to each other. The weld that joins fillet joints is called a fillet weld and it has an approximately triangular crosssection. Figure 8 shows a lap joint, a tee joint, and a corner joint with fillet welds. Figure 8 also shows the nomenclature of fillet welds including base metal, face of fillet weld, root of fillet weld, toe of fillet weld, throat of fillet weld, equal leg fillet weld, unequal leg fillet weld, and the leg and size of fillet weld.


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Figure 8. Fillet Welds


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Butt Weld Joint A butt weld joint is a joint between two members that are aligned approximately in the same plane. The weld that joins butt joints is called a groove weld. Figure 9 identifies the nomenclature of complete penetration butt welds including base metal, face of weld, toe of weld, root of weld, external weld reinforcement, and root reinforcement.



Figure 9. Complete Penetration Butt Weld Nomenclature


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Figure 10 shows single-groove butt welds, including a single-square groove weld, a single-bevel groove weld, a single-V groove weld, and a single-U groove weld with complete penetration.



Figure 10. Single-Groove Butt Welds


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Figure 11 shows double-groove butt welds including a double-square groove weld, a doublebevel groove weld, a double-V groove weld, and a double-U groove weld.



Figure 11. Double-Groove Butt Welds


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Partial Penetration Weld Joint A partial penetration weld joint is any joint design between two members in which complete weld penetration is not possible. Figure 12 shows a partial penetration single-square groove weld, a partial penetration single-V groove weld, and a partial penetration double-V groove weld. Figure 12 also shows the unique nomenclature of partial penetration welds including root penetration and joint penetration (also called the effective throat). Joint penetration of a partial penetration weld is the minimum distance (less any reinforcement) from the root of a weld to the face of the weld.



Figure 12. Partial Penetration Welds


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Combination A combination weld joint is any joint with more than one joint design. The welds that join combination joints are known by their individual names such as butt weld and fillet weld. Figure 13 shows the most common type of combination weld, a single-V groove weld with a fillet weld cover. A single-V grove weld is used almost exclusively to weld pipe nozzles to pressure vessels. Figure 13 also shows the nomenclature of a combination weld including base metal, root of weld, face of weld, single-V-groove weld, and fillet weld.



Figure 13. Combination Weld


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Stud Weld Joint A stud weld joint is a joint between a metal stud (of any shape) and a base metal. The weld that joins a metal stud to base metal is called a stud weld. Stud welds have essentially the same configuration as a fillet weld. Figure 14 shows stud joints with different stud shapes and the stud welds. Figure 14 also identifies the nomenclature of stud welds including base metal, stud, stud weld, toe of weld, and size of weld.



Figure 14. Stud Weld


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AWS Weld Symbols The American Welding Society (AWS) Standard A2.4, Standard Symbols for Welding, Brazing, and Nondestructive Examination, is the standard for weld symbols in Saudi Aramco. A weld symbol represents the weld joint on a drawing so that the details of the weld joint do not have to be shown. The specific weld symbols described in this Module include fillet, butt, partial penetration, combination, and stud. To aid in the understanding of these weld symbols, each weld symbol will include an illustration of the desired weld detail that matches the weld symbol. Elements of a Weld Symbol Figure 15 shows the basic components of a weld symbol including the reference line, arrow leader, tail, arrow side of the reference line, other side of the reference line, weld-all-around symbol, and field weld symbol. The reference line is the horizontal line from which all elements of a weld symbol are positioned. The arrow leader points to the joint to be welded. The tail is used only if additional reference information needs to be included. Weld symbols below the reference line are on the "arrow side", and the weld is made on the same side of the joint where the arrow leader points. Weld symbols above the reference line are on the "other side", and the weld is made on the opposite side of the joint from where the arrow leader points. Weld symbols that are placed both above and below the reference line are considered to be "both side", and the weld is made on both sides of the joint where the arrow leader points." The weld-all-around symbol means that a weld that extends around a series of connected joints must be completely welded around the entire series of connected joints. The weld-all-around symbol is not required for circumferential butt welds. The field weld symbol identifies those welds that must be made in the field.



Figure 15. Components of a Weld Symbol


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Additional elements can be added to this basic weld symbol to provide more weld joint information. To establish uniformity among all possible weld symbols, the AWS has standardized the location of these elements on a weld symbol. Figure 16 shows the standard location of elements for any weld symbol that includes the finish symbol, contour symbol, root opening, groove weld size, depth of penetration, welding procedure specification (or other reference), basic weld symbol, number of spot, stud, or projection welds, pitch, length of weld, and groove angle. Finish Symbol Groove Angle; Included Angle of Counter Sink for Plug Welds

Contour Symbol Root Opening; Depth of Filling For Plug and Slot Welds Depth of Bevel: Size or Strength for Certain Welds Specification, Process, or other Reference

T

S (E)

Basic Weld Symbol or Detail Reference

(BOTH SIDES)

Groove Weld Size

F A R L-P

Length of Welds Pitch (Center-To-Center Spacing) of Welds

(N)

Number of Spot, Stud, or Projection Welds

Figure 16. Standard Location of Elements on a Weld Symbol


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Fillet Weld Symbol The dimensions of fillet welds are always shown on the same side of the reference line as the weld symbol, and they generally identify the size of the weld, the length of the weld, and the pitch of the weld. Figure 17 shows a weld symbol for a 5/16" fillet weld on the arrow side of the joint. Figure 17 also shows the desired weld.

Figure 17. Arrow Side Fillet Weld Symbol Figure 18 shows a weld symbol for a 1/2" fillet weld on the arrow side of the joint and a 1/4" fillet weld on the other side of the joint. Figure 18 also shows the desired weld.



Figure 18. Both Side Fillet Weld Symbol


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Figure 19 shows a weld symbol for a 1/4" (Member A) by 1/2" (Member B) fillet weld (unequal leg) on the arrow side of the joint. Figure 19 also shows the desired weld. Because the weld symbol convention does not provide sufficient detail, a note is required when it is necessary to accurately locate the 1/2" leg.



Figure 19. Unequal Leg Fillet Weld Symbol Figure 20 shows a weld symbol for a 1/4" intermittent fillet weld on both sides of the joint that is 2" long with a pitch of 5". Figure 20 also shows the desired weld.



Figure 20. Intermittent Fillet Weld Symbol


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Figure 21 shows a weld symbol for a 3/8" staggered intermittent fillet weld that is staggered on both sides of the joint and that is 3" long with a pitch of 10". Figure 21 also shows the desired weld.

Figure 21. Staggered Intermittent Fillet Weld Symbol


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Butt Weld Symbol The dimensions of butt welds are also shown on the same side of the reference line as the weld symbol. Butt weld symbols generally identify the root opening, groove angle, contour symbol, and finish symbol. Figure 22 shows a weld symbol for a single-V groove weld with zero root opening, a reference to WPS 16, a 60° included bevel on the arrow side of the joint, and the desired weld. Use Word 6.0c or later to


Figure 22. Single-V Groove Weld Symbol


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Figure 23 shows a weld symbol for a double-V groove weld with a 1/8" root opening, a 60° included angle, a 3/4" depth of preparation, a ground convex face. Figure 23 also shows the desired weld.



Figure 23. Double-V Groove Weld Symbol Figure 24 shows a weld symbol for a single-U groove with a 1/16" root opening, a 40° included angle, and a 7/8" depth of preparation on the other side of the joint. Figure 24 also shows the desired weld.

Figure 24. Single-U Groove Weld Symbol


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Partial Penetration Weld Symbol The dimensions of partial penetration welds generally identify the root opening, groove angle, depth of preparation, groove weld size, contour symbol, and finish symbol. Figure 25 shows a weld symbol for a single-V groove weld with zero root opening, a 60° included angle on the arrow side of the joint, a depth of preparation of 1/4" and a weld size of 3/8". Figure 25 also shows the desired weld.



Figure 25. Partial Penetration Single-V Groove Weld Symbol Figure 26 shows a weld symbol for a double-V groove weld with a zero root opening, a 60° included angle, a depth of preparation of 1/4" and a weld size of 7/16". Figure 26 also shows the desired weld.



Figure 26. Partial Penetration Double-V Groove Weld Symbol


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Combination The dimensions of combination welds may include the root opening, groove angle, depth of preparation, groove weld size, contour symbol, finish symbol, fillet weld size, the length of the weld, and the pitch of the weld. Figure 27shows a weld symbol for a square groove weld (both sides) with zero root opening, a weld size of 3/8", and a 1/4" fillet weld (both sides). Figure 27 also shows the desired weld.



Figure 27. Combination Square Groove/Fillet Weld Symbol


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Figure 28 shows a weld symbol for a single-V groove weld (both sides) with a zero root opening, a 60° included angle, a depth of preparation of 1/4", a weld size of 9/16", and a 3/8" fillet weld (both sides). Figure 28 also shows the desired weld. Use Word 6.0c or later to


Figure 28. Combination Single-V Groove/Fillet Weld Symbol


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Stud Weld Joint Symbol The symbol for a stud weld is a circle with a cross in the center. Figure 29 shows a stud weld symbol with a 0.25" stud diameter, a pitch of 1", and five stud welds. The stud weld symbol does not indicate the welding of a joint in the ordinary sense; therefore, it has no arrow or other side significance. The stud weld symbol must be placed below the reference line and an arrow must clearly point to the surface to which the stud is to be welded. As with other weld symbols, the dimensions must be placed on the same side of the reference line as the stud weld symbol. Because a stud weld symbol cannot locate the first and last stud weld, the drawing must also specify the exact location of the first and last stud welds that are in a single line. In Figure 29, the first and last studs are positioned 3/4" from the edges of the plate.

Figure 29. Stud Weld Symbol


36



WELDING CONSUMABLES The following section will describe the identification standards for welding consumables. Because there are so many types of welding consumables, each type has been assigned a unique classification number by the American Welding Society (AWS). The information in this section provides some background on welding consumable identification and includes the following topics: •

Types of Welding Consumables

•

AWS Classifications

•

Storage and Handling Requirements

•

Shielding Gases and Fluxes

Types of Welding Consumables Several types of welding consumables, generally referred to as "filler metal," are available for different welding processes and materials. During welding, the filler metal melts in the heat of the welding arc and is consumed in the finished weld. The following basic types of welding consumables are described in this Module: •

Coated Electrodes

•

Bare Rods

•

Bare Wires

•

Flux Cored Electrodes

Coated Electrodes Coated electrodes are the most popular type of filler metal that is used in arc welding. Coated electrodes are also readily adaptable to field welding applications that use the shielded metal arc welding (SMAW) process. Coated electrodes have a solid metal rod as core and the electrodes have a coating of baked-on flux. The solid metal rod is made of various materials such as carbon steel, low carbon alloys, stainless steel, and nickel alloys. The formulation of the electrode flux is very complex. The flux determines the usability of the electrode, the composition of the deposited weld metal, and the specification of the coated electrode. The original purpose of the flux was to shield the welding arc from atmospheric oxygen and nitrogen. Researchers determined that ionizing agents that are added to the flux to help stabilize the arc to make the electrodes suitable for alternating current. Researchers also found that silicates and metal oxides helped to form slag. Slag improves the weld bead shape due to the reaction at the surface of the weld metal. In addition, alloy agents that are added to the flux improve the strength and provide specific weld metal deposit composition. Most recently, iron powder has been added to the flux to improve the weld metal deposition rate. Saudi Aramco DeskTop Standards

37



Today, the flux on a coated electrode is designed to achieve the following desirable characteristics. It is designed to: •

Provide a specific composition and enhance mechanical properties of the deposited weld metal

•

Reduce weld metal porosity

•

Reduce weld metal cracking

•

Provide a desirable weld deposit contour

•

Provide a desirable weld metal surface finish (e.g., smooth with even edges)

•

Reduce undercut adjacent to the weld

•

Reduce spatter adjacent to the weld

•

Control slag in all positions of welding

•

Provide a stable welding arc

•

Provide penetration control (e.g., deep or shallow)

•

Provide for immediate arc initiation and re-initiation capabilities

•

Reduce electrode overheating while welding

Two common types of coated electrodes are the cellulosic and low hydrogen-iron powder. When burned in the electric welding arc, the flux coating on a cellulose-sodium coated electrode (e.g., E6010 and E7010) produces both CO2 and hydrogen. The solid metal rod of the cellulosic electrode must contain sufficient deoxidizers to counteract the effects of oxygen from the flux. The cellulosic-coated electrodes tend to have an arc that produces deep penetration into the base metal. The weld deposit is somewhat rough and the spatter is at a higher level than other coated electrodes. Cellulosic-coated electrodes are one of the earliest types of coated electrodes that were developed. Cellulosic-coated electrodes are widely used for welding cross-country pipelines, using the downhill welding technique. The low hydrogen-iron powder-coated electrodes do not use cellulose, clays, asbestos, or other minerals that contain combined water. These components are not used to ensure the lowest possible hydrogen content in the arc atmosphere. The low hydrogen-iron powder coated electrodes provide superior weld metal properties, such as resistance to cracking, better bead appearance, and improved strength with moderate penetration.


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Bare Rods Bare rods are typically manufactured in 36" straight lengths with diameters that range from 0.045" to 3/16". Bare rods were first used with oxyacetylene welding to add filler metal to the weld joint. Today, bare rods are predominantly used with the gas tungsten arc welding (GTAW) process and the torch brazing process. Bare rods are similar to the coated electrode in that bare rods are made of various materials such as carbon steel, low carbon alloys, stainless steel, nickel alloys, and aluminum alloys. Bare Wires Bare wire electrodes are similar to bare rods except that bare wire is manufactured in continuous lengths with diameters that range from 0.020" to 1/8." The solid bare wire was developed for use with automatic and semi-automatic welding processes such as gas metal arc welding (GMAW), GTAW, and submerged arc welding (SAW). The bare wire is wound onto spools that range from 4" to 30" in diameter; however, for high volume applications, the bare wire may even be provided in large drums. Bare wire is similar to bare rods in that bare wire is made of various materials such as carbon steel, low carbon alloys, stainless steel, nickel alloys, and aluminum alloys. The carbon and low alloy steel wires are also coated with a thin layer of copper to prevent rusting and to improve the current pick-up between the contact tip and the electrode. Flux Cored Electrodes Flux cored electrodes consist of tubular wire that is manufactured in continuous lengths with diameters that range from 0.045" to 5/32." The tubular wire is actually a metal sheath that is filled with a flux material and alloying compounds. Figure 30 shows several different types of flux cored electrodes. As with coated electrodes, the flux inside the electrodes improves the welding characteristics of the electrode. The majority of flux cored electrodes are carbon steel; however, some low carbon alloys, stainless steel, and chromium-nickel alloys are also manufactured and used in flux cored arc welding (FCAW) applications.


39





Figure 30. Various Types of Flux Cored Electrodes


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AWS Classifications The American Welding Society (AWS) has established specifications for filler metals and fluxes. Each specification contains information about the chemical and physical properties of the filler metal such as manufacturing, packaging, and identification requirements; testing requirements and acceptance criteria; and additional information about the use of welding consumables. Currently, 30 specifications in the AWS A5.x series prescribe the requirements for filler metals and fluxes. These welding material specifications are identical to the specifications in Section II, Part C of the ASME B&PV Code, Specifications for Welding Rods, Electrodes, and Filler Metals. Each AWS specification covers numerous types of metallurgically similar filler metals. To readily identify each type of filler metal, the AWS has developed a unique filler metal classification system. The AWS classification system provides a unique identification for all standard filler metals including coated electrodes, bare rods, bare wires, and flux cored electrodes. Due to the large number of AWS classifications and variations within the classification system, only the most common types of filler metal classifications will be described in this module. Coated Electrodes The AWS classifications for carbon and low alloy steel coated electrodes are based on an "E" (electrode) prefix with a four or five digit number (EXXXXX). The first two digits (or three digits in a five digit number) indicate the minimum required tensile strength in the “as deposited” weld metal (not stress relieved) in thousands of pounds per square inch. For example, 60 = 60,000 psi and 100 = 100,000 psi. The next to the last digit indicates the welding position(s) in which the coated electrode can make satisfactory welds: 1 = all positions and 2 = flat and horizontal fillet welds. Table 1 is a tabulation of AWS/ASME Section II, Part C classification of E6XXX and E70XX carbon steel electrodes which shows type of electrode coating, welding position , and type of welding current and polarity. A tabulation of typical welding current ranges for these carbon steel electrodes is shown in Table 2.


41



Page left blank intentionally Table 1. AWS/ASME, Section II, Part C Classification of Coated Carbon Steel Welding Electrodes AWS CLASSIFI CATION

TYPE OF COVERING

SATISFACTORY WELDING POSITIONS(a)

TYPE OF CURRENT(b)

E60 Series Electrodes E6010

High cellulose sodium (Organic)

F, V, OH, H

DCEP

E6011

High cellulose potassium (Organic)

F, V, OH, H

AC or DCEP

E6012

High titania sodium (Rutile)

F, V, OH, H

AC or DCEN

E6013

High titania potassium (Rutile)

F, V, OH, H

AC or DC either polarity

E6020

High iron oxide

H-fillets

AC or DCEN

E6022( c )

High iron oxide

F


E6027

High iron oxide, iron powder

H-fillets, F

AC or DCEN


42



E70 Series Electrodes

a.

E7014

Iron powder, titania

F, V, OH, H


E7015

Low hydrogen sodium

F, V, OH, H

DCEP

E7016

Low hydrogen potassium

F, V, OH, H

AC or DCEP

E7018

Low hydrogen potassium, iron powder

F, V, OH, H

AC or DCEP

E7024

Iron powder, titania

H-fillets, F


E7027

High iron oxide, iron powder

H-fillets, F

AC or DCEN

E7028


H-fillets, F

AC or DCEP

E7048


F, OH, H, down

V-

AC or DCEP

The abbreviations, F, V, V-down, OH, H, AND H-fillets indicate the welding positions as follows: F= Flat H= Horizontal H-fillets = Horizontal fillets V-down = Vertical down *V = Vertical *OH = Overhead

*Note: For electrodes 3/16in.(4.8mm) and under, except 5/32 in. (4.0 mm) and under for classifications E7014, E7015, E7016. And E7018 b.

The term DCEP refers to direct current, electrode positive (DC reverse polarity). The term DCEN refers to direct current, electrode negative (DC straight polarity).

c.

Electrodes of the E6022 classification are for single-pass welds.


43



Page left blank intentionally


44



Table 2. Typical Welding Current Ranges For Mild Carbon Steel Electrodes

Electrode diameter

in. 1/16

E6010 and E6011

E6012

E6013

20 to 40

20 to 40

E6027 and E7027

E7014

E7015, E7016,and E7016-1

E7018 and E7018-1

E7024-1, E7024, and E7028

E7048

E6020

E6022

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

—

80 to 125

65 to 110

70 to 100

100 to 145

mm 1.6

5/64

— 25 to 60

25 to 60

—

3/32 a

2.4 a

40 to 80

35 to 85

45 to 90

1/8

3.2

75 to 125

80 to 140

80 t0 130

100 to 150

110 to 160

125 to 185

110 to 160

100 to 150

115 to 165

140 to 190

80 to 140

5/32

4.0

110 to 170

110 to 190

105 to 180

130 to 190

140 to 190

160 to 240

150 to 210

140 to 200

150 to 200

180 to 250

150 to 220

3/16

4.8

140 to 215

140 to 240

150 to 230

175 to 250

170 to 400

210 to 300

200 to 275

180 to 255

200 to 275

230 to 305

210 to 270

7/32

5.6

170 to 250

200 to 320

210 to 300

225 to 310

370 to 520

250 to 350

260 to 340

240 to 320

260 to 340

275 to 365

—

¼

6.4

210 to 320

250 to 400

250 to 350

275 to 375

—

300 to 420

330 to 415

300 to 390

315 to 400

335 to 430

—

5/16 a

8.0 a

275 to 425

300 to 500

320 to 430

340 to 450

—

375 to 475

390to 500

375 to 475

375 to 470

400 to 525

—

a.

—

These diameters are not manufactured in the E7028 classification


45


The AWS classifications for stainless steel coated electrodes are also based on an "E" prefix with only a three digit number and a two digit suffix (EXXX-XX). The first three digits represent the American Iron and Steel Institute's (AISI) numbering system for stainless steels. The 300 series designates austenitic stainless steels and the 400 series designates martensitic and ferritic stainless steels. The two digits in the suffix indicate the type of current and the type of coating for the electrode. These two digits are similar to the final two digits of carbon and low alloy steel coated electrodes that were shown in Table 1. Most stainless steel coated electrodes have suffixes of either 15 (lime coating) or 16 (titania coating). For example, an E308-15 coated electrode is a 308 stainless steel material that can be welded in all positions with dc+ (reverse polarity) only whenE308-16 can operate on AC or dc+. E308-15 coated electrodes have a low hydrogen coating. In the AWS classification E308L-15, the "L" represents a low carbon version of the filler metal with a maximum of 0.03% C and a minimum of 0.08% C for conventional grades. Bare Rods and Wire The AWS classifications for carbon and low alloy steel bare rods and wire have an "ER" prefix with a two or three digit number and a one or two digit suffix (ERXXXS-XX). The "E" indicates an electrode, and the "R" indicates a welding rod; therefore, "ER" indicates either an electrode or a welding rod. The first three digits "XXX" indicate the minimum required tensile strength in thousands of pounds per square inch. These three digits are similar to the first three digits of the carbon coated electrodes. The "S" indicates a solid electrode or rod. The one or two digits in the suffix indicate the chemical composition of the deposited weld metal. The AWS classifications for stainless steel bare rods and wire have an "ER" prefix with a three digit number (ERXXX). The first three digits represent the American Iron and Steel Institute's (AISI) numbering system for stainless steels. The 300 series designates austenitic stainless steels, and the 400 series designates the martensitic and ferritic stainless steels. In the AWS classification ER308L, the "L" represents a low carbon version of the filler metal. Flux Cored Electrodes The AWS classifications for carbon and low alloy steel flux cored electrodes have an "E" prefix with a two digit number "XX and a one digit suffix (EXXT-X). The "E" indicates an electrode and the "T" indicates a tubular electrode. The first two digits indicate the minimum required tensile strength in thousands of pounds per square inch. These first two digits are similar to the first two digits of the carbon coated electrodes. The one digit suffix indicates the chemical composition of the deposited weld metal, gas type, and usability factor. For example, an E70T-1 flux cored electrode has a 70,000 psi tensile strength and it uses CO2 shielding gas for flat position welding. The method of classification of carbon steel and stainless steel electrodes for flux-cored arc welding is illustrated in Figure 31.


46


Designates an electrode. Indicates the minimum tensile strength of the deposite weld metal in a test weld made with the electrode and in accordance with specified welding conditions. Indicates the primary welding position for which the electrode is designed:

EXXT-X

0-flat and horizontal positions 1-all positions Indicates usability and performance capabilities.

Indicates a flux cored electrode. Note:The letter " X " as used in this figure and in electrode classifications this specification substitutes for specific designations indicated by this figure.

(A) Carbon Steel flux-cored electrodes such as E70T-1 Indicates an electrode. Designate classification according to compposition Designates a flux cored electrode Designates the external shielding medium to be employed during welding (SeeNotes)

EXXXT-X (B) Stainless steel flux-cored electrodes such as E316T-3. EXXXT-1 – designates an electrode using carbon dioxide shielding plus a flux system. EXXXT-2 – designates an electrode using a mixture of argon with 2% oxygen plus a flux system. EXXXT-3 – designates an electrode using no external shielding gas wherein shielding is provided by the flux system contained in the electrode core (self-shielding). EXXXT-G – indicates an electrode with unspecified method of shielding, no requirements being imposed except as agreed between purchaser and supplier. Each producer of an EXXXT-G electrode shall specify the chemical composition and mechanical property requirements for his electrode.

Figure 31. Classification Method of Carbon Steel and Stainless Steel Flux-Cored Electrodes.


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Shielding Gasses and Fluxes All welding processes require some protection for the molten weld metal while welding. This protection is called weld shielding and it takes the form of either shielding gas or flux. The primary purpose of the shielding gas or flux is to protect the molten weld metal from contamination by the oxygen and nitrogen in the atmosphere. Gases Weld shielding gases are considered consumables and are mostly used with the GTAW,GMAW, and the FCAW welding processes. The quality of commercial shielding gases are governed by specifications that were developed by the Compressed Gas Association. Although argon and helium are the only two inert gases that are available in sufficient quantities to support welding operations, the inert gases argon, helium, neon, krypton, and xenon are commercially available. Argon – is an inert gas that is most widely used by Saudi Aramco with the GTAW process. Because it is heavier than air, argon forms a protective blanket over the weld area. Helium – because it is lighter than air, i6t does not make a good shielding gas when welding outdoors. If it is used, helium must be highly controlled to ensure that the shielding gas can actually protect the molten weld metal and not rise from the weld and disperse. When butt welding the stainless and nickel alloy steels, a purge gas must also be used to protect the molten weld metal that is at the root of the weld. Carbon Dioxide (CO2)– This gas is most widely used by Saudi Aramco to weld carbon and low alloy steels with the GMAW and FCAW processes. Because CO2 contains oxygen, the filler metals must contain sufficient deoxidizers to counteract the effects of oxygen from the shielding gas. CO2 is a used mainly because of its relative low cost. Fluxes Fluxes are also considered consumables and are used mostly with the SMAW, FCAW, and the SAW processes. The coatings on carbon and low alloy steel electrodes may have from 6 to 12 of the following ingredients: •

Cellulose - to provide a gaseous shield with a reducing agent. The gas shield that surrounds the are is produced by the disintegration of the cellulose.

•

Metal Carbonates - to adjust the basicity of the slag and to provide a reducing atmosphere.

•

Titanium Dioxide - to help form a highly fluid but quick-freezing slag and to provide ionization for the welding arc.


48


•

Ferromanganese and Ferrosilicon - to help oxidize the molten weld metal and to supplement the manganese and silicone content of the deposited weld metals.

•

Clays and Gums - to provide the elasticity for extruding the plastic coating material and to help provide strength to the coating.

•

Calcium Fluoride - to provide shielding gas to protect the welding arc, to adjust the basicity of the slag, and to provide fluidity and solubility of the metal oxides.

•

Mineral Silicates - to provide slag and to give strength to the electrode.

•

Alloying Metals -to provide alloy content, (such as, nickel molybdenum, and chromium) to the deposited weld metal.

•

Iron or Magnesium Oxide - to adjust the fluidity and other properties of the slag.

•

Iron Powder - to increase the productivity by providing additional metal to be deposited in the weld.

Through combinations of various amounts of the above ingredients, an infinite variety of electrode coatings are possible. The flux in flux cored electrodes is essentially the same as the flux on coated electrodes, however, to do the same job, more flux is required on the coated electrodes than in flux cored electrodes. When electrodes are manufactured, binders (other ingredients) are added to keep the coating intact. The binders allow for the extrusion f the coating. Because the flux on a coated electrode contains additional binders a greater volume of flux is required. The flux that is used with the submerged arc welding (SAW) process is separate from the filler metal. The flux is a granular form that is made up of may of the same ingredients that were listed previously for the SMAW coated electrodes. Three type of SAW fluxes used are as follows: •

Fused Flux - the ingredients are dry mixed, melted in a furnace at approximately 2,900°F, and quenched to form a glassy material This material is then dried crushed, sized, and packaged.

•

Agglomerated Flux - - ingredients are wet mixed and dried in a rotary kiln at approximately 1,800°F to form small balls of flux. After the flux balls are cooled the balls of flux are sized and packaged.

•

Bonded Fluxes - are very similar to the agglomerated fluxes with the exception that the flux mixture is bonded at a lower temperature. When alloy materials are not added to the flux, the flux is considered “neutral”.

In accordance with the Saudi Aramco standards, agglomerated fluxes and bonded fluxes should not be used because of their high affinity for moisture. Welds that are made by using these fluxes are know to develop weld metal hydrogen delay cracks.


49


Storage and Handling Requirements The proper storage and handling of filler metals is crucial to maintain the cleanliness and quality of the filler metal. The storage of filler metal includes not only the packaging requirements of the filler metal but also the interim storage requirements of the packages until the filler metal is actually used. The handling of filler metals refers to the following movements: •

Movement of the filler metal from the receipt location to the storage location

•

Movement of the filler metal from the storage location to the issuance location

•

Movement of the filler metal from the issuance location to the work location

Coated Electrodes Because of the flux coating, coated electrodes can be easily damaged when improperly stored or handled. Rough handling in shipment or in storage can cause a portion of the flux coating to crack loose from the solid metal rod, which can make the electrode unsuitable for welding. When most coated electrodes are bent, the flux coating will crack loose from the solid metal rod. When opening a sealed container of coated electrodes, the container must be inspected for evidence of damage. A dented or punctured container can indicate possible damage to the coating of the electrodes. When the solid metal rod is exposed, coated electrodes should not be used. Although coated electrodes may look similar, not all coated electrodes have the same packaging and storage requirements. When the flux coating is exposed to moisture for an extended period of time, some coated electrodes may become unusable. The popular low hydrogen coated electrodes are extremely sensitive to moisture (e.g., rain, humidity). The cellulosic-coated electrodes are less sensitive to moisture pick-up and actually require moisture levels of three to seven percent to maintain the flux coating. Today, the majority of coated electrodes are packaged in hermetically-sealed metal containers to prevent excessive moisture intrusion. Coated electrode containers must be stored in a clean, dry area. Coated electrode containers should be stored separately by AWS classification. Once the low hydrogen coated electrodes are removed from the container, the coated electrodes must be placed in storage ovens at approximately 250°F to ensure that the coated electrodes do not absorb any moisture. When the low hydrogen coated electrodes are required to be taken into the field (for more than four hours) to perform welding, the low hydrogen coated electrodes must be placed in small portable storage ovens at approximately 150°F. SAES-W-001 specifies storage temperatures for types of L.H. electrodes. The lids on the portable storage ovens must remain closed at all times except when electrodes are removed.


50


When the low hydrogen coated electrodes are used in the field for less than four hours, the low hydrogen coated electrodes must be kept dry but they do not need to be stored in a heated oven. All low hydrogen coated electrodes that are exposed to the atmosphere must be rebaked for a period of at least four hours. Saudi Aramco procedures SAES-W-001 and SADP-W-001 specify the proper rebake temperature for a variety of coated electrodes. The cellulosic-coated electrodes do not require storage in heated ovens, but they must be kept in a dry environment. Coated electrodes should never be placed directly on cold concrete or steel that can draw heat out of the coated electrode and allow moisture to enter the coating. Coated electrodes that become wet should never be used for welding and must be discarded. Bare Rods and Wire Bare rods and wire must be stored in sealed containers, which must be placed in a clean, dry environment. Any bare rods or wires with visible rusting or contamination must not be used for welding and must be discarded. After removal from the sealed containers, bare rods and wires must remain dry and clean in the field. Contamination on bare wire can cause porosity in the weld or operation difficulties in the wire feed mechanism. Flux Cored Electrodes Flux cored electrodes must also be stored in sealed containers in a clean, dry environment. Any flux cored electrodes with visible rusting or contamination must not be used for welding and must be discarded. After removal from the sealed containers, flux cored electrodes must remain dry and clean in the field. Contamination on flux cored electrode wire can cause porosity in the weld or operation difficulties in the wire feed mechanism. Consideration must be given to the storage of flux cored electrodes in heated ovens based on the type of flux and the electrode manufacturer's recommendations.


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HEAT INPUT EFFECTS The following section introduces the relation of heat input to welding. Heat input will be described in terms of the welding process variables that contribute to variations in the amount of heat input to a weld. This section includes the following topics: •

Parameters

•

Effects of Heat Input

Parameters Heat is required for all welding processes to melt the surface of the metal to be welded and the filler metal that is added to the weldment so that coalescence can occur. The most common heat source for welding is the electric arc. The three key welding parameters affect the amount of heat input to a weld are: •

The welding current

•

The welding arc voltage

•

The travel speed of the welding process

Preheat and interpass temperatures are additional variable that can affect the amount of heat input to a weldment. Because preheat is not directly associated with a welding process, preheat will be described in the next section. Each of the three welding parameters is described below. Current The current in a welding circuit is the amount of electric charge that flows through the welding cable in one second. The amount of electric per second that flows through the welding cable is called an ampere and it is designated by the letter "I". High welding current results in greater base metal penetration, and a lower welding current results in shallow base metal penetration. An increase in the welding current increases the heat input to the weld. An increase in the welding current also increases the melt-off rate of the electrode and improves productivity because more weld metal is deposited.


52


Voltage Voltage is the force that causes a current to flow. The measure of electrical pressure is the volt. The difference in potential or voltage causes the current to flow in an electric circuit. As Equation 1 shows, both current and voltage affect the heat input to a weld. The letter "V" is used to designate voltage. High welding voltage results in greater base metal penetration, while lower welding voltage is indicative of shallow base metal penetration. An increase in the welding voltage increases the heat input to the weld. Travel Speed The rate that a welding electrode progresses along a weld joint while welding is called the travel speed. The letter "S" designates the travel speed. Several welding variables affect the travel speed, such as, the welding process, the position of the weld, the welder, and the wire feed speed. Slow travel speeds make wide weld beads with deep base metal penetration. Fast travel speeds make narrow weld beads and shallow base metal penetration. A decrease in the travel speed increases the heat input to the weld. Effects of Heat Input The electric arc welding process uses a high temperature heat source that can melt the base metals. An extensive difference in temperature between a high temperature heat source and the base metal can cause thermal expansion and contraction between the base metal and the weld metal. The following are some of the disadvantages of excessive heat input that can affect weld quality: •

Warpage and distortion caused by high residual differential shrinkage stresses

•

Cracks caused by a reduction of ductility or a degree of hardening

•

Premature failure caused by the deterioration of the toughness properties of the weld joint

•

Premature failure caused by the loss of strength of certain work hardened, quenched, and tempered materials

Even though the electric welding arc is a heat source that moves continuously, steady state conditions are established and the temperature distribution relative to the heat source is relatively stable.


53


Because many variables are involved, the heat input-time-temperature relationship (the thermal cycle of a weld) cannot be precisely determined. However, fairly accurate estimates can predict the effects of heat input from a specific welding process. The total heat input to a weldment must be balanced to produce the desired weld properties. Extra heat is required, over and above the heat that is needed to melt the base metal and filler metal, to compensate for the heat that is conducted away from the weld and into the adjacent base metal. The heat input to a weldment (in joules per inch of weld) by a welding process can be estimated with Equation 1, located in Work Aid 4 EXAMPLE: If a weld is to be made with the SMAW process and a 1/8" coated electrode at 145 amperes, 22 volts, and a travel speed of ten (10) inches per minute, the amount of heat input to the weld could be estimated as follows: Heat Input =

(145 × 22)( 60) 10

= 19,140 joules per inch

If a 5/32" coated electrode is used at 210 amperes and 24 volts with a travel speed of only six (6) inches per minute, the heat input is significantly increased. Heat Input =

( 210 × 24)( 60) 10

= 30,240 joules per inch

This heat input equation is used to calculate the heat that is developed in an electric arc and can be used to compare welding procedures when heat input is a consideration. The base metal temperature changes in an arc welding operation are much quicker and more abrupt than for most metallurgical processes (e.g., heat treatments). The metallurgical reactions from welding heat input do not follow the normal heat treating relationships due to the short time duration at temperature. More specifically, in the arc welding process, melting and solidification occur with relative quickness; and equilibrium is not achieved as it is achieved in direct treatment processes.


54


HEAT TREATMENT EFFECTS With some welding processes and materials, heat must be applied to a weld prior to welding (preheat treatment). Other welding processes and materials require the use of heat after the welding is complete (postweld treatment). The information in this section provides background on the heat treatment of welds and includes the following topics: •

Preheat

•

Postweld Heat Treatment

Preheat Preheat is defined as the heat that is applied to the base metals of a weld joint immediately before welding. The construction standards that are associated with systems and components at Saudi Aramco facilities identify the required preheat temperatures for the various base metals. However, the preheat temperature depends on many factors such as the composition of the base metal, the ambient temperature, and the welding procedure. Several methods of preheat determination and application, along with typical preheat requirements for commonly used materials, are described in more detail below. Purpose The main purpose of preheat is to reduce the rate at which a weld cools. The preheat of weldments has the following advantages: •

Reduces shrinkage stresses in the weld and heat-affected zone that lead to cracks

•

A slower rate at which the weld metal cools through the critical temperature range (approximately 1,600°F to 1,330°F), which prevents excessive hardening and loss of ductility of both the weld metal and the heat-affected zone

•

A slower rate at which the weld metal cools through the 400°F range, which allows more time for any hydrogen that is present to diffuse away from the weld and adjacent base metal to avoid underbead cracking

•

Maintains sufficient heat at the weld area on highly conductive or thick base metals

•

Removes moisture from the weld joint


55


Figure 32 is a graph of temperature versus time that shows the effect of preheat on a plate butt weld. As the graph shows, a greater temperature drop in one second exists at a given temperature (T1) when the initial temperature (To) of the plate is 70°F than when To is 300°F. In other words, the cooling rate (°F/sec) is slower when preheat is used. Preheat should also be considered before thermal cutting (i.e., oxyacetylene cutting). Thermal cutting tasks may include the preparation of weld joint bevels, the removal of attachments, or the removal of defective material. For thick carbon and low alloy steels, preheat ensures that the base metal cools at a slower rate and prevents excessive hardening and loss of ductility in the base metal. Use Word 6.0c or later to


Figure 32. Effect of Preheat on Cooling Rate Methods Several methods are available for preheat treatment. The most common tools are oxyacetylene torches and electric resistance heaters. The choice of the preheat tools depends on factors such as the following: •

The preheat temperature

•

The duration of the preheat cycle

•

The size and shape of the weldment

•

The need for a one-of-a-kind or a continuous production preheat operation


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For the majority of pipe welds, an oxyacetylene torch provides sufficient preheat to support the welding operation. Oxyacetylene torches are usually limited to small weldments or circumferential pipe welds that are less than 12" in diameter and that are less than 3/4" thick. For small weldments or circumferential pipe welds, actual preheat temperature is measured with a temperature indicating crayon. When an oxyacetylene torch is used to preheat a weld, the surface of the base metal is generally much hotter than the average temperature in the base metal. When possible, temperature measurements should be made on both sides of the weld joint. Because the welder must constantly switch between the welding process and the preheat process, the use of an oxyacetylene torch is not the most productive tool to apply preheat. When preheat for a single weld or for multiple welds that are in close proximity is required for a long period of time, electric resistance heaters are often more convenient to use than oxyacetylene torches. Gas burners are more effectively and more widely used in pressure vessels (and in building industries in general) than handheld torches. The resistance heater elements are commonly available in either rope, rope pads, or ceramic pads. Figure 33 shows a typical arrangement for rope resistance heaters and the power connections that provide preheat for a circumferential butt weld in pipe. Use Word 6.0c or later to


Figure 33. Typical Arrangement of Rope Resistance Heaters


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Figure 34 shows a typical arrangement for resistance heating pads and the power connections that provide preheat for a circumferential butt weld in pipe. Use Word 6.0c or later to


Figure 34. Typical Arrangement of Resistance Heating Pads Figure 35 shows a typical arrangement for ceramic resistance heater pads and the power connections that provide preheat for a circumferential butt weld in pipe. The limitations of electric resistance heaters include the inability to adapt to small intricate parts and to adequately heat materials greater than six inches thick. Use Word 6.0c or later to


Figure 35. Typical Arrangement of Ceramic Resistance Heater Pads


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Figure 36 shows an electric resistance heater system that consists of a power supply, a temperature controller, a temperature recorder, resistance heating wires, power cables, thermocouples, and thermocouple signal cables. In electric resistance heaters, thermocouples attach directly to the base metal that is adjacent to the weld to measure the exact preheat temperature of the weld. The thermocouples provide signals to the temperature controller that regulates the electrical power that is required for the preheat. The temperature recorder makes a permanent record of the exact preheat temperature throughout the preheat and welding operation.



Figure 36. Electric Resistance Heater System


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Determination The necessity to preheat weld joints and the temperature requirements should be established by Engineering and should be demonstrated by a welding procedure specification (WPS). The preheat temperature depends upon these factors: •

Type of base metal and its composition

•

Joint thickness degree of restraint

•

Type and composition of filler metal

The interpass temperature should also be considered. The interpass temperature is the highest temperature in the weld joint immediately prior to welding. Usually, the minimum interpass temperature will be the same as the preheat temperature. The weldment temperature should never be allowed to become lower than the preheat or the interpass temperature. When welding is interrupted for any reason, the preheat temperature must be attained before welding is started again. Preheat and interpass temperatures must be maintained through the entire thickness of the welding area. The interpass temperature is usually specified as a maximum temperature to prevent excessive heat input to a weldment. When welds are made on a small weldment, the interpass temperature increases due to the heat input from welding. Under certain conditions, allowing the interpass temperature to exceed a specific temperature is usually not desirable; therefore, a maximum interpass temperature is specified. When the heat build up becomes excessive, the weldment must be allowed to cool but must not cool below the minimum preheat temperature. Otherwise, distortion and lowered ductibility of the weld joint can result. The temperature of the welding area must be maintained within the minimum preheat and the maximum interpass temperature. Higher preheat temperatures should be considered when the base metal has a carbon content in excess of 0.30%, when the base metal is thick (over 1-1/2"), or when the weld joint is highly restrained (e.g., a piping closure weld). Carbon Equivalent (CE) – While the material thickness, the type of base metal to be welded, the degree of joint restraint, and the filler metal are taken into account when preheat requirements are determined, some adjustment may be needed for specific material composition. Generally, as the carbon content of a material increases, the necessity for preheat also increases but the critical cooling rate decreases. However, carbon is not the only element that influences the critical cooling rate.


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Other elements in steel materials are responsible for the hardness and the loss of ductility that occur with rapid cooling. The determination of preheat requirements must account for the total hardenability of a material. This total hardenability can be represented by a "carbon equivalent". This common measure of the effects of carbon and other alloy elements on hardening is the basis for preheat and interpass temperature estimates. CE empirical values that represent the sum of the effects of various elements on the hardenability. One of the most widely used carbon equivalent formulas is shown in Equation 2, Work Aid 5. For carbon steels (all ASME p-No.2 materials, including API pipe grades up to and including X60), the minimum preheat is calculated using Standard Drawing AE-036451. For carbon steel piping and pipeline welds, the minimum preheat requirements are listed in the preheat tables of SAES-W-011 and SAES-W-012. For all other materials, the minimum preheat shall be no less than what is listed or specified in the applicable codes and standards, such as ASME/ANSI B31.3, B31.4, B31.8, and AWS D1.1, Structural Welding Code. Metal Thickness –As previously stated, the thickness of the material to be welded also affects the required amount of preheat. In general, thicker materials require more preheat. The required amount of preheat can be determined through review of the applicable construction standards or industry guidelines that are based on carbon equivalency. The preheat requirements of applicable construction standards will be discussed in the following paragraphs. The ASME standards present the minimum preheat temperature based upon the applicable P Number (from ASME Section IX) of the materials to be welded. When materials of two different P Numbers are welded together, the preheat temperature is normally the highest preheat temperature that is recommended for either of the materials. The AWS D1.1 standard also presents the preheat temperature requirements based on similar material (specific to structural construction) groups. The API standards provide more generic preheat temperature information. The preheat temperature information that is presented below is based on the applicable construction standards. Table 3 summarizes the ASME preheat requirements for the following materials: •

P Number 1 (carbon steel)

•

P Number 3 (low-alloy steels)

•

P Number 4 (1-1/4 chrome-moly steels)

•


•

P Number 8 (stainless steels)


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Table 3. ASME Construction Standard Preheat Temperature Requirements



Where: C Cr t Tp Ts

= = = = =

carbon content chromium content material temperature preheat temperature minimum specified tensile strength


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The AWS D1.1 standard categorizes structural steel base materials into four main groups that are, for the most part, considered carbon steels. Slight differences exist between each of the categories, and base metal thickness is the primary determinant of the minimum preheat temperatures. Table 4 shows the preheat temperature ranges have been developed and are based upon the four standard thickness ranges that are listed in AWS D1.1. Table 4. Preheat Temperature Ranges for the Four Thicknesses Ranges Listed In AWS D1.1. Thickness of Thickest Part at Point of Welding (Inches

Preheat Temperature (°F) Range ????????

Up to 3/4 Over 3/4 through 1-1/2 Over 1-1/2 through 2-1/2 Over 2-1/2

????????? None 50 150 225

50 150 225 300

API 620 and 650 do not specifically require preheat treatment when welding tanks. However, both construction standards do acknowledge the benefit of preheat and suggest that all preheat treatments be qualified with the welding procedure specification prior to production welding. Filler Metal Coating Type and Composition – Minimum preheat temperature is also affected by the type of electrode coating (low versus non -low hydrogen) types as well as its chemical composition. When used to weld materials of the same thickness and chemical composition, the minimum preheat temperature is much higher with the non-low hydrogen electrodes than those of the low hydrogen type. Similarly, welding electrodes containing alloys are elements that contribute to the weld ????????? such as Cr, Mo, etc. will require higher preheat than those that don’t contain these elements. The difference in preheat temperatures levels as a function of welding electrode types is shown by preheat tables of AWS D1.1 structural welding and Saudi Aramco piping and pipeline standards SAES-W-011 and -012. Postweld Heat Treatment A number of postweld heat treatments exist for weldments but stress relief is the most widely used postweld heat treatment. Some other postweld heat treatments include annealing and normalizing. For the purposes of this Module, postweld heat treatment will be synonymous with stress relief. Postweld heat treatment is any heat treatment that is applied to a weld or weldment after welding to reduce residual stresses. The construction standards for systems and components at Saudi Aramco facilities identify the required postweld heat treatments for the various base metals. However, the postweld heat treatment depends on many factors such as the *composition* of the base metal joint thickness and type of service. Several methods of postweld heat treatment temperature determination and application, along with typical postweld heat treatment requirements for commonly used materials are described in more detail below.


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Purpose The purpose of postweld heat treatments is to reduce the residual stresses that are within weldments. The following are the advantages of postweld heat treatment of weldments: •

Reduce residual stresses that are inherent to any weldment, casting, or forging

•

Soften hardened weld zones

•

Improve resistance to corrosion and caustic embrittlement

•

Improve dimensional stability of the weldment when machined

•

Increase service life of the weldment

Methods With the exception of welding torches, similar methods as that utilized for preheat are used for use for postweld heat treatment (PWHT) of welds Electric resistance heaters are the most popular method of PWHT for field applications. Postweld heat treating furnaces are generally very large permanent structures that can accommodate an entire pressure vessel. Furnaces are generally used by manufacturers of large components that require PWHT. Temporary furnaces are also used and are usually built around a field constructed component such as a pressure vessel. The choice of the PWHT methods is similar to preheat treatment methods and depends on factors such as the following: •

The postweld heat treatment temperature

•

The duration of the postweld heat treatment cycle

•

The size and shape of the weldment or component

•

The need for a one-of-a-kind or a continuous production postweld heat treatment operation

Electric Resistance Heaters – For most pipe welds, electric resistance heaters suffice for postweld heat treatment operations. These resistance heaters are identical to the preheat devices that were described earlier in this Module. Figure 36 showed an electric resistance heater system that consisted of a power supply, a temperature controller, a temperature recorder, resistance heating wires, power cables, thermocouples, and thermocouple signal cables. As with preheating operations, thermocouples directly attach to the base metal adjacent to the weld to measure the exact PWHT weldment temperature. The thermocouples provide signals to the temperature controller to regulate the electrical power that is required for the PWHT. The number and location of thermocouples that should be attached to the weldment and other PWHT requirements are outlined in SAES-W-010, SAES-W-0011, and SAES-W-012.


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Figure 36. Electric Resistance Heater System To reduce the time that is necessary to get the weldment up to the PWHT temperature and to provide a slower cooling down period, welds that require PWHT are usually well insulated. As with preheat, the temperature recorder provides a permanent record of the exact PWHT temperature throughout the PWHT. The major limitations of electric resistance heaters include the inability to adapt to small intricate parts and the inability to adequately heat very thick materials. Furnace PWHTs – When using the furnace PWHT, the following factors must be consider: •

The support requirement of the component to be heat treated

•

Freedom of the material to expand and contract

•

The placement of sufficient thermocouples to verify the accuracy of the PWHT

•

The type of heat source


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The heat source for furnaces is either natural gas or fuel oil. For uniform application of heat, the component to be treated is carefully and strategically surrounded by heating nozzles. To expedite the heating process, heating nozzles may also be arranged inside the component to improve the heat-up rate and thorough heat soak of the component. The component's location in the furnace must also be considered to avoid hot or cold spots. The location of severe temperature gradients depends upon the arrangement of the furnace and the location of the heat source. The duration of a PWHT cycle includes the heating time to maximum temperature (ramp up), the holding time at the specified maximum temperature (soak), and the cooling time to ambient temperature (ramp down). A typical soak time that is specified in the construction standards is one hour per inch of material thickness. In some cases, obtaining the required maximum PWHT temperature or soak time is not possible. In these cases, the construction standards have provisions for alternate temperatures and soak times. Regardless of the PWHT method that is employed, the heating and subsequent cooling rates are critical to the success of the operation. When the heating rate is too high, the temperatures of thin sections of material increase faster than thick sections. Similarly, when the cooling rate is too low, the temperatures of thin sections of material decrease faster than thick sections. Nonuniform heating and cooling can cause distortion, residual stresses, and cracks. For this reason, the heating and cooling rates are specified by the applicable construction standards. Requirements The majority of postweld heat treatment applications in Saudi Aramco must be in accordance with ASME Section VIII, Division 1, Paragraph UCS-56. All construction standards that were addressed in earlier Modules contain mandatory PWHT requirements for specific types of materials. When a PWHT temperature must be determined for a particular welding operation, the applicable construction standard is the first place to look. Typically, the construction standards require that the heating rate must not exceed 300°F to 400°F per hour when the base metal of the weld or component is above 800°F. While the weld or component is ramping up in temperature, the heating rate is not critical below 800°F. The cooling rate must not exceed 400°F to 500°F per hour when the base metal of the weld or component is above 800°F. While the weld or component is ramping down in temperature, the cooling rate is not critical below 800°F.


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The postweld heat treatment temperature information presented below is based on the applicable construction standards. This information has been greatly simplified because the construction standards provide many exemptions from the mandatory PWHT requirements. The PWHT temperature requirements for materials that are covered by ASME construction standards are similar to each other but care must be taken because the differences are very subtle. Specific PWHT requirements must be directly derived from the applicable construction standards. Figure 39 summarizes the ASME PWHT requirements for the following materials: •

P Number 1 (carbon steel)

•

P Number 3 (low-alloy steels)

•


•


Carbon Steels – The PWHT temperature requirements for mild carbon steels (P Number 1) that are covered by ASME Section I, Section VIII, B31.1, and B31.3 are similar. The PWHT temperature for materials with a thickness that exceeds 3/4" is generally between 1,100°F and 1,200°F. ASME Section VIII is the only exception in that PWHT is not required until the material thickness (t) exceeds 1-1/2". Materials between 1-1/4" and 1-1/2" in thickness also require PWHT per Section VIII when the preheat temperature (Tp) is less than 200°F. Table 5 summarizes the ASME preheat requirements for P Number 1 materials.


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Table 5. ASME Construction Standard PWHT Temperature Requirements Use Word 6.0c or later to


Where: C Cr O.D. NPS t Tp Ts

= = = = = = =

carbon content chromium content outside diameter nominal pipe size material temperature preheat temperature minimum specified tensile strength


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The AWS D1.1 construction standard does not specifically require PWHTs of weldments. AWS D1.1 acknowledges that the need for PWHT must be identified on the contract drawings or specifications. However, AWS D1.1 does provide guidelines for PWHT temperature between 1,100°F and 1,200°F for most carbon steels and heating and cooling rates that are similar to those previously discussed. Because the size and weight of field erected tanks do not permit adequate support at PWHT temperatures, API 620 does not specifically require the use of PWHT when welding tanks. API 650 does describe PWHT for all flush-type cleanout fittings and shell connections. In general, the PWHT temperatures are between 1,100°F and 1,200°F. Variations in the PWHT requirements are based on the diameter of the opening, the material group, and the thickness of the tank shell material. Low Alloy Steels – The postweld heat treatment temperature requirement for low alloy steels (P Number 3) in ASME Section I and B31.1 is 1,100°F when the material thickness (t) is in excess of 5/8" and the carbon content (C) is in excess of 0.25%. ASME Section VIII requires PWHT at 1,100°F for all P Number 3, Group Number 3, materials. Group Number 1 and 2 materials require PWHT at 1,100°F when the material thickness exceeds 1/2" and the carbon content exceeds 0.25%. ASME B31.1 requires PWHT at temperatures between 1,100°F and 1,200°F when the material thickness exceeds 5/8" and the carbon content exceeds 0.25%. ASME B31.3 requires PWHT at temperatures between 1,100°F and 1,200°F when the material thickness exceeds 3/4" or the minimum specified tensile strength (Ts) exceeds 71 ksi. Figure 39 summarizes the ASME PWHT requirements for P Number 3 materials. The postweld heat treatment temperature requirement for low alloy steels (PNumber 4) in ASME Section I and VIII is 1,100°F when the outside diameter (O.D.) is greater than 4", when the material thickness (t) is greater than 5/8", or when the carbon content (C) is greater than 0.15%, when the minimum preheat temperature(Tp) is less than 250°F. ASME B31.1 requires PWHT at temperatures between 1,300°F and 1,375°F when the nominal pipe size (NPS) is greater than 4", the material thickness (t) is greater than of 1/2", the carbon content (C) is greater than 0.15%, or the minimum preheat temperature(Tp) is less than 250°F. ASME B31.3 requires PWHT at temperatures between 1,300°F and 1,375°F when the material thickness (t) is greater than 1/2" or the minimum specified tensile strength (Ts) exceeds 71 ksi. Figure 39 summarizes the ASME PWHT requirements for P Number 4 materials.


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The postweld heat treatment temperature requirement for low alloy steels (PNumber 5) in ASME Section I and VIII is 1,250°F when the chromium content is greater than 3.0%, when the outside diameter (O.D.) is greater than 4", when the material thickness (t) is greater than 5/8", when the carbon content (C) is greater than 0.15%, or when the minimum preheat temperature(Tp) is less than 300°F. ASME B31.1 requires PWHT at temperatures between 1,300°F and 1,400°F when the nominal pipe size (NPS) is greater than 4", when the material thickness (t) is greater than 1/2", when the chromium content is greater than 3.0%, when the carbon content (C) is greater than 0.15%, or when the minimum preheat temperature(Tp) is less than 300o F. ASME B31.3 requires PWHT at temperatures between 1,300°F and 1,400°F when the material thickness (t) is greater than 1/2", the chromium content is greater than 3.0%, or the carbon content is greater than 0.15%. Figure 39 summarizes the ASME PWHT requirements for P Number 5 materials.


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GLOSSARY acidity

A chemical term relating to the quality or condition of being acidic in composition.

annealing

A heat treatment that increases the temperature of steel above the critical temperature and then is slowly cooled to remove internal stresses that result in a steel of lower strength and higher ductility.

basicity

A chemical term relating to the quality or condition of being basic in composition.

coalescence

The growing together or growth into one body of the materials being welded.

interpass temperature

The highest temperature in the weld joint immediately prior to welding, or, in the case of multiple pass welds, the highest temperature that is in the section of the previously deposited weld metal immediately before the next pass is started.

liquidus temperature

The lowest temperature at which a metal or alloy is completely liquid, i.e., the temperature at which freezing starts.

normalizing

A heat treatment that increases the temperature of steel above the critical temperature and is then air cooled to remove internal stresses that result in a steel of higher strength and lower ductility than annealing.

postweld heat treatment

For stress relieving applications, any heat treatment that is applied to a weld or weldment subsequent to welding in order to reduce stresses or to alter the weld structure.

preheat

Heat that is applied to the base metals of a weld joint immediately before welding.

reducing agent

A material that adds hydrogen to an element or compound.

reducing treatment atmosphere

An atmosphere of hydrogen (or other substances that readily provide electrons) surrounding a chemical reaction or physical device. The effect is the opposite to that of an oxidizing atmosphere.

root penetration

The depth of fusion that is obtained with the root pass.


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solidification temperature

Synonymous with solidus temperature.

solidus temperature

The highest temperature at which a metal or alloy is completely solid, i.e., the temperature at which melting starts.

temperature indicating crayon

A temperature measuring device that is made from a chalk-like material and that is formulated to melt at specific temperatures with a ± 1% accuracy

thermocouple

A thermoelectric device that measures temperature differences.

weldment

An assembly of component parts that is joined by welding.


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WORK AID 1:

HOW TO IDENTIFY THE MOST COMMONLY

The following Work Aid will assist you in identifying the most common metals used in the oil and gas industry. TYPE OF METAL OR SUBGROUP Carbon Steel (Three Types)

DESCRIPTION Carbon content is less than 1 percent Manganese content is less than 1.65 percent Copper and silicon content are each less than 0.60 percent (1). Low-carbon Steel – to 0.25% carbon ( c ) -0.25 to 1.5 % magnesium (Mn) (2). Medium-carbon Steel –0.25 to 0.50% c, 0.60 to 1.65% Mn.

•

Low Alloy Steels

(3). High-carbon Steels –0.50 to 1.03% c, 0.30 to 1.00% Mn. The amount of manganese is greater than 1.65 percent. The amount of silicon is greater than 0.60 percent. The amount of copper is greater than 0.60 percent. A definite minimum quantity of any of the following elements is specified or required in alloy steels: aluminum, boron, chromium up to 3.99 percent, cobalt, columbium, molybdenum, nickel, titanium, tungsten, vanadium, or zirconium. Any other alloying agent is added to obtain a desired alloying effect.


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•

Stainless Steels (Five groups)

All stainless steels contain iron as the main element and chromium in amounts that vary from about 11 percent to 30 percent. Chromium provides corrosion resistance (1) Austenitic – most commonly used, corrosive resistant, easily welded, does not require pre or post heat. (2) Chromium Martensitic – are magnetic steels that contain 12 to 14 percent chromium and up to 0.35 percent carbon (3) Chromium Ferritic – are also magnetic and readily welded; however, the gas welding processes are not recommended (4) Duplex – combine the corrosion resistance properties of austenitic S. S. grade, especially stress corrosion cracking (SCC), and the mechanical properties of the ferritic stainless steel grades (5) Precipitation-hardened – can develop high strength with reasonably simple heat treatments; Precipitationhardened stainless steels that are readily welded require no preheat or solution annealing heat treatment.


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WORK AID 2:

HOW TO IDENTIFY WELD JOINT DESIGNS AND SYMBOLS

This Work Aid consists of Figure 8 through Figure 29 in the Information Section of this module. Use the Table of Contents for Figures to locate the page numbers.


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WORK AID 3:

HOW TO IDENTIFY WELDING CONSUMABLES

This Work Aid will assist you in identifying welding consumables. Following is a synopsis of the four basic types of welding consumables: 1. Coated Electrodes •

Most popular type of filler metal for arc welding

•

Also used in the SMAW welding process

•

The AWS classifications for carbon and low alloy steel coated electrodes are based on an "E" (electrode) prefix with a four or five digit number (EXXXXX). −

The first two digits (or three digits in a five digit number) indicate the minimum required tensile strength in the “as deposited” weld metal (not stress relieved) in thousands of pounds per square inch.

−

The next to the last digit indicates the welding position(s) in which the coated electrode can make satisfactory welds: (a) – 1 = all positions (b) – 2 = flat and horizontal fillet welds

•

The AWS classifications for stainless steel coated electrodes are also based on an "E" prefix with only a three digit number and a two digit suffix (EXXX-XX). −

The first three digits represent the American Iron and Steel Institute's (AISI) numbering system for stainless steels. (a) – The 300 series designates austenitic stainless steels (b) – The 400 series designates martensitic and ferritic stainless steels

−

The two digits in the suffix indicate the type of current and the type of coating for the electrode.

2. Bare Rods and Wires •

Bare rods are typically manufactured in 36" straight lengths with diameters that range from 0.045" to 3/16".

•

Bare rods are predominantly used with the gas tungsten arc welding (GTAW) process and the torch brazing process.

•

Bare wire electrodes are similar to bare rods except that bare wire is manufactured in continuous lengths with diameters that range from 0.020" to 1/8."


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•

The AWS classifications for carbon and low alloy steel bare rods and wire have an "ER" prefix with a two or three digit number and a one or two digit suffix (ERXXXS-XX). The "E" indicates an electrode, and the "R" indicates a welding rod; therefore, "ER" indicates either an electrode or a welding rod.

−

The first three digits "XXX" indicate the minimum required tensile strength in thousands of pounds per square inch.

−

The "S" indicates a solid electrode or rod.

− •

The one or two digits in the suffix indicate the chemical composition of the deposited weld metal.

The AWS classifications for stainless steel bare rods and wire have an "ER" prefix with a three digit number (ERXXX). −

The first three digits represent the American Iron and Steel Institute's (AISI) numbering system for stainless steels.

−

The 300 series designates austenitic stainless steels, and the 400 series designates the martensitic and ferritic stainless steels.

−

In the AWS classification ER308L, the "L" represents a low carbon version of the filler metal.

3. Flux cored Electrodes. •

Tubular wire that is manufactured in continuous lengths with diameters that range from 0.045" to 5/32."

•

The AWS classifications for carbon and low alloy steel flux cored electrodes have an "E" prefix with a two digit number "XX and a one digit suffix (EXXT-X). −

The "E" indicates an electrode and the "T" indicates a tubular electrode.

−

The first two digits indicate the minimum required tensile strength in thousands of pounds per square inch.

−

The one digit suffix indicates the chemical composition of the deposited weld metal, gas type, and usability factor. (Refer to Figure 31)

−

Example, An E70T-1 flux cored electrode has a 70,000 psi tensile strength and it uses CO2 shielding gas for flat position welding.


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WORK AID 4:

HOW TO DESCRIBE HEAT INPUT EFFECTS

This Work is a synopsis of the effects of heat input. 1. Parameters of heat input •

The three key welding parameters affect the amount of heat input to a weld are: −

The welding current (a) (b) (c)

−

The welding arc voltage (a) (b) (c) (d) (e)

−

High welding current results in greater base metal penetration, and a lower welding current results in shallow base metal penetration. An increase in the welding current increases the heat input to the weld. An increase in the welding current also increases the melt-off rate of the electrode and improves productivity because more weld metal is deposited

Voltage is the force that causes a current to flow. The difference in potential or voltage causes the current to flow in an electric circuit. The letter "V" is used to designate voltage High welding voltage results in greater base metal penetration, while lower welding voltage is indicative of shallow base metal penetration. An increase in the welding voltage increases the heat input to the weld

The travel speed of the welding process. (a) (b) (c) (d) (e) (f)

The rate that a welding electrode progresses along a weld joint while welding is called the travel speed. The letter "S" designates the travel speed. Several welding variables affect the travel speed, such as, the welding process, the position of the weld, the welder, and the wire feed speed. Slow travel speeds make wide weld beads with deep base metal penetration. Fast travel speeds make narrow weld beads and shallow base metal penetration A decrease in the travel speed increases the heat input to the weld


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2. Disadvantages of excessive heat input that can affect weld quality. •

Warpage and distortion caused by high residual differential shrinkage stresses

•

Cracks caused by a reduction of ductility or a degree of hardening

•

Premature failure caused by the deterioration of the toughness properties of the weld joint

•

Premature failure caused by the loss of strength of certain work hardened, quenched, and tempered materials

3. Use the following equation to determine heat input. Equation 1. Heat Input

Where: I = the welding current in amperes V = the arc voltage in volts S = the travel speed in inches per minute


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WORK AID 5:

HOW TO DESCRIBE HEAT TREATMENT EFFECTS

This Work Aid is a synopsis of the effects of heat treatment. 1. Preheat •

Heat that is applied to the base metals of a weld joint immediately before welding.

•

Preheat temperature depends on many factors such as the composition of the base metal, the ambient temperature, and the welding procedure.

•

The main purpose of preheat is to reduce the rate at which a weld cools.

•

Preheating the weld has these advantages:

•

−

Reduces shrinkage stresses in the weld and heat-affected zone that lead to cracks

−

A slower rate at which the weld metal cools through the critical temperature range (approximately 1,600°F to 1,330°F), which prevents excessive hardening and loss of ductility of both the weld metal and the heat-affected zone

−

A slower rate at which the weld metal cools through the 400°F range, which allows more time for any hydrogen that is present to diffuse away from the weld and adjacent base metal to avoid underbead cracking

−

Maintains sufficient heat at the weld area on highly conductive or thick base metals

−

Removes moisture from the weld joint

The choice of the preheat tools depends on factors such as the following: −

The preheat temperature

−

The duration of the preheat cycle

−

The size and shape of the weldment

−

The need for a one-of-a-kind or a continuous production preheat operation

•

For the majority of pipe welds, an oxyacetylene torch provides sufficient preheat to support the welding operation

•

When preheat for a single weld or for multiple welds that are in close proximity is required for a long period of time, electric resistance heaters are often more convenient.

•

Gas burners are more effectively and more widely used in pressure vessels (and in building industries in general) than handheld torches.

•

The necessity to preheat weld joints and the temperature requirements should be established by Engineering and should be demonstrated by a welding procedure specification (WPS).


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•

The preheat temperature depends upon these factors: −

Type of base metal and its composition

−

Joint thickness degree of restraint

−

Type and composition of filler metal

•

The interpass temperature should also be considered. The interpass temperature is the highest temperature in the weld joint immediately prior to welding.

•

The weldment temperature should never be allowed to become lower than the preheat or the interpass temperature.

Use the following formula to determine carbon equivalent of base metals Equation 2. Carbon Equivalent Formula Ni + Cu CE = %C + (%Mn/6) + ( Er = Mo + v ) + ( 1.5 ) 5 Where: %C = Percent of Carbon %Mn = Percent of Manganese %Ni = Percent of Nickel %Mo = Percent of Molybdenum %Cr = Percent of Chromium %Cu = Percent of Copper 2. Postweld Heat Treatment •

A number of postweld heat treatments exist for weldments but stress relief is the most widely used postweld heat treatment..

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Postweld heat treatment is any heat treatment that is applied to a weld or weldment after welding to reduce residual stresses.

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Advantages of postweld heat treatment of weldments: −

Reduce residual stresses that are inherent to any weldment, casting, or forging

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Soften hardened weld zones

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Improve resistance to corrosion and caustic embrittlement

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Improve dimensional stability of the weldment when machined

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Increase service life of the weldment


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•

Similar methods as that utilized for preheat are used for use for postweld heat treatment (PWHT) of welds. T he choice of the PWHT methods is similar to preheat treatment methods and depends on factors such as the following: −

The postweld heat treatment temperature

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The duration of the postweld heat treatment cycle

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The size and shape of the weldment or component

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The need for a one-of-a-kind or a continuous production postweld heat treatment operation

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The majority of postweld heat treatment applications in Saudi Aramco must be in accordance with ASME Section VIII, Division 1, Paragraph UCS-56.

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Typically, the construction standards require that the heating rate must not exceed 300°F to 400°F per hour when the base metal of the weld or component is above 800°F.

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The cooling rate must not exceed 400°F to 500°F per hour when the base metal of the weld or component is above 800°F


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BIBLIOGRAPHY AWS A5.x series, 1991 ASME Section I, Rules for Construction of Power Boilers, 1966 ASME Section II, Part C, Specifications for Welding Rods, Electrodes, and Filler Metals, 1966 ASME Section VIII, Pressure Vessels, 1966 ASME Section IX, Qualification Standards for Welding, 1966 ASME/ANSI B31.3, Process Piping, 1966 ASME/ANSI B31.4, Liquid Transportation of Hydrocarbons, 1992 AWS D1.1, Structural Welding Code, 1996


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Welding Parameters

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