INTRODUCTION TO WELDING
DEFINITION Welding is a fabrication process is defined as an operation by which two (or) more parts are united by means of heat or pressure in such a way that there is continuity of the nature of the material between these parts to be welded and a nd forms a weld. A filler material, the melting temperature of which is of the same s ame as that of the parent material, may or may not be used. Welding is one of the important and versatile means of fabrication available in industry. Welding is used to join all types of materials including plastics, ferrous and non-ferrous alloys etc. Many of the high temperature metals and super alloys and fabricated processes used in industries are Gas Welding, Arc welding and Resistance welding.”
“ HEAT ” - Primary requirement for welding, and, medium of classification of welding processes
CONTENTS
1 Welding Process & types of welding 2 Welding Equipment 3 Weld Design 4 Weld Defects 5 Distortion & its control 6 Inspection & Non-destructive testing of welds 7 Special Welding techniques 8 WPS, PQR and Welder qualifications
Classification of welding processes 1. 2. 3. 4. 5. 6.
Gas welding Arc welding Resistance welding Solid state welding Thermo-chemical welding Radiant energy welding
Gas welding:
Arc welding:
Resistance welding:
- Oxy-acetylene welding - Air acetylene welding - Oxy-hydrogen welding - Pressure gas welding
- Carbon arc - Flux cored arc - TIG or GTAW - Plasma arc - Electro slag & electro gas - Shielded metal arc - Submerged arc - MIG or GMAW - Stud arc
- Spot - Seam - Projection - Resistance butt - Flash butt - Percussion - HF resistance
Solid state welding: - Cold welding - Explosive welding - Friction welding - Roll welding - Diffusion welding - Forge welding - Hot-pressure welding - Ultrasonic welding
Thermo-chemical: - Thermit welding - Atomic hydrogen welding
Radiant energy welding: - Electron beam welding - Laser beam welding
Advantages of welding - A good weld is as strong as the base metal - Low cost of general-welding equipment ( economical ) - Portable equipment - Permits freedom in design - Lighter, smoother structure - Both similar & dissimilar metals can be joined - Simplicity in design, ease of modification/additions - Mechanization of welding processes
Disadvantages of welding - Harmful light radiations, fumes, splatter etc. - Residual stresses & distortion, due to high heat - Change in metallurgical properties of base metal - Jigs n fixtures required to hold job - Edge preparation required - Skilled labour - High standards of testing and inspection, etc.
Oxy-Acetylene Flame - Produced by the combustion of a mixture of oxygen and acetylene. - Temperature and characteristics of the flame depend on the ration of the gas mixture
Oxy- Acetylene gas is preferred because… …is a well controlled flame with high temperature -- 3200°C …flame can be manipulated as per requirement …does not change the chemical properties of base metal …does not require components to be forced together under pressure
Parts of the Flame Parts of the flame are based on the temperature zones. - Inner cone - Inner reducing cone - Outer zone or envelope
The greatest amount of heat of produced just ahead of the inner cone.
Stages of combustion: - Primary combustion - Secondary combustion
Primary combustion: - Takes place in the inner cone, at the tip of the nozzle - Max heat just in front of the inner cone
Secondary combustion: - Takes place at the outer envelope of the flame
The chemical reaction 1 volume of Acetylene combines with 2½ volumes of Oxygen and burns to form 2 volumes of CO 2 and1 volume of water-vapour, plus heat
For complete combustion : 1 volume of Acetylene + 2 ½ volumes of Oxygen
The flame takes an additional 1.5 volume of oxygen from the atmosphere
Types of Flame: - Neutral flame - Oxidising Flame - Carburizing or Reducing flame
Selection of Flame is based on metal to be welded
Neutral Flame: - Most commonly used flame - O2 & C2H2 in equal proportion - Complete combustion - No bad effect on base metal, **base metal not oxidized **no carbon available for reacting with metal - Used to weld metals like Mild Steel, Cast Iron, Stainless Steel, Copper, Aluminium
Oxidising Flame: - More of Oxygen (1.5:1) - Oxidising effect on metals - Burns with loud roar - Used to weld Brass and for brazing of ferrous metals
Reducing Flame: - Volume of oxygen less than that of neutral flame - A reducing flame does not carburize metal, it ensures absence of oxidising condition - A carburizing flame has a little more oxygen than a reducing flame and has carburizing effect on steel, i.e., causes hard, brittle & weak weld - Reducing flame is Used for flame cleaning,
Advantages of Oxy-acetylene Gas welding - It can be applied to a wide variety of manufacturing & maintenance situations - Welder has considerable control over the temperature of the metal in the weld zone. - Equipment is versatile, low cost, self-sufficient and portable. - Equipment can also be used for… …pre-heating …braze welding …oxygen cutting
Disadvantages of Oxy-acetylene Gas welding - Heavy sections cannot be joined economically - Flame temperature is less than arc temperature - Reactive metals cannot be gas welded - Gas flame takes longer time to heat metal - Flux shielding is less effective than inert gas shielding used in Arc welding processes
Other gas welding processes Welding flame may also be produced by the combination of Oxygen and gases such as Hydrogen, Propane, Butane, etc. Factors that influence the selection of another gas… …type of material to be welded …welding temperature required …the fuel-gas that is readily available …cost of supply of the fuel-gas Hydrogen: …used to weld metals with low melting points …Temperature at hottest point of flame is only 2500°C 2500°C.. If higher temp is reached by adjusting O2 supply, flame becomes unsuitable for welding.
…requires O2 – H2 ratio of 1:2 …flame is scarcely visible, there are no zones in the flame Propane & Butane: …used primarily in brazing …not used for welding of Steel because atmosphere is oxidising
Braze welding …intermediate between true welding and brazing …method of welding where a weld is made using a non-ferrous filler metal having a melting point below that of the base metal, but above 427°C …filer metal is deposited directly in the required region
Brazing …Used with close-fitted surfaces …Filler alloy is fed to one or few points and is drawn into the rest of the joint by capillary action
ARC WELDING
Arc welding is a group of welding process where in coalescence is produced by heating with an electrical arc or arcs, mostly without the application of pressure and with or without the use of filler metal depending upon the base plate thickness.
ARC WELDING PROCESS CARBON ARC WELDING
Welding may be carried out in air or in inert atmosphere. Filler material may or may not be used. Dc straight polarity is preferred to restrict electrode disintegration and amount of carbon going in to the weld metal and for the same larger arc length (up to 30 mm) are employed in carbon arc welding. This process is used for welding steel, aluminum, nickel, copper and other alloys. This process is manually operated. It is an arc welding process where in coalescence is produced by heating the work peace with an electric arc struck between carbon electrode and the work peace.
SHEILDED METAL ARC WELDING
It is an arc welding process where in coalescence is produced by heating the work peace with an electric arc step up between a flux-coated electrode and the wok peace. The flux covering decomposed due to arc heating and performs many functions like arc stability, Weld metal protection etc. The electrode itself melts and supplies and necessary filler metals. A big range of metal and their alloys can be welded. Welding can be carried out in any position. This process is manually operated.
SMAW PROCESS Direction of Travel Gaseous Shield Electrode Coating
Metal Droplets Deoxidizers
ARC
Slag Formers
Weld Bead & Slag
Electrode Wire
Base Metal
Molten Pool
SUBMERGED ARC WELDING
It is an arc welding process where in coalescence is produced by heating with an electric arc or arcs step up between a bare metal electrode/electrode at the job. The arc end of the electrode and molten pool remain completely hidden and are visible being submerged a blanket of granular material (flux). The continuously fed bare metal electrode metals and acts as filler rod. No pressure is applied for welding purpose the flux serves as a shield and protects the molten metal from atmospheric contamination. The process may be semi- automatic or automatic.
SUBMERGED ARC WELDING
TUGSTEN INERT GAS WELDING
It is an arc welding process where in coalescence is produced by heating the job with an electric arc struck between a tungsten electrode and the job. A shielding gas (argon, helium, nitrogen, etc) is used to avoid atmospheric contamination of the molten weld pool. A filler metal may be added if required. In this process a non- consumable electrode (tungsten) is used. TIG welding is very much suitable for high quality welding of thin materials as thin as 0.125 mm.
COMPONENTS OF A TIG SYSTEM REGULATOR/FLOWMETER HELIARC TORCH (AIR or WATER COOLED)
SHIELDING GAS WORK PIECE
CONSTANT CURRENT POWER SUPPLY
WATER COOLER (OPTIONAL)
PROCESS ADVANTAGES
Cleanest weld metal
Excellent mechanical properties
Excellent puddle control
PROCESS DISADVANTAGES
Low deposition rates
Higher weld cost
Greater operator skill required
BASIC PROCESS Gas Cup Shielding Gas Electrode
( ) CC POWER
Arc Puddle Work
SUPPLY ( )
Types of Tungsten Electrodes Pure Least expensive, low current capacity
Thoriated Better arc starting, higher current carrying capacity, resistant to comtamination
Zirconiated Has properties of both pure and thoriated with greater stability for AC welding
Ceriated Similar properties to thoriated tungsten, gaining use as a safer alternative
Lanthanated Similar properties to ceriated tungsten, gaining use as a safer alternative
TUNGSTEN ELECTRODES Most Commonly Used Type Color Current Shape Uses
Pure Green AC Ball Al, Mg
2% THORIATED Red DC Point CS, SS, Cu, Ni, Ti
Benefits
Self balling
Higher currents Better starts
SHIELDING GASES
ARGON Best arc stability Best shielding Best cleaning action
HELIUM Increases penetration and heat Increases welding speed
ARGON/HELIUM MIXTURES
Combines benefits of both gases
Applications
6Al 4V Forgings
Joints
Bearing Support Fitting Assembly
Applications Courtesy of Martin Marietta Aero & Naval Systems
Nose cone Section of Cruise Missile Material: Inconel 718
uel Tank Section of Cruise Missile aterial: 6Al 4V Titanium
METAL INERT GAS WELDING It is an arc welding process where in coalescence is produced by heating the job with an a n electric arc established between a continuously fed metal electrode and the job. No flux is used but the arc and molten metal are shielded by an inert gas, which may ma y be argon, helium, carbon dioxide or a gas mixture. Because of continuously fed electrode MIG welding process is much faster as compared to TIG welding and can produce deep penetration joints. All commercial metals and alloys can be welded.
MIG WELDING SYSTEM Regulator/ flowmeter: R-50-FM-580 High Pressure Gas Cylinder, Shield Gas Filler Metal, (Electrode): Spoolarc 86 Wire Feeder: Digipulse Constant Voltage Power Source: Digipulse 450 Welding Torch/Mig Gun: MT400
MAJOR PROCESS COMPONENTS
(+) CV POWER SUPPLY (-)
Advantages & Limitations of GMAW Overcomes the restriction of limited electrode length (SMAW) Welding can be done in all positions Welding speeds are higher than SMAW; higher depositions rates Deeper penetration than SMAW which allows for smaller fillets for an equivalent strength SMA weld of same size Can be used on all the major commercial alloys Equipment is more complex, more costly, and less portable Welding arc must be protected against air drafts that may disperse the shielding gas causing porosity
Process Fundamentals Fundamentals Process
(+)
” 4 / 3
Increase Electrode Extension
(-)
” 8 / 3
Decrease Electrode Extension
SH I E L D I N G G A S Purpose: Protect the molten weld metal and HAZ from contamination from surrounding atmosphere. Will also affect metal transfer characteristics, penetration etc. Types Inert, i.e. Argon, Helium Reactive, i.e. Carbon Dioxide Mixtures of inert and reactive, i.e. Argon/Oxygen,
Argon/CO2,
Tri-mixes, i.e. Argon/Helium/Carbon Dioxide
SHIELDING GAS PROPERTIES
Argon Inert; Excellent arc stability, heavier than air Helium Inert; Increased heat input; Faster travel speeds, lighter than air Carbon Dioxide - CO2 Increased heat input; Good shielding, oxidizing Oxygen Improves bead wetting; Arc stabilizer, oxidizing Hydrogen Improves bead wetting; Increases heat input
MODES OF METAL TRANSFER
Four basic modes of metal transfer
High
Spray Arc y g r e n E
Low
Pulsed Spray Arc Globular Arc Short Arc Deposition Rates
SHORT ARC TRANSFER
Generally uses small diameter solid wires Uses low currents and voltages Low heat input
Good for welding thin gauge and out-of-position
SHORT ARC PIPE WELD - ID Smooth uniform penetration
Uniform fusion at toes of weld
WELD UNDERBEAD
SHORT ARC - (V (VERT) ERT)
Macro Section
GLOBULAR TRANSFER
The type of transfer achieved when using CO2 shielding gas.
Good penetration and higher speeds
Excessive weld spatter
Increased Increased clean-up cost
CO2 WELD vs Arg Argon-CO on-CO2 Mix
Argon/CO2
CO2
SPRAY TRANSFER
Spray transfer occurs when:
Welding current is above the transition point Shielding gas is greater than 80% argon Arc voltage is high enough that it does not short circuit - about 26-30 volts
SPRAY TRANSFER
High currents and high voltages give high heat inputs Used in the flat and horizontal positions on heavy plate High currents give good penetration and high deposition rates Almost no spatter Very smooth weld beads
SPRAY ARC PIPE WELD
Fixed ST21M
Rotating Pipe Assembly
SPRAY ARC - (H (HORZ) ORZ)
.045” WIRE @ 300 IPM - 280 AMPS @ 27.5 VOLTS
PULSED SPRAY TRANSFER
Requires a special power supply that pulses the welding current above the transition level at a given frequency to achieve spray transfer at lower average currents. Requires the use of a spray arc gas. Less heat input than spray arc Welds out-of-position Almost no spatter
PULSED SPRAY ARC PIPE WELD
PULSED ARC WELD - PIPE
MECHANIZED MECHANIZ ED W / OSCILLATION - 7.5” SCH. 80
PULSED ARC WELD - (HO (HORZ) RZ)
.045” WIRE @ 300 IPM - 200 AMPS @ 25 VOLTS
PULSED ARC WELD - (V (VERT) ERT)
Macro Section
ELECTRO SLAG WELDING
It is an arc welding process where in coalescence is produced by molten slag which melts the filler metal and surface of the work to welded. Welding flux is added and melts it to form the slag arc heats. Much thicker steel can be welded in single pass. Thickness up to 450mm in plain and alloy steel can be welded.
ELECTRO GAS WELDING
It is an arc welding process where in coalescence is produced by the gas or heat require for welding is obtained by maintaining a continuous electric arc between the electrodes and the moten weld poll during welding operation. An inert ( ex:CO2) shields the molten weld pool from oxidation, only DC can be used for electro gas welding. Plates 12.5mm to 275mm thick are most commonly welded. Welding is done in one pass.
PLASMA GAS WELDING
Plasma arc welding is an arc welding process where in coalescence is produced by heat obtained from a constructed arc step up between be tween tungsten/ alloy tungsten. Electrode and the water-cooled nozzle as between tungsten/alloy tungsten electrode and the job. The process employs two inert gases, one forms the arc plasma and the second shields the arc plasma. Filler metal may or may not be added. All metal and alloys can be welded.
ARC SPOT WELDING
It is an arc welding process at zero arc travel speed where in coalescence between two coating surfaces is obtained with the help of a spot formed by heating with air electric arc step up between a tungsten electrode and one of the two closely fit surface. Filler metals may or may not be used depending upon the materials to be welded. The process involves less maintenance cost. The process is normally free from smoke and spatter. All commercial metals and alloys can be easily welded.
STUD WELDING
It is an arc welding process where in coalescence is produced by heating with an electric arc drawn between a metal stud, bolt, rivet or a similar part and the base metal. Arc is maintained till a molten pool is i s created in the base metal and the arcing end of the stud is heated to melting point. At this stage, arc extinguishe extinguished d and the stud is forced in to the molten metal pod of the metal to form the weld. The molten metal is shielded by a ferrule and a flux or inert shield gas. A simple, efficient economic and very fast method of joining pins, bolts, studs, rivets, rods etc. to a plate or a structure. Most of the ferrous and nonferrou nonferrous s materials and their alloys can be stud welded easily.
RESISTANCE WELDING PROCESSES
Resistance welding is a group of welding processes wherein coalescence is produced by the heat obtained from the resistance offered by the work piece to the flow of electric current in a circuit of which the work is a part, and, by the application of pressure. No filler metal is needed. The different types of resistance welding are as follows:
SPOT WELDING
It is a welding welding process in w hich la pping in which whi ch over lapping lappin g sheets are joined by local fusion at one or more spots by the heat generated by resistance to the flow of electric current through work piece that are held together under force by two electrodes one above the other below the two overlapping sheets. High speed of welding, the skilled workers, and operation may be made automatic or semiautomatic are the advantages. Spot welding of two 12.5mm thick steel pl plates ates has been done satisfactorily.
SEAM WELDING
Seam welding is a resistance welding process where in coalescence at the saying surfaces is produced by heat obtained from resistance to electric current (flow) through the work parts held together under pressure by electrodes. The resulting weld is a series of overlapping resistance spot welds made progressively along a joint by rotating the circular electrodes. All commercial metals and their alloys are successfully welded by seam welding.
PROJECTION WELDING PROJECTION
Projection welding is a resistance welding process where in coalescence in produced by heat obtained from resistance to electric current (flow) through the work parts held together under pressure by electrodes. The resulting welds are localized predetermined points by projections, embossment or intersections. Projection welds can be made in metals that are too thick to be joined by spot welding. A number of welds can be made simultaneously.
RESISTANCE BUTT WELDING
Upset butt welding is a resistance or the entire area of butting surfaces by the heat obtained from the the resistance of electric current through the area of contact of these surfaces. Pressure is supplied before heating is started and is maintained throughout the heating period. This pressure or force lateral on increased to give give a forging squeeze when the welding temperature t emperature temperature (1600deg to 1700deg) has been reached when sufficient upset has been produced the the welding current is cutoff and the force is removed. For welding of small ferrous and non-ferrous strips and rods for welding longitudinal and transverse butt joint.
Flash welding is a resistance welding process where in coalescence is produced. Simultaneously over the entire area of abutting surface by the heat obtained from the resistance to electric current between the two surfaces and by the application of pressure after heating is substantially completed. Flashing and upsetting are accomplished by explosion of metal from the joint. Many different metals with different melting temperatures can be flash welded.
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OXY ACETYLENE WELDING EQUIPMENT
Cylinders containing Oxygen gas are black in colour. They are usually capable of storing around 7 m3 of oxygen with pressure ranging between 120 & 150 kg/cm2. Oxygen cylinder pressure regulators are right hand threaded. Acetylene gas cylinders are usually maroon in colour. They are stored in a dissolved state with pressure ranging between 15 & 16 kg/cm2. The pressure regulator on the Acetylene cylinder is left-hand threaded. Oxy-acetylene welding equipment
ARC WELDING EQUIPMENT
When high current passes through an air gap from one conductor to another, it produces very intense and concentrated heat in the form of a spark. The temperature of this spark (arc) is approximately 3600oC, which can melt or fuse the metal very quickly to produce a homogeneous weld. Metallic-Arc Welding is the most commonly used process. Heat is obtained from an arc formed between a metallic (consumable) electrode and the welding job. The metal electrode itself acts as a filler metal
Simple Welding circuit
CARBON-ARC WELDING EQUIPMENT
Welding current is provided by either an AC or DC machine. The Welding speed and the feed of the electrode is controlled manually by the welder.
Carbon Arc welding equipment
TIG WELDING EQUIPMENT The TIG welding system consists of the following:
A power source: A shielding gas supply A torch Controls and interlocks
TIG Welding equipment
MIG WELDING EQUIPMENT
The arc is formed between a continuous, automatically fed, metallic consumable electrode and the welding job in an atmosphere of an inert gas. The best results are obtained using DC power source, electrode positive. This also permits the arc cleaning action to take place. The wire is continuously fed into the arc at a constant rate by a wire feeding system
MIG Welding equipment
SUBMERGED-ARC WELDING EQUIPMENT
The components of a submerged Arc welding setup are: Welding head Wire feed system Power source Flux feed and recovery units
Submerged-arc welding equipmen
Welding Head: It consists of a wire spool, wire feed system, flux hopper, conveyor and electrical contact nozzle Wire Feed System: The wire feed control may be of the following types: (i) Voltage sensitive system (ii) Constant speed system Power Sources: Submerged arc welding can be carried out with both DC and AC current power sources Flux feed and recovery unit: This consists of a system of conveyor belts arranged to supply the hopper
Submerged-arc welding equipment
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WELD DESIGN
Before an arc is struck on metal, the product must be designed to serve its purpose, the material chosen and the method of welding has to be defined in detail.
Environmental considerations leading to brittle fracture, creep, corrosion of welds etc have also to be considered
PRINCIPLES OF GOOD WELD DESIGN
Material selection.
Weld Process selection.
Joint design. Welding Positions
MATERIAL SELECTION The primary requirement for selection of any structural material is its “ Weldability ”. ”.
Weldability is the capacity of a material to be welded, under the fabrication conditions imposed, into a specific suitably designed structure and to perform satisfactorily in the intended service
Weldability encompasses the following: (a) Metallurgical compatibility of a metal or alloy with any specific welding process.
(b) Ability of the metal or alloy to be welded with mechanical soundness.
(c) Serviceability of the resulting welding joint.
Carbon content has the greatest effect on weldability of Steels. The tendency to crack increases with the increase in Carbon content, particularly above 0.2% The The relative effect of other alloying elements is expressed by a “Carbon Equivalent ” (CE) CE = % C + (Mn%)/6 + (Cr% + Mo % + V %)/5 + (Cr % + Ni )/15 Steel with CE upto 0.4% can be welded, without any special precautions against cracking, upto 1 inch thickness.
WELD PROCESS SELECTION
The process of welding similar or dissimilar metals is selected according to the type of base metals (ferrous or non ferrous) to be welded. For ferrous metals of less thickness gas welding can be selected. For ferrous metals of larger thickness arc welding processes are chosen. For nonferrous based metals brazing is chosen. (d) Soldering is chosen for joining metals or wires, which are not exposed to external forces or pressure.
Submerged arc welding should be used, u sed, where the molten metal arc pool can be submerged in a pool of flux (Typically down hand welding). (e) Metal Inert Gas (MIG) and Tungsten Metal Inert Gas (TIG) welding process are selected to weld Aluminium-based metals. Welding processes are also selected according to the position of welding. Sometimes there are requirements to carry out over head welding and some so me times even underwater.
Following factors should be considered for process selection :Base metal material. Environment. Capacity of facility. Positions/locations of welding. Availability of facilities. Optimum utilization of technologies etc.
WELD JOINT JOINT DESIGN:
Selection and preparation of weld joints is and important step in the fabrication of a weldment and is very essential if the welded members are to perform within the load service, corrosive atmosphere and safety requirements. The final product should have sufficient strength to perform well under loaded conditions and should also be pleasing in appearance.
The selection of Weld-Joint for a particular type of weldment depends upon the following factors:Base-plate thickness. Geometry of structure. Magnitude and type of loading. Cost of edge preparation. Number of passes. Electrode consumption and cost of welding. Chances and magnitude of distortion. Operational ease.
Welding Joints
Figure 1 Types of Weld Joints
connect plates in the same plane. The joint is superior to all the other types, particularly under bending, cyclic and dynamic loadings, and is adopted wherever practicable for connecting structural members Butt welds are usually built up of several runs so that a slight 'over-fill' or 'reinforcement' exists on both surfaces of the finished weld.
• The welds that form a “T” joint are called “Fillet Welds”. • These welds are the cheapest type of welds to manufacture, because no edge preparation is required •Single fillet “T” joints are preferred for smaller plate thickness when subject to low or almost no-load conditions, whereas, double “T” joints are recommended for most severe loading conditions
• These are used only for joints of minor importance. e.g. collars for making decks watertight where pierced by frames.
• The fillet welds should be continuous and of equal size on both sides of the joint .
•Closed and half open joints are recommended for plates of smaller thickness not subjected to severe loading. • Full, open corner joints can be used on plates of practically all thickness under severe load conditions. • The load-bearing capacity increases when the joint is welded from both sides.
Two pieces of metal are lapped, with their edges in line and the pieces are joined together as shown in the figure.
Edge Joint
PLATE AND PIPE WELDING POSITION
PLATE POSITIONS
PIPE POSITIONS ROTATED OR ROLLED
PIPE POSITION : FIXED
WELDING DIRECTIONS OR POSITIONS
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WELD DEFECTS
Need to study weld defects? A defective weldment fails under service conditions and causes damage to property and loss of human lives. This makes it necessary to study defects in weld joints and analyse their causes.
COMMON WELD DEFECTS
The main causes for crack formation in a weldment are as follows: 1. Rigid Rigidity ity of the joint joint 2. Poor ductilit ductility y of base metal metal Solidification Crack
3. Harde Hardenabili nability ty (high Sulphur Sulphur & Carbon content) 4. Concav Concave e weld weld bead bead 5. High arc arc travel travel speed speed 6. Elect Electrode rode with high Hydrogen content
Types of cracks
Some remedies to reduce appearance of cracks are as follows :
(a) Apply preheat to the base metal. (b) Relieve residual stresses mechanically. (c) Minimize shrinkage stresses using back step or block welding sequence. (d) Change weld current and travel speed (to effect slower cooling rate). (e) Bake electrodes to remove moisture. (f) Reduce root opening; opening; build-up build-up edge edge with weld metal. (g) Increase electrode size for small sm all weld bead, raise small welding current, and reduce travel speed. (h) For high sulphur base metal, use filler metal low in sulphur. (j) Use of jigs and fixtures. (k) Reduce welding time. (l) Weld outward from the centre point. (m)Removal of shrinkage forces during or after welding. (n) Breaking down of forge weld mends into sub assemblies.
LAMELLAR TEARING
•“Lamellar Tearing” Fracture face ofsevere Lamellar Tearing” is the result of very Tearing and due to the restraint on the joint, poor ductility presence of non-metallic inclusions running parallel to the plate surface. •It can be seen in the material of the parent plate and the HAZ, the cracks usually running parallel to the plate surface. •‘T’ and corner joints are more susceptible to lamellar
Suggested configuration to avoid Lamellar Tearing
Buttering to avoid Lamellar Tearing
INCOMPLETE PENETRATION is defined as failure of weld metal to penetrate into the root of the joint. 1.Small root gap. 2.Small Causes :- angle. bevel
3.Less arc current. 4.Fast arc-travel speed. 5.Small angle of inclination of electrode (‘ ’ angle). 6.Large electrode diameter. 7.Longer arc length. 8.Incorrect polarity when Large root face. 9.Welding with DC. 10.Wrongly held electrode.
Result : Weakens the joint and becomes a potential fatigue initiation site.
SLAG INCLUSIONS
Inclusions may be in the form Slag or any other foreign material. They are impurities which do not get a chance to float on the surface get trapped in the solidifying weld metal. Inclusions lower the strength of the joint
Slag inclusions in a butt weld
The factors that promote inclusions are as follows: Too high or too low arc current. Long arcs. Too large electrode diameter. Insufficient surface Slag inclusion due to a poor bead - Co preparation. Insufficient cleaning of previous passes in multi-pass welding. Under cutting. Wrongly placed tack welds. Radiograph of Slag Small joint angle. Inclusions
POROSITY AND BLOW HOLES ‘Porosity ’ is a group of small voids, whereas, a ‘Blow hole’ or ‘Gas Pocket’ is a comparatively bigger isolated hole or cavity. They occurplate mainly due to urface porosity in a ‘T’ joint on a primed entrapped gasses.
Elongated pores or worm holes
Porosities in the weld bead
Formation of pores
The parent metal under the arc tends to absorb Hydrogen, Carbon monoxide, nitrogen and oxygen. These gasses may be produced due to electrode coatings or by moisture, rust, oil, grease, etc. existing on the base plate. The gasses spread through the molten metal by diffusion and convection, and their solubility decreases as the weld metal cools. The bubbles generally form at the solid-liquid interface, remain there, grow to critical size, move through the molten pool and get liberated to the atmosphere. Voids are formed due to the trapped gas bubbles. The terms ‘Porosity’, ‘Blow holes’ or ‘Gas Pockets’ apply depending on the size and the concentration of the bubbles/voids bubbles/voids..
Factors leading to these defects are as follows:
Improper electrode (or coating) or damaged/damp coating. Longer arcs. Faster arc travel speeds. Too high/low currents. Incorrect welding techniques. Impurities present on the job surface. Improper base-metal configuration (high ‘S’ or ‘C’ content).
LACK OF FUSION
Lack of bonding between the parent metal and weld metal or between weld metal passes.
Incomplete inter-run fus Incomplete Fusion of Side Wall
CAUSES :Lower arc current. current. Faster arc travel speed. Improper weaving technique. Presence of oxide, rust, scale and other impurities (on the surfaces to be welded), which do not permit the deposited metal to fuse properly with the base metal. Incorrect joint preparation (i.e. small included angle). Incorrect electrode manipulation
Measures to avoid/reduce poor fusion are: (a) Follow correct welding procedures. (b) Maintain proper electrode position. (c) Reposition work, lower current or increase arc travel speed. (d) Clean weld surface prior to welding.
SPATTER
Small metal particles thrown out of the arc during welding cause ‘Spatter ’. ’. The particles get deposited on the base metal, around the weld area along the length of the weld. The causes are: Excessive welding current. Wrong polarity (DC). Long arc. Arc blow. Uneven flux coating on electrode.
UNDERCUT
Undercut is a ‘groove’ or channel formed in the parent metal as the toe of a weld. The groove reduces the thickness of the plate and and thus reduces the area along the bead, which, in turn, weakens the weld.
The causes for undercutting are as follows: •Current too high. •Welding speed it too fast. •Overheating due to continuous welding. •Faulty electrode manipulation. Undercut in fillet and but welds we lds •Acute welding angle.
OVERLAP
An ‘Overlap’ occurs when the molten metal from the electrode flows over the parent metal surface without fusing into it. The causes for overlap are as follows: Low current. Slow arc travel speed. Long Arc. Too large diameter electrode. Improper joint geometry (ex: large root gap)
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DISTORTION
Distortion is the result of uneven expansion and contraction of heated metals As metals are heated they expand and when they are cooled they contract. During welding, heating and cooling of metals occur unevenly. This results in high stresses and the metal distorts.
TYPES OF DISTORTION:
Longitudinal distortion.
Transverse distortion.
Angular distortion
FACTORS INFLUENCING DISTORTION
Weld design. Parent metal. Edge preparation. Assembly procedure. Welding process. Deposition techniques. Welding Sequence. Unbalanced heating about the neutral axis. The restraint imposed
DISTORTION CONTROL
Precautions can be taken to avoid or reduce weld distortions before, during or after welding. The control of distortion before welding can be facilitated by:Tack Welding. Using Jigs, clamps and fixtures. Ensuring uniform Pre-heating. Pre-setting.
A Tack Weld is a short weld, made prior to welding, to hold the plates in perfect alignment with uniform root gap. Tack welds are made at regular intervals along the joint with high current (to obtain proper penetration). They are necessary where plates cannot be held by a fixture. The thumb rule for tack welds is as follows P = 100 mm + 16T L = 3T where ‘T’ - plate thickness in mm ; ‘L’ - length of each tack in mm
JIGS AND FIXTURES
They are used to hold the work in a rigid position during welding. By using them, the shrinkage forces of the weld are balanced with a sufficient counterforce.
PRE - HEATING
Some metals would normally fracture if welded in a cold state. They may be welded successfully by preheating and subsequent controlled cooling. Pre-heating is also used to avoid cracks
Preheating to avoid HIC
PRE-SETTING
Distortion may be avoided by pre-setting the plates in the opposite way so that the weld pulls them to the desired shape. When the weld shrinks, it will pull the vertical plate to its correct position. The fig shows presetting to avoid angular distortion
DURING WELDING
Back-step welding.
Intermittent “Chain” & “Staggered ” welding.
Planned wandering method.
A correct welding procedure to reduce the size of the weld beads.
Excessive Welding should be avoided.
BACK STEP WELDING
The general direction of welding progression is from left to right, but, the bead is deposited from right to left. The result being that the plates expand to a lesser degree with each bead because of the locking effect of each weld.
Back-step welding method
INTERMITTENT WELDING
The amount of weld metal deposited can be minimized with the use of intermittent welds instead of continuous welds. This method can be used for fillet welds only. “Chain” welding and “Staggered” welding are the two types of intermittent fillet welding.
Chain Intermittent Welding
Staggered Intermittent Welding
PLANNED WANDERING METHOD
In this method, welding starts at the centre, and thereafter portions are completed on each side of the centre in turn Planned wandering method
AFTER WELDING The control of distortion after welding can be facilitated by the following: Slow cooling. Flame straightening or contra heating. Annealing. Stress relieving. Normalising. Mechanical straightening
TIPS TO AVOID DISTORTION
It is possible to reduce the effect of shrinkageshrinkageforce by correct edge preparation. This will ensure proper fusion at the root of the weld with a minimum of weld metal. The correct welding procedure used a greater number of welded runs positioned to refine the grain size of the weld metal in the previous layer. A small number if heavy runs will cause more distortion due to the greater heat input, and the contraction stresses set up by the cooling of the larger deposit of weld metal.
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INSPECTION & NON-DESTRUCTIVE TESTING OF WELDS
Welded joints in a structure are expected to possess certain service-related capabilities. They are required to carry loadings of various types in which the weld is subjected to stress of either a simple or complex character. Moreover, a finished weld is not always as good as or bad as it may appear to be on its it s surface. Hence, it is necessary to inspect a weld-job on completion. The purpose of inspection is to locate and determine the type of fault, quality of joint and quality of workmanship
METHODS OF NON-DESTRUCTIVE TESTING
Non-Destructive methods
Common Testing Methods
Special Testing methods
COMMON TESTING METHODS
Visual inspection Leak or Pressure test Stethoscopic test
VISUAL INSPECTION
It is the simplest, fastest, the most economical and most commonly used method for detecting defects on the surface of the weld. Visual Inspection may be carried out in three stages: (i) Before welding (ii) During welding (iii) After welding
BEFORE WELDING A qualified operator should be given the job. The material to be welded should be of weldable quality. Proper edge preparation – as per the plate thickness and other factors. Setting of root gap. Proper distortion control procedures (jigs, fixtures, etc.). Electrode storing and drying procedures. p rocedures. procedures. Polarity of electrodes (in the case of DC current). Current setting as per the size of the Electrode electrode and the weld-position.
dryin
DURING WELDING Visual inspection during welding will help reduce the chances of rejecting the weldment at the final stage The aspects that are to be observed during welding are as follows: Studying the sequence of the weld deposit. Examine surface cleaning before every run ( multi-run welding). Check shape of bead, angle of electrode, amperage, speed of travel, arc length, correct manipulation of electrode and control of splatter.
AFTER WELDING
Surface defects in and around the welds, such as cracks, undercut, overlap, excessive contour, the smoothness of the weld surface, penetration and distortion control are to be inspected. The shape of the profile can be be checked checked by measuring the dimensions of the weld – the surface of butt welds should be slightly convex; fillet welds may be either concave of convex. The degree of undercut, surface cavities and trapped slag. Deposition of runs, single or multiple. Penetration bead in butt welds. Quality of the weld metal.
Some of the ‘templates’ used to check weld profiles and determine size and shape of welds are shown below :-
plate for testing the bead contour of welds
Template for checking the max. & min. convexity of butt
Template to measure the size of a fillet weld
Template used to determine the size of a concave fillet
LEAK OR PRESSURE TESTS
This test is used tto o test welded pressure vessels, used u sed to tanks and pipelines for leaks. The welded vessel is sealed and subjected to internal pressure using air, water or kerosene. The internal pressure built-up depends on the working pressure of the joint, generally, twice the tthe he working pressure of the vessel. Any drop in pressure would indicate a leak or leaks. Soap solution may be applied to check for leaks in an Air-pressure test.
STETHOSCOPIC TEST:
The principle of this test is that a defect-free weld metal gives a good ringing sound when struck with a hammer, whereas that with defects gives a flat sound. An ordinary physician’s stethoscope and a hammer may be used to magnify and identify the sound
Special NonDestructive tests
Magnetic Particle test
Liquid Penetrant test
Radiography (X-ray) test
Gamma ray test
Ultrasonic test
MAGNETIC PARTICLE TEST
This test is used to detect both surface and sub-surface ( upto 6 mm) defects in ferrous materials. When the test piece is magnetized, iron particles gather at the edges of the defect and can be seen as dark hair-line marks Magnetic particle testing of welds with the naked eye.
LIQUID PENETRANT TEST LIQUID
This test is based on the principle that coloured liquid dyes and fluorescent liquids penetrate the cracks. A solution of the coloured dye is sprayed on the clean welded surface and allowed to soak. The dye on the surface is then washed off using a cleaner and the surface dried with a soft cloth. A liquid ‘developer ’ (white in colour) is then sprayed on the weld and the coloured dye comes out in the shape of the t he the defect
Different stages of “Liquid penetrant test”
RADIOGRAPHY (X-RAY/GAMMA (X-RAY/GAMMA RAY) TEST
X-Rays or Gamma rays can be used to take internal photographs of the weld joint. Radium & its compounds, Cobalt-60 etc. are used in Gamma radiography. Hidden defects will be visible on the developed film. Defects appear in the same manner as bone fractures . Gamma rays penetrate greater thickness than X-rays and the main advantage of this process is ‘ portability portability ’. ’. Gamma ray tests can be done even where electricity is not available. Radiography tests are conducted on high-quality jobs such as boilers, pressure vessels, pipes, etc
X-ray Radiography
ULTRASONIC TEST
Sound waves are used to identify defects. The waves can penetrate 6 to 10 metres of steel. A transmitter is placed on the job surface. The echo of the sound waves is displayed on a calibrated screen attached to the machine. The test can also be used to find out the thickness of metal, this is especially useful in ship hull inspection during refits
Ultrasonic testing
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SPECIAL WELDING TECHNIQUES TECHNIQU ES
Welding of “Titanium” “Titanium” is possible in protected inert-gas chambers using an electron-beam process. The main difficulty in welding is the high reactivity of Titanium At elevated temperatures it reacts with most elements, ex: Hydrogen, Oxygen, Nitrogen, etc. Even if small quantities of impurities are picked-up during welding, the material is noticeably embrittled, thus stringent protection from the atmosphere is called for.
Titanium welding enclosure (deflated) used at ND(V)
The purity of the inert gas used must be at least 99.95%. Argon shielding must be provided till the HAZ has cooled to below 300°C. The recommended gas-flow rate is 3 to 5 litres per min and the arc length should be held as short as possible. If it is not possible to provide a protective chamber, effective protection can be obtained by providing argon, not only with the welding torch but also on the trailing side through a separate feed-tube & nozzle arrangement attached to the torch. Gas should also be supplied on the underside through a longitudinal groove in a copper backing.
Welding torch with attachment to protect trailing end during welding
Equipment used for inert-gas protection of pipes and plates (Purging units)
UNDERWATER WELDING Underwater welding can be classified as follows: Wet Welding Dry Welding In wet welding the welding is performed underwater, directly exposed to the wet environment. In dry welding, a dry chamber is created near the area to be welded and the welder does the job by staying inside the chamber
WET WELDING
A special electrode is used and welding is carried out manually just as one does in open air welding. The increased freedom of movement makes wet Underwater ‘wet’ welding welding the most effective, efficient and economical method. Welding power supply is located on the surface with connection to the diver/welder via cables and hoses. Wet welding process schematics
The power source should be a direct current curr ent current machine rated at 300 or 400 Amperes The welding machine frame must be grounded to the ship The welding circuit must include a positive type of switch, usually a knife switch operated on the surface and commanded by the welder-diver. The knife switch in the electrode circuit must must be capable of breaking the full welding current and is used for safety reasons Special welding electrode holders with extra insulation against the water are used. The underwater welding electrode holder utilizes a twist type head for gripping the electrode. The electrode types used conform to AWS E6013 classification The electrodes must be waterproofed. All connections must be thoroughly insulated so that the water cannot come in contact with the metal parts.
Advantages of Wet Welding Advantages
The versatility and low cost of wet welding makes makes this method highly desirable. Other benefits include the speed. With which the operation is carried out. It is less costly compared to dry welding. The welder can reach portions of offshore structures that could not be welded using other methods. No enclosures are needed and no time is lost in designing these enclosures ( as in dry welding ). Readily available standard welding machine and equipment are used. The equipment needed for mobilization of a wet welded job is minimal.
DISADVANTAGES OF WET WELDING
There is rapid quenching of the weld metal by the surrounding water. Although quenching increases the tensile strength of the weld, it decreases the ductility and impact strength of the weldment and increases porosity and hardness. Hydrogen Embrittlement – Large amount of hydrogen is present in the weld region, resulting from the dissociation of the water vapour in the arc region. The H2 dissolves in the Heat Affected Zone (HAZ) and the weld metal, which causes embrittlement, cracks and microscopic fissures. Cracks can grow and may result in catastrophic failure of the structure. Another disadvantage is poor visibility. The welder some times is not able to weld properly
HYPERBARIC WELDING (DRY (DRY WELDING)
Hyperbaric welding is carried out in chamber sealed around the structure to be welded. The chamber is filled with a gas (generally helium containing 0.5 bar of oxygen) at the prevailing pressure. The habitat is sealed onto the pipeline and filled with a breathable mixture of helium and oxygen, at or slightly above the ambient pressure at which the welding is to take place. This method produces high-quality weld joints that meet X-ray and code requirements. The TIG welding process is employed for this process. The area under the floor of the Habitat is open to water. Thus the welding is done in the dry but at the the hydrostatic pressure of the sea water surrounding the Habitat.
ADVANTAGES OF DRY WELDING
Welder/Diver Safety – Welding is performed in a chamber, immune to ocean currents and marine animals. The warm, dry habitat is well illuminated and has its own environmental control system (ECS). Good Quality Welds – This method has ability to produce welds of quality comparable to open air welds because water is no longer present to quench the weld and H2 level is much lower than wet welds Surface Monitoring – Joint preparation, pipe alignment, NDT inspection, etc. are monitored visually. Non-Destructive Testing (NDT) – NDT is also facilitated by the dry habitat environment.
DISADVANTAGES OF DRY WELDING
The habitat welding requires large quantities of complex equipment and much support equipment on the surface. The chamber is extremely complex. Cost of habitat welding is extremely high and increases with depth. Work depth has an effect on habitat welding. At greater depths, the arc constricts and corresponding higher voltages are required. The process is costly and the same chamber cannot be used for another job, if it is a different one.
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SCOPE OF PRESENTATION
Welding Quality assurance is divided into the following: -
WPS – Welding Procedure Specification S pecification
PQR -Performance Qualification Record
WPQ –Welder Performance Qualification
QUALITIES OF A GOOD WELD
he good qualities that are looked for in a weld comprise of the following:
a)Physical soundness discontinuities)) discontinuities b)Metallurgical compatibility
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Physical soundness is related to the mode of deposition, in other words, the process techniques as influenced by the skill of the welder or welding machine operator.
Metallurgical compatibility depends on the chemical composition of the basemetal/filler-metal/flux or gas, the heat cycles that the weldment is going to undergo while welding and post-weld heat treatment.
The ultimate aim of all Quality assurance schemes, i.e. Welder qualification, Welding procedure specifications and Procedure qualification records, is to ensure the soundness of welding. No matter what equipment is used to achieve these standards, the skill of the operator is of primary importance, hence the requirement of “Welder Performance Qualification” tests.
WELDING PROCEDURE SPECIFICATION From a designer’s point-of-view, the properties of the weld joint are designated as follows: (a) Weld metal chemistry (b) Ultimate tensile strength (c) Yield point (d) Percentage elongation (e) Hardness (f) Impact strength, etc.
•Welding procedure specifications are written to exactly translate the above property requirements onto relevant welding variables.
• Thus, “a Welding Procedure Specification (WPS) is a written qualified welding procedure, prepared to provide direction for making production welds to code requirements”
•Each of these welding procedure specifications shall be qualified by the welding of the test coupons and the mechanical testing of specimens cut from these coupons (the data for the tests and the results are recorded in a PQR). •It is pre-supposed that the welder or, the welding machine operator is qualified to handle the job. Therefore, welding procedure qualification is strictly to establish the metallurgical compatibility of the weldment through mechanical tests and not the skill of the welder of the welding operator.
WRITING A WELDER PROCEDURE SPECIFICATION (WPS) A typical working procedure specification is written with all factors of production in mind. Both experience and technological awareness help to write an acceptable or successful WPS. The WPS is a list of the following welding variables: (a) Welding process – which process is to be used (b) Base metal (c) Joint design (d) Filler metal (e) Welding position
(f) Pre-heat/interpass/post-heat (g) Electrical characteristics (h) Post-weld heat treatment (j) Gases – for shielding (k)Techniques (stringer, weave bead, transfer mode, orifice diameter, etc.) The variables Depending on their influence on obtaining a desired weldment, welding variables are then classified and listed. Some are classified as ‘Essential ‘ Essential variables’ , some as ‘Supplementary essential variables’ variables ’ and others as ‘Non-essential ‘Non-essential variables’. variables’.
(a) Essential variables: These, when changed beyond the allowable limits while welding, alter the prescribed weld properties. Hence, production welds with such variables should not be continued without re-qualification and certification. (b) Supplementary essential variables: More number of variables from the above list are declared as essential, incase where a procedure qualification is applied for, ex: a weldment going for notch-toughness applications. These are called supplementary essential variables, i.e. there arises a supplementary requirement that makes these variables essential.
(c) Non-essential variables: Those welding variables which when changed during welding (within permissible limits) do not cause to alter the desired weld properties are classified under non-essential category. •They are just altered in the WPS; hence, production welds with such altered parameters could be continued without any re-qualification or certification
A WPS may require the support of more than one PQR, while alternatively, one PQR may support a number of WPS.
WPS will be applicable equally for a plate, pipe and tube joints.
A single WPS may cover several essential variable changes as long as a supporting PQR exists for each essential and supplementaryessential variable.
PROCEDURE QUALIFICATION RECORD (PQR)
A Procedure qualification record (PQR) is essentially a record of welding data used to weld a test coupon.
It also contains the test results of the tested specimen. Recorded variables normally fall within a small range of the actual variables that will be used in production welding.
Changes to PQR are generally not permitted, except in cases of editorial correction to the entries or in the case of an agenda being added. All changes in a PQR would call for re-qualification
When more than one welding process or filler metal is used to weld a test coupon, the deposit weldmetal thickness of each process and the filler metal would be recorded.
WELDER PERFORMANCE QUALIFICATIONS (WPQ) (WPQ)
The basic attempt is to establish the ability of the welder to deposit sound quality of metal.
In the case of welding operator qualification, his ability to operate the welding machine is tested and acknowledged.
During qualification tests, only essential variables, as applicable to the welder’s skill, are considered. Production welds with altered variables should not be continued without additionally qualifying the welder. A welding machine operator usually gets qualified along with a procedure test. The welder’s performance test has to be limited only to verify his ability to deposit sound metal and the requirements of metallurgical compatibility are to be attributed to the welding process characteristics
CODES FOR PERFORMANCE QUALIFICATIONS AND ACCEPTANCE OF DEFECTS
A large number of industrial codes are available which recognize pre-qualified welding procedures, as the manufacturing firms are to produce the documentary evidence for such procedures as in vogue in their organizations.
The selection, training and qualification of welders is the next step
Some popular codes used are as follows: f ollows:
(a) Indian boiler regulations (IBR – chapter viii)
(b) ASME Boiler and Pressure vessel code
(c) DIN 8560 (German standards - testing of welders section)
(d) BS – 4872 (British standards)
Welders are to weld a test coupon, which may be a pipe or a plate.
The test coupons are then subjected to Radiography testing.
The metallurgical compatibility of the weld is revealed only by running mechanical tests; hence face-bend tests and root bend tests are also carried out in addition to Radiographic testing
“Welders are out-rightly failed for the following defects” (All other defects ate acceptable within limits)
Severe root porosities/Blow holes.
Cluster of porosities.
Cracks.
Lack of fusion/Incomplete penetration.
Hydrogen crack examples
Incomplete inter-run fusion
Incomplete penetration
Surface porosity in a ‘T’ joint on a primed plate
WELDER QUALIFICATION QUALIFICATION AS PER ASME
The welder may be qualified based on the results of mechanical tests or by radiographic examination of a minimum length of 150 mm. The welding positions in which the test may be conducted are classified as 1G, 2G, 3G, 4G to 6G, in ascending order of expertise. A qualification in a plate or pipe butt joint shall also qualify the welder for fillet welding of all plate thicknesses and pipe diameters.
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