ASME-Notch-Toughness & Supplementary Essential Variables

BPV Code Section IX: Welding, Brazing, and Fusing Qualifications

Module 7: Notch-Toughness & Supplementary Essential Variables This module provides a brief review of notch-toughness, followed by an explanation of supplementary essential variables and their use in creating a WPS when impact testing is required. Design application, metallurgical, and geometrical factors which can affect the notch-toughness of materials are discussed. Welding variables, such as travel speed, heat input, base metal thickness, and electrode (filler metal) composition, which can affect the notch-toughness of materials, are also discussed. Supplementary essential variables become essential variables for procedure qualification when notchtoughness is required by the end user, the construction Code, or the application. The supplementary essential variables for the qualification of SMAW WPSs will be defined and their qualification ranges is explained. After, the process by which an existing WPS can be ‘supplemented’ by a PQR proving in the notch-toughness of the weldment is described. The objective of this module is for the student to understand and be able to apply the supplementary essential variables necessary to qualify welding procedures when notch-toughness is required. By now, the student should be well aware that the qualification of welding procedures is concerned with proving in the mechanical properties of the weldments produced by that welding procedure. Since the ASME BPVC is primaril y concerned with safety as it relates to t he retention of pressure, the m echanical property of primary interest is tensile strength. However, when tensile loads are applied rapidly or at low temperature, failure may occur at a much lower load than it would under the slow loading typical of standard tension tests. There are conditions under which a material’s performance is based as much on its behavior under rapid loading or at low temperature as it is on the simple maximum load it can bear in tension at room temperature. Since notch-toughness is associated with a material’s ability to absorb energy when loaded rapidly, it is determined by a category of mechani cal testing referred to as ‘impact testing.’ The first part of this module will review notch-toughness and some of the metallurgical and welding factors which affect it. Notch-Toughness Refer to Module 3 for a general discussion of notch-toughness. A brief review will be given here as a prelude to a discussion of the design, metallurgical and welding variables that can affect notchtoughness. Figure 33 below shows a typical stress-strain curve for mild steel. The tensile strength, known as the ultimate tensile strength (UTS), is determined by the maximum load, as shown on the curve. The total area under the stress-strain curve is is the total amount of energy that was absorbed by that tension test specimen during the process of failing it.

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Welding Inspection Technology , 5th Edition, 2008, American Welding Society Education Department, Miami, Florida, page 6-7.

Figure 33 Typical stress-strain curve for a mild steel. The ultimate tensile stress (UTS) is determined at the maximum load

Materials that absorb a relatively large amount of energy during failure are referred to as ductile materials – they stretch or elongate considerably during the tension test. This is an important engineering feature, because it allows a structure under load to redistribute stresses, thus absorbing a significant amount of energy. For example, copper is a ductile material; it will routinely display 40 % elongation or more in a tensile test. Materials that do not absorb significant energy during failure, however, are referred to as brittle materials – they will fail with little or no elongation during a tension test. For example, cast iron is a brittle material; it routinely displays less than 0.5 % elongation in a tension test. The classic example of a brittle material is glass, which typically fails by shattering, with little or no deformation beforehand. With brittle materials, localized stresses continue to increase when there is no local yielding or stretching. These high local stresses create a crack at one or more points of stress concentration and the crack propagates quickly through the section. Even if no stress risers are present in a brittle material, fracture will still occur suddenly because the yield strength and the tensile strength are practically the same. Materials which fail with little or no deformation beforehand provide no warning of the imminent failure. As a result, the release of st ored energy can be sudden; when this happens, the r esults can be catastrophic. Certain materials may display adequate ductility when tested in tension slowly at room temperature. When the load is applied more rapidly, or at low temperature, or in the presence of an embrittling agent, such as hydrogen, or in the presence of a stress riser, significantly less ductility is displayed. Since it is the designer or construction Code that usually knows when these conditions are likely to be present, the requirements for qualification of welding procedures with notch-toughness considerations rests with them, not with Section IX.

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As an aside, the reader should assume t hat ALL engineering structures contain str ess risers, either in the form of welds, machined features, discontinuities, or changes in thickness, geometry, or strength. The construction Codes identify the temperatures below which the above conditions could reasonably be expected to promote brittle behavior in an otherwise ductile base metal. As a result, it is useful to understand some basic characteristics associated with materials for which notch-toughness requirements are usually specified. Notch Effects A notch, or stress riser, m agnifies the applied stress at the location of the notch. However, the most important effect of the notch is that it causes a ‘triaxial’ state of stress at the notch; this is a stress state in which significant stresses exist in all three directions: parallel to the applied tensile stress, transverse to the applied tensile stress, and, most importantly, through the thickness of the plate or sheet. For sections which are relatively thick compared to the size of a through-thickness notch, a condition referred to as ‘plane strain’ is created, in which the through-thickness tensile stress is increased significantly in the presence of the notch. For a brittle material, the increased through-thickness stresses from the plastic constraint caused by the relatively large thickness can exceed the critical value for fracture before the material undergoes general plastic yielding. This can result in catastrophic failure. In addition, the increased stresses resulting from the notch produce accompanying increased strains. These strains harden the material locally. These local strain-hardened areas are subject to the formation of small cracks, without the expenditure of much plastic deformation. These cracks can propagate. As these local strain-induced cracks grow, they multiply the local strain rate. One way in which the notch effect is translated into requirements for procedure qualification is through limitations on the base metal thickness qualified to reflect the fact that thin sections do not generally experience plane strain loading [see variable QW-403.6, which gives a minimum qualified base metal thickness of 5/8 inch (16 mm)].

Crystal Structures As a general rule, medium and low strength metals having a f ace-centered cubic (FCC) structur e have such a high notch-toughness that brittle fracture is not a concern unless the application involves some special reactive or corrosive chemical environment. The types of alloys that fall into this category are the austenitic stainless steels and almost all nickel-based alloys. For instance, the ASME B31.3 Process Piping Code allows the use of many austenitic stainless steels to temperatures down to -325 º F (-200 º C) (or lower in some cases) without impact testing. Many high-strength materials have such low notch-toughness that brittle fracture can occur at stresses even in the elastic range, in the presence of flaws, regardless of the design temperature and strain rates. Aluminum alloys, titanium alloys, and high-strength steels oft en fall into this category. In the ASME B31.3 Process Piping Code, the titanium alloys listed may not be used at temperatures below -75 º F (-60 ˚ C) without impact testing. Many aluminum alloys, however, may be used at temperatures down to -452 º F (270 ˚ C) without impact testing – but there are other considerations and rules for aluminum alloys. The notch-toughness of low and medium strength metals having a body-centered cubic (BCC) structure is strongly dependent on temperature. Carbon steels and low-alloy steels are the construction materials of interest which fall into this category. For these materials, fracture at low temperature occurs by cleavage, but fracture at high temperature occurs by ductile rupture. The temperature at which this transition occurs is called the ‘Ductile-to-Brittle Transition Temperature’ or DBTT and was defined and discussed earlier in Module 3. It is useful here to describe the metallurgical and welding-related factors which affect the DBTT of engineering materials, particularly steels. Chemical Composition Changes in the DBTT of more than 100 º F (55 º C) can be produced by simply changing the chemical composition or the microstructure of mild steel. The elements which have the biggest effect on the DBTT 3 ASME ©2014


of mild steel are carbon and manganese. The DBTT is increased (notch-toughness is decreased) by approximately 25 º F (14 º C) for each 0.1 wt. % of carbon added. Conversely, the DBTT is decreased (notch-toughness is increased) by roughly 10 º F (5 º C) for each 0.1 wt. % of manganese added. For adequate notch-toughness, the Mn:C ratio of carbon and low-alloy steels should be at least 3:1. The significance of the effect of manganese and carbon on the notch-toughness of steels (P-Numbers 1 through 15F) is reflected in the subdivision of P-Numbers for steels into Group Numbers. For instance, PNumber 1 base metals, which includes carbon steels, is further divided into Group 1 through Group 3 base metals. These Group Numbers reflect compositional ranges whose notch-toughness varies based largely on carbon and manganese contents. When notch-toughness is required by the construction Code, Group Number is a supplementary essential variable for qualification of welding procedures, in addition to the P-Number (see variable QW-403.5).

Other alloying elements also affect the notch-toughness of steels. Nickel is added to steels to increase notch-toughness, in amounts up to 2 wt. %. Silicon, on the other hand, decreases the notch-toughness (by raising the DBTT) when present in amounts greater than 0.25 wt. %. Molybdenum decreases the notch-toughness of steels almost as much as carbon. Chromium, however, has little, if any, effect on the notch-toughness of steels. The effect of alloying elements on the notch-toughness of steels is one reason why the electrode classification of consumable electrodes and filler metals is a supplementary essential variable for qualification of welding procedures. Electrode classifications for SMAW electrodes can have suffixes, such as E9018- B9, in which the ‘B9’ indicates that it has 9 wt. % Cr and 1 wt. % Mo, added, in part, to produce a weld metal that will respond to PWHT designed to improved notch-toughness to match the base metal. Electrode classifications for FCAW electrodes can also have suffixes, such as E71TX-K6X, in which the ‘K6X” indicates the addition of 1 wt. % Ni to improve notch-toughness. Bare wire filler metal classifications can have similar suffixes. For instance, ER80S-Ni1 could be used for either GTAW (as a filler metal classification) or GMAW (as an electrode classification) of low-alloy steels where notch-toughness is required because the ‘Ni1’ indicates the addition of 1 wt. % Ni for improved notch-toughness. The electrode classification system for SAW electrode/flux consumables has similar capability, specifying certain alloying elements in the electrode portion of the classification. However, the notch-toughness designator is in the flux half of the classification (see Module 2). Tramp elements, however, have a pronounced negative effect on the notch-toughness of steels. Phosphorus has a strong effect in decreasing notch-toughness. The DBTT is raised approximately 13ºF (7ºC) for every 0.01 wt. % of phosphorus. Similarly, oxygen is also very detrimental to notch-toughness; an increase from 0.001 wt % to 0.057 wt. % oxygen increases the DBTT from 5ºF to 650ºF (15ºC to 345ºC). Nitrogen is generally considered to be detrimental to notch-toughness, but its effect is difficult to measure because of its interaction with other elements. Because of the detrimental effect that oxygen has on notch-toughness and other properties of weldments, deoxidants are often added to filler metals. Electrode/filler metal classifications describe the level of deoxidants in the electrode/filler metal. ER70S-2 filler metals generally produce weld metal of excellent notch-toughness in GTA welds on carbon steels. Higher levels of deoxidants are found in classifications having a higher suffix, such as ER70S-6, but these filler metals are designed to be used on heavilyoxidized base metals and the increased levels of deoxidants may not necessarily improve notchtoughness. Similarly, SAW consumables also rely on alloy additions to increase the notch-toughness of the weld deposits. Additions of manganese to the electrode for increased deoxidation may result in increased manganese in the weld metal, which will actually decrease the notch-toughness of the weld deposit due to the formation of stringers and other manganese-bearing nonmetallic inclusions. Regardless, the electrode/filler metal classification usually has a significant effect on the notch-toughness of the weldment and that is why it is a supplementary essential variable for welding procedure qualification (see variables QW-404.33 and QW-404.12).

Grain Size

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Grain size also has a strong effect on notch-toughness. As a general rule, fine-grained materials have better notch-toughness than coarse grained materials. An increase of 1 ASTM number in the ferrite grain size (a decrease in the average grain diameter) can produce a decrease in the DBTT of up to 30 º F (17 º C) in a mild steel. Decreasing the grain size from an ASTM 5 to an ASTM 10 can reduce the DBTT from room temperature to well below 0 º F (-18 º C). For higher-alloyed, heat-treated steels, a similar change can be observed in the DBTT by reducing the austenite grain size. During welding, the heat input of the welding process has a substantial influence on the peak temperature and thermal cycle experienced by the heat-affected zone. As the heat input increases, the peak temperature experienced by the HAZ increases and the width of the HAZ increases, but the cooling rate experienced by the HAZ decreases. Increased heat inputs and the corresponding higher peak temperature in the HAZ will promote an increase in grain size at the peak temperature location. This increase in grain size becomes significant when the peak temperature exceeds 1900 ˚ F (1040 ˚ C). Extended times at temperatures in excess of 1900 ˚ F (1040 ˚ C), will produce significant grain growth. This grain growth can result in severe degradation in the notch-toughness (increase in the DBTT) at that location. Furthermore, in hardenable alloys (like low-alloy steels), in the absence of multipass welds which produce tempered martensite, the cooling rate at this location may be rapid enough to promote the formation of harder phases such as martensite and/or bainite. The combination of increased grain sizes and hardened microstructural phases will significantly degrade the notch-toughness at this location. However, when multipass stringer-bead welds are used in thick sections of hardenable steels, the HAZ of each weld bead tends to temper the martensite in the previous weld beads (and the base metal HAZ), resulting in tempered martensite, which has excellent notch-toughness. Although the reduced cooling rate associated with higher heat input welds will t end to produce an annealed microstructure across the HAZ, the ‘coarse-grained’ region produced by the higher peak temperature will always experience the most rapid cooling rate in the HAZ and, as a result, will tend to display lower notch-toughness. This effect is worse for base metal thicknesses less than 5/8 inch (16 mm), as reflected in supplementary essential variable QW-403.6. When the test coupon is 1 inch (25 mm) thick, the minimum thickness qualified is not 3/16 inch (5 mm) as permitted by table QW-451.1, supplementary essential variable QW-403.6 limits it to ⅝ inch (16 mm), meaning that any thickness less than this would see even more extended time at temperatures in excess of 1900 ˚ F (1040 ˚ C), resulting in even worse notch-toughness and that would require additional testing (a new PQR) to prove in. As base metal thickness increases, traditional arc-welding processes must use multipass welds to fill these joints and the multipass welds used tend t o produce tempered martensite. To address the rapid HAZ cooling rates associated with single pass welds in thick sections (particularly for high energy density processes like PAW, EBW, LBW or hybrid welding), or the presence of thick individual passes in multipass welds in thick sections, essential variable QW-403.9 limits the base metal thickness qualified to 1.1T for any single-pass or multipass welding in which any pass is greater than ½ inch (13 mm) thick. Because of the effect of weld heat input on the peak temperature in the HAZ, increasing the heat input (or the volume of weld metal deposited per unit length of weld) is identified as a supplementary essential variable for the qualification of welding procedures in Section IX (see variable QW-409.1). The equation for the calculation of heat input is shown below: Heat = (Current) x (Voltage) x 60 Input Travel Speed

For a given heat input, an increase in the thickness of the base metal will produce a ‘quenching’ effect, which increases the cooling rate experienced by the HAZ. In these cases, hardenable base metals can be subject to the formation of hardened microstructural phases that would reduce notch-toughness. As long as multipass stringer beads are used for these welds, any hardened phases produced will tend to be tempered by succeeding beads, resulting in excellent mechanical properties, including notch-toughness. Increasing the maximum interpass temperature has the same effect as increasing the heat input of welding: it increases the grain size in the ‘coarse-grained’ region of the HAZ in previously deposited weld 5 ASME ©2014


beads, thus decreasing notch-toughness. As a result, increasing the maximum interpass temperature more than 100 º F (55 º C) is a supplementary essential variable for the qualification of welding procedures in Section IX (see variable QW-406.3).

Post-Weld Heat Treatment (PWHT) Sub-critical heat treatments (those conducted at temperatures below the lower transformation temperature) are often used in steels to temper martensite formed in the HAZ on cooling after welding. This tempering process restores ductility and notch-toughness at the expense of tensile strength. Because of the effects this has on both tensile strength and notch-toughness, it is both an essential and a supplementary essential variable for the qualification of welding procedures in Section IX (see the variables in QW-407). Another sub-critical PW HT which is governed by the QW -407 variables is stress-relieving after welding. Since this heat treatment removes the residual stress caused by welding, it can also improve notchtoughness. Super-critical heat treatments (those conducted at temperatures above the upper transformation temperature) remove the effects of microstructural transformations that occur in the weld and HAZ as the result of cooling after welding. Martensite, bainite and hardening precipitates are ‘put back into solution’ in austenite and then the slow cooling to room temperature results in a lower-strength microstructure, often pearlitic in nature, with improved ductility and notch-toughness. As a result, the application of or limits on certain essential and supplementary essential variables are removed when a weldment is qualified using a PWHT above the upper transformation temperature or when the material does not have a transformation temperature, such as is the case for austenitic stainless steels. Examples of this are:

(a) Variable QW-404.7, Increase in electrode diameter to over ¼ inch, which is a supplementary essential variable for the qualification of welding procedures for some welding processes. This variable states ‘a change in the nominal diameter of the electrode to over ¼ in. (6 mm). This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic material is solution annealed after welding.’ The use of large diameter electrodes increases the welding current so much that extensive grain coarsening can be seen in the HAZ, resulting in a loss in notch-toughness. However, the application of a solution annealing heat treatment after welding will refine the grain size and restore the notchtoughness of the HAZ. (b) Variable QW-409.1, Increase in heat input, which is another supplementary essential variable for the qualification of welding procedures for some welding processes. This variable states, in part, ‘an increase in heat input. . .over that qualified. . . . .The requirement for measuring the heat input or volume of deposited weld metal does not apply when the WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P-No. 10H material is solution annealed after welding.’ Again, the damage done to the HAZ by high heat inputs is eliminated when the weldment is subject to a solution heat treatment, which refines the grain size and dissolves any martensite or detrimental precipitates that have formed. (c) Variable QW-410.9, change in multiple to single pass per side, which is a supplementary essential variable for the qualification of welding procedures for some welding processes. This variable states ‘A change from multipass per side to single pass per side. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P- No. 10H material is solution annealed after welding.’ In a multipass weld, each weld bead tempers or softens the HAZ in the previously deposited weld bead. In a single pass weld in the same material, however, no such softening occurs. In hardenable materials, this can degrade notch-toughness. However, the detrimental microstructural phases or precipitates responsible for this are dissolved during a solution annealing process, so this heat treatment removes this variable from consideration. 6 ASME ©2014


For most other alloy systems, with the exception of the non-heat treatable aluminum alloys, PWHT has a significant effect on the notch-toughness of weldments. For all of these reasons, some variant of PWHT is always a supplementary essential variable for the qualification of welding procedures in Section IX. Supplementary Essential Variables The three categories of welding variables used for procedure qualification in Section IX have already been defined: Essential, Nonessential, and Supplementary Essential. Essential and nonessential variables for the SMAW process have been described in great detail in Module 4. Supplementary essential variables will now be discussed and explained so that their application in the qualification of welding procedures where impact testing is required can be understood. Supplementary essential variable: “a change in a welding condition which will affect the notch-toughness properties of a weldment. . . .Supplementary essential variables are in addition to the essential variables for each welding process.” (see QW -401.3). So, variables identified in the tables in QW-250 as supplementary essential variables become essential variables for procedure qualification when notchtoughness is required by other Sections or by the end-user. The same rules that govern the application and enforcement of essential variable rules on PQRs and WPSs also apply to supplementary essential variables. As an example of how to identif y, define, and apply these variables, we will go through the supplementary essential variables listed for the SMAW process in table QW-253, using the same methodology as was used in Module 4.

With that introduction, let’s look at each supplementary essential variable listed in table QW-253 for the qualification of welding procedures using the SMAW process.

QW-403 Base Metals QW-403.5 Change in Group Number The first supplementary essential variable listed for the qualification of SMAW procedures is QW-403.5, ‘Change Group Number.’ To understand what this means, the detailed definition/explanation in paragraph QW-403.5 must be read from Article IV: ‘

QW-403.5 Welding procedure specifications shall be qualified using one of the following:

(a) the same base metal (including type or grade) to be used in production welding (b) for ferrous materials, a base metal listed in the same P-Number Group Number in table QW/QB-422 as the base metal to be used in production welding (c) for nonferrous materials, a base metal listed with the same P-Number UNS Number in table QW/QB-422 as the base metal to be used in production welding For ferrous materials in table QW/QB-422, a procedure qualification shall be made for each PNumber Group Number combination of base metals, even though procedure qualification tests have been made for each of the two base metals welded to itself. If, however, two or more qualification records have the same essential and supplementary essential variables, except that the base metals are assigned to different Group Numbers within the same P-Number, then the combination of base metals is also qualified. In addition, when base metals of two different PNumber Group Number combinations are qualified using a single test coupon, that coupon qualifies the welding of those two P-Number Group Numbers to themselves as well as to each other using the variables qualified. This variable does not apply when impact testing of the heat-affected zone is not required by other Sections.’

When notch-toughness is required, qualification with any given P-Number base metal does NOT qualify the procedure for all base metals having that P-Number. The base metals qualified are now limited to the

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specific type and grade combinations qualified because the notch-toughness of base metals often varies from one type/grade combination to another. QW-403.5(a) reflects a conservative approach in which a welding procedure would only be qualified for welding the material type and/or grade which was used during qualification. In this way, the WPS would be restricted to a single grade (or combination of grades if two different material grades were welded for qualification). This approach can also be used for base metals which are not listed in table QW/QB-422. QW-403.5(b) applies to steels in which the P-Numbers are further subdivided into Group Numbers, each of which reflects a different level of notch-toughness. So, a WPS qualified with impact testing by welding a P-Number 1 Group Number 2 base metal is ONLY qualified for welding P-Number 1 Group Number 2 base metals where notch-toughness is required. If notch-toughness was NOT required, this same WPS would be qualified for welding P-Number 1, Group Number 1, 2, and 3 base metals. QW-403.5(c) applies to nonferrous base metals. For these base metals, P-Numbers are not subdivided into Group Numbers; rather, several different alloy types and grades, each having a different UNS number, may be assigned to a single P-Number based on compositional similarity. For instance, UNS N06625 and UNS N10276 are both P-No. 43 base metals. Where notch-toughness is concerned, however, the two alloys (UNS number) are different. For this reason, when notch-toughness is required, the WPS is only qualified for base metals having the same UNS number as that used during the qualification process.

The last paragraph of QW-403.5 further clarifies the rules for P-Number Group Number combination qualifications of ferrous base metals. For example, if WPSs exist to weld a P-No. 1 Gr. No. 1 base metal to itself and to weld a P-No. 1 Gr. No. 2 base metal to itself, an additional WPS must be qualified to weld a P-No. 1 Gr. No. 1 base metal to a P-No. 1 Gr. No. 2 base metal. However, if a PQR exists for welding P-No. 1 Gr. No. 1 base metal to itself and another PQR exists for welding P-No. 1 Gr. No. 2 base metal to itself an d all of the essential variables and supplementary essential variables for the two PQRs are identical, then these two PQRs may be used to support a WPS for welding P-No. 1 Gr. No. 1 to P-No. 1 Gr. No. 2 base metals. Finally, if a PQR exists for welding P-No. 1 Gr. No. 1 to P-No. 1 Gr. No.2 base metals, then this PQR can be used to support two WPSs: one for welding P-No. 1 Gr. No. 1 base metal to itself and one for welding P-No. 1 Gr. No. 2 base metal to itself.

QW-403.6 Thickness Limits impact The next supplementary essential variable listed for the qualification of SMAW procedures is QW-403.6, ‘Thickness Limits impact.’ To understand what this means, the detailed definition/explanation in paragraph QW-403.6 must be read from Article IV: ‘

QW-403.6 The minimum base metal thickness qualified is the thickness of the test coupon T or 5/8 in. (16 mm), whichever is less. However, where T is less than ¼ in. (6 mm), the minimum thickness qualified is ½ T. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P-No. 10H material is solution annealed after welding.’

This variable reflects the fact that HAZ time at temperatures above 1900 ˚ F (1040 ˚ C) increase substantially for thicknesses less than ⅝ inch (16 mm), resulting in significant degradation of HAZ notchtoughness, as previously described. For test coupon thicknesses of 1 inch (25 mm) or 2 inches (50 mm) , for example, the minimum thickness qualified when notch-toughness is specified would be ⅝ inch (16 mm). But, for test coupon thicknesses less than ⅝ inch (16 mm) down to ¼ inch (6 mm), the minimum thickness qualified is that tested, meaning that the test will reveal exactly what the level of notchtoughness degradation in the HAZ is f o r t h a t t h i c k n e s s . For test coupon thicknesses less than ¼ inch (6 mm), the minimum thickness qualified is ½ T, reflecting the fact that there is not much difference in time at peak temperature between the two thicknesses, as well as the fact that plain strain conditions (to aggravate notch-toughness sensitivity) do not exist in thin materials.

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QW-404 Filler Metals

QW-404.7 Change in Diameter > ¼ in. (6 mm) The next supplementary essential variable listed for the qualification of SMAW procedures is QW-404.7, ‘Change in Diameter > ¼ in. (6 mm).’ To understand what this means, the detailed definition/explanation in paragraph QW-404.7 must be read from Article IV: ‘

QW-404.7 A change in the nominal diameter of the electrode to over ¼ in. (6 mm). This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic material is solution annealed after welding.’ This variable reflects the fact that large diameter electrodes require large currents, which are sufficient to cause excessive grain growth in the HAZ. This grain growth can degrade notch toughness when combined with the typical rapid cooling rates common to single pass welds made with large diameter electrodes in thick sections. Clearly, this effect is negated when the weldment is given a post-weld solution heat treatment that can refine the grain size in the HAZ.

QW-404.12 Change in Classification The next supplementary essential variable listed for the qualification of SMAW procedures is QW-404.12, ‘Change in Classification.’ To understand what this means, the detailed definition/explanation in paragraph QW-404.12 must be read from Article IV: ‘

QW-404.12 A change in the filler metal classification within an SFA specification, or for a filler metal not covered by an SFA specification or a filler metal with a “G” suffix within an SFA specification, a change in the trade designation of the filler metal. When a filler metal conforms to a filler metal classification, within an SFA specification, except for the “G” suffix classification, requalification is not required if a change is made in any of the following: (a) from a filler metal that is designated as moisture-resistant to one that is not designated as moisture-resistant and vice versa (i.e., from E7018R to E7018) (b)

from one diffusible hydrogen level to another (i.e., from E7018-H8 to E7018-H16)

(c) for carbon, low alloy, and stainless steel filler metals having the same minimum tensile strength and the same nominal chemical composition, a change from one low hydrogen coating type to another low hydrogen coating type (i.e., a change among EXX15, 16, or 18 or EXXX15, 16, or 17 classifications) (d) from one position-usability designation to another for flux-cored electrodes (i.e., a change from E70T-1 to E71T-1 or vice versa) (e) from a classification that requires impact testing to the same classification which has a suffix which indicates that impact testing was performed at a lower temperature or exhibited greater toughness at the required temperature or both, as compared to the classification which was used during procedure qualification (i.e., a change from E7018 to E7018-1) (f) from the classification qualified to another filler metal within the same SFA specification when the weld metal is exempt from Impact Testing by other Sections This exemption does not apply to hard-facing and corrosion- resistant overlays.’

These rules reflect the fact that individual electrode classifications can specify different chemical compositions, which can produce different levels of notch-toughness, so a change in an electrode classification would require requalification of a welding procedure where impact testing is required. 9 ASME ©2014


Of particular interest here is the restriction on the use of electrodes having t he ‘G’ suffix. This suffix identifies electrodes which may not meet one or more characteristics of an electrode specification, such as chemical composition. So, two electrodes having the ‘G’ suffix, but made by different manufacturers, could have different chemical compositions. For this reason, a WPS requiring impact testing qualified with an electrode having the ‘G’ suffix from manufacturer ‘A’ could not use an electrode having a ‘G’ suffix from manufacturer ‘B’ because the chemical composition could be different, resulting in different notchtoughness properties of the weld metal. The WPS would have to be requalified to use the electrode made by manufacturer ‘B’ even though both electrodes correctly carry the ‘G’ suffix. The exemptions simply state that changes in diffusible hydrogen levels, low-hydrogen types, moisture resistant designations, position usability for FCAW electrodes, or changes to a filler metal producing weld metal of better notch-toughness do not decrease notch-toughness and, therefore do not require requalification of the WPS when impact testing is required.

QW-405 Positions QW-405.2 Change in Position The next supplementary essential variable listed for the qualification of SMAW procedures is QW-405.2, ‘Change in Position.’ To understand what this means, the detailed definition/explanation in paragraph QW-405.2 must be read from Article IV: QW-405.2 A change from any position to the vertical position uphill progression. Vertical-uphill progression (e.g., 3G, 5G or 6G position) qualifies for all positions. In uphill progression, a change from stringer bead to weave bead. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic material is solution annealed after welding.’ ‘

Welds made in the uphill progression tend to be hotter than welds made in other progressions. As a result, the heat input into the HAZ and associated grain growth can result in reduced notch-toughness in the HAZ. For this reason, changing the progression to the vertical-uphill progression will require requalification of the WPS when notch-toughness is specified. Changing from a stringer bead technique to a weave bead technique decreases the effective travel speed and, as a result, increases the heat input, thereby reducing notch-toughness in the HAZ. A change from the stringer bead progression to the weave bead progression will require requalification of the WPS when notch-toughness is required. Since the application of a post-weld solution annealing heat treatment will refine the grain size in the HAZ, this variable would not apply when the WPS is qualified with a post-weld solution heat treatment.

QW-406 Preheat QW-406.3 Increase > 100 º F (55 º C) (IP) The next supplementary essential variable listed for the qualification of SMAW procedures is QW-406.3, ‘Increase > 100 º F (55 º C) (IP).’ To understand what this means, the detailed definition/explanation in paragraph QW-406.3 must be read from Article IV: QW-406.3 An increase of more than 100 º F (55 º C) in the maximum interpass temperature recorded on the PQR. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P-No. 10 H material is solution annealed after welding.’ ‘

An increase of more than 100 º F ( 55 º C) in the maximum interpass temperature over than recorded o n the PQR could result in significant grain coarsening or other microstructural changes in the HAZ of the previously deposited bead or the base metal of a multipass weld, resulting in reduced notch-toughness. As a result, this would requir e requalification of the W PS when notch-toughness is required. Again, the application of a post-weld solution annealing heat treatment will homogenize the microstructure, dissolving any precipitates, and negate the application of this as a variable requiring requalification of the WPS when notch-toughness is required. 10 ASME ©2014


QW-407 PWHT QW-407.2 Change in PWHT (T & T range) The next supplementary essential variable listed for the qualification of SMAW procedures is QW-407.2, ‘Change in PWHT (T & T range).’ To understand what this means, the detailed definition/explanation in paragraph QW-407.2 must be read from Article IV: ‘

QW-407.2 A change in the postweld heat treatment (see QW-407.1) temperature and time range The procedure qualification test shall be subjected to PWHT essentially equivalent to that encountered in the fabrication of production welds, including at least 80 % of the aggregate times at temperature(s). The PWHT total time(s) at temperature(s) may be applied in one heating cycle.’ This variable acknowledges the strong effect that PWHT has on the notch-toughness of many materials, particularly low-alloy steels. It also recognizes that the effect of time at PWHT temperature on notchtoughness decreases with long times. As a result, this variable allows production welds to experience slightly more time at temperature during PWHT than that used during the qualification of the original WPS used to make them. This permits the PWHT of repair welds near existing welds, without violating the maximum time at temperature limits for the PWHT of the pre-existing welds.

QW-409 ELECTRICAL CHARACTERISTICS QW-409.1 Increase in Heat input The next supplementary essential variable listed for the qualification of SMAW procedures is QW-409.1, ‘Increase in Heat input.’ To understand what this means, the detailed definition/explanation in paragraph QW-409.1 must be read from Article IV: ‘

QW-409.1 An increase in heat input, or an increase in volume of weld metal deposited per unit length of weld, over that qualified. The increase may be determined by (a), (b), or (c) for nonwaveform controlled welding, or by (b) or (c) for waveform controlled welding. See Nonmandatory Appendix H. (a)

Heat input [J/in. (J/mm)] Voltage x Amperage x 60 = Travel Speed [in./min (mm/min)]

(b)

Volume of weld metal measured by (1) an increase in bead size (width x thickness), or (2) a decrease in length of weld bead per unit length of electrode

(c)

Heat input determined using instantaneous energy or power by (1) for instantaneous energy measurements in joules (J) Heat input [J/in. (J/mm)] Energy (J)_______ = Weld Bead Length [in. (mm)]

(2) for instantaneous power measurements in joules per second (J/s) or Watts (W) heat input [J/in. (J/mm)]

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Power (J/x or W) x arc time (s) = Weld Bead Length [in. (mm)]

The requirement for measuring the heat input or volume of deposited weld metal does not apply when the WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic material is solution annealed after welding.’ This variable enforces the rule that increased heat input degrades the notch-toughness of the HAZ because of the microstructural changes (primarily grain growth) that occurs. The degrading effects of increased heat input can be measured either by the traditional equation shown in QW-409.1(a), by measuring the volume of weld metal deposited per length of unit weld as shown in QW-409.1(b), or by using energy or power outputs from solid-state power sources using controlled waveform welding. Again, the application of a post-weld solution heat treatment would homogenize the microstructure, refining the coarse-grained HAZ and dissolving any deleterious precipitates in the HAZ. This would negate the application of this variable as a requirement for requalification of the WPS when notch-toughness is required.

QW-409.4 Change in Current or polarity The next supplementary essential variable listed for the qualification of SMAW procedures is QW-409.4, ‘Change in Current or polarity.’ To understand what this means, the detailed definition/explanation in paragraph QW-409.4 must be read from Article IV:

‘

QW-409.4 A change from AC to DC, or vice versa; and in DC welding, a change from electrode negative (straight polarity) to electrode positiv e (reverse polarity), or vice versa.’ This variable is necessary for the qualification of welding procedures where notch-toughness is required because the type of current and the polarity have a direct effect on the amount of heat input imparted to the workpiece. In DCEN (straight polarity), approximately 70 % of the heat of the arc is transmitted to the work (not counting for heat expended in melting the filler metal); whereas in DCEP (reverse polarity), approximately 30 % of the heat of the arc is transmitted to the work (again, not counting for heat expended in melting the filler metal). In balanced waveform welding (50 % EP and 50 % EN) and in AC welding, the heat is divided roughly evenly between the electrode and the work. Similar to heat input, change in current type and polarity then is a supplementary essential variable for the qualification of welding procedures when notch-toughness is required.

QW-410 Technique QW-410.9 Change Multiple to single pass/side The next supplementary essential variable listed for the qualification of SMAW procedures is QW-410.9, ‘Change Multiple to single pass/side.’ To understand what this means, the detailed definition/explanation in paragraph QW-410.9 must be read from Article IV: ‘

QW-410.9 A change from multipass per side to single pass per side. This variable does not apply when a WPS is qualified with a PWHT above the upper transformation temperature or when an austenitic or P- No. 10H material is solution annealed after welding.’ In a weld joint thick enough to be welded using a multipass weld, changing the technique from a multipass weld to a single pass weld would significantly increase the weld heat input, increase HAZ time at temperatures in excess of 1900 ˚ F (1040 ˚ C) (increasing grain growth) and eliminate any tempered martensite or grain refinement in the HAZ of the base metal and previously deposited weld beads. As a result, the increased heat input would degrade the notch-toughness of both the base metal HAZ and weld metal. For this reason, this change would require requalification of the WPS when notch-toughness is required. Again, the application of a post-weld solution heat treatment to the single pass weld would 12 ASME ©2014


homogenize the microstructure of the base metal HAZ, restoring the notch-toughness, and negating this variable as a requirement for requalification of the WPS when notch-toughness is required.

Qualification of a WPS with Notch-Toughness As a general rule, construction Codes will identify the conditions under which notch-toughness is required during the qualification of welding procedures. When construction Codes, minimum design temperatures, base metals, or client requirements require notch-toughness, the procedure qualification test coupon shall be made large enough that impact testing specimens (usually Charpy V-Notch specimens) can be removed from it in addition to the tension tests and bend tests normally required for procedure qualification. In addition, other welding variables which affect notch-toughness, but do not affect tensile strength, referred to as the supplementary essential variables just described and listed in the tables in QW-250 for each welding process, become invoked as essential variables for procedure qualification. The process of qualifying a WPS with impact testing is identical to that described in Module 5 for any other WPS qualification with the following additional requirements: (a) In addition to recording the essential variables during the welding of the test coupon, the supplementary essential variables for the specific welding process must be recorded also. (b) In addition to conducting the traditional bend and tension tests required by Section IX, impact testing must be conducted as well. (c)

Finally, the WPS m u s t include the supplementary essential variables and their specific limits on qualification ranges.

Ideally, the need for notch-toughness will be known when the procedure is originally qualified, in which cases, the three items above can be addressed during the qualification process. However, there may be cases where notch-toughness is required by the Code, but the WPS available for that welding process and base metal was qualified without impact testing. A good practice is to make the initial test coupon large enough that there is additional material left over for impact testing at a later date to allow for this possibility. The other solution is to weld an additional test coupon later for impact testing. In both cases, these additional coupons can be used to generate a ‘supplemental’ PQR, which can be used to support a new WPS qualified with impact testing. These two techniques will be described at the end of this Module, under ‘Supplemental PQRs.’ Construction Code Considerations The most important consideration is to fully understand the notch-toughness requirements of the applicable construction Code. Although the details of that subject are beyond the scope of this course, some examples will be provided here as background information. For instance, the ASME B31.3 Process Piping Code requires Charpy V-Notch impact testing of the procedure qualification test plate at the lowest design temperature for certain base metals. It also specifies the number of specimens to be tested. The specimens (and more importantly, their notches) are to be located as dictated by material specifications based on the product form being tested. ASME B31.3 also specifies the acceptance criteria. In Section VIII, depending on the base metal, Charpy V-Notch impact testing of the procedure qualification test plate is also required to be conducted at the lowest design temperature. The procedure qualification test coupon shall be subjected to heat treatment, including cooling rates and total time at temperature or temperatures as established by the material Manufacturer for use in actual manufacture. Two sets of three specimens must then be tested: one set having the notch in the weld metal and one set having the notch in the HAZ. The samples must meet the minimum notch toughness requirements specified in UG-84(c)(4) for the thickest material of the range of base metal qualified. Without going into any more detail about these requirements, suffice it to say that these requirements must be known when welding the test coupon so sufficient material can be welded to allow the minimum number of impact specimens required by the construction Code to be removed from the specified 13 ASME ©2014


locations and tested. Bear in mind that other construction Codes may have different requirements for the number of specimens, their notch location relative to the weld and HAZ, the test temperature, and the acceptance criteria. Welding the Test Coupon Now that the welding process, base metal, filler metal, and variables required by Section IX, the construction Code and the client have been identified, and the test coupon size and shape has been identified, the test coupon may be welded. Section IX requires that a representative of the organization must supervise the welding. All other conditions described in Module 5 for welding of PQR test coupons must be observed. For WPSs qualified with impact testing, b o t h essential an d supplementary essential variables m u s t be monitored and recorded. If impact testing is performed on the test coupon during the original WPS qualification, then the WPS can be qualified with notch-toughness. If material from this original test coupon is withheld and not impact tested until a later date, the original WPS would be qualified without notch-toughness. When the extra material is impact tested at a later date, a supplemental PQR could be generated and combined with the original PQR having the bend and tension test results to support a new WPS, which would now be qualified with notch-toughness. Nonessential variables may be recorded if they are monitored. If required by the client or the applicable construction Code, the test coupon must then be subject to PWHT. After this, the test coupon must be subjected to the tests required by Section IX.

Testing the Test Coupon The tests required for qualifying a WPS with impact testing are the same as those previously described in Module 5 (tension and bend tests), with the addition of the impact tests as prescribed by the applicable construction Code. Section IX recognizes two types of impact testing: Charpy V-Notch testing and Drop Weight Testing. Charpy V-Notch testing was described in detail in Module 3 and will not be discussed further here. Drop-Weight Testing is a method of determining the nil-ductility temperature (NDT) of ferritic materials 5/8 inch (16 mm) thick and greater. This is similar to, but different than, the DBTT described earlier. The NDT is the maximum temperature below at which the specimen fractures during the test. The probability of a ductile fracture below the NDT is negligible. Drop-Weight Testing was specifically developed to determine the NDT on full-thickness plates. A short single-pass weld of brittle weld metal is deposited on the surface of a plate 5/8 inch (16 mm) to 1 inch (25 mm) thick. A small notch is made in the weld bead and the specimen is supported as a simple beam in a bath held at a known temperature. With the weld bead (and notch) on the bottom of the plate, a standardized weight is dropped from a standardized height. Successive tests are conducted at a series of gradually decreasing temperatures. When the starter crack (notch) propagates across the width of the plate on the tension surface to the edges, the test temperature is the NDT. Complete separation on the compression side of the specimen is not required. The NDT is the maximum temperature at which a nil-ductility break (all the way across the plate surface) is produced. Based on the construction Code requirements, impact testing must be conducted using either Charpy VNotch testing in accordance with ASME SA-370, Drop-Weight Testing in accordance with ASTM E208, or other test techniques as required by the applicable construction Code. For all impact testing, when the test coupon is pipe welded in the 5G or 6G position, the impact specimens must be removed from the shaded regions shown in figure QW-463.1(f), which is basically the top half of the pipe. Other than this location requirement for pipe coupons, Section IX does not have guidelines for the location for the removal of impact test specimens from procedure qualification test coupons. Both QW-171.3 (for Charpy V-Notch testing) and QW-172.3 (for drop-weight testing) state that the test specimen location and notch location and orientation requirements are found in the construction Codes. These requirements will determine how many and what type of test coupons are to be welded during procedure qualification. When qualification is done using plate material, it may be possible to 14 ASME ©2014


make the plate large enough to remove Charpy V-Notch specimens as well, depending on the construction Code requirements. In many cases, however, separate coupons may need to be welded. Similar to guidelines in Module 5 for the performance of bend tests and tension tests, it is not necessary to use a certified laboratory for the impact tests required when qualifying a WPS with notch toughness. Writing the PQR Now that the test coupon has been welded and the bend tests, tension tests, and impact tests have been conducted, the procedure qualification record (PQR) can be generated. Using a form similar to that shown in QW-483 in Nonmandatory Appendix B, or any other suitable form, the following information should be entered as a minimum:

(a) (b) (c) (d) (e) (f) (g) (h) (i) (j)

suitable information to identify the organization the date of welding identification of the test coupon base metal to determine the minimum specified tensile strength all of the essential variables all of the supplementary essential variables, since the WPS is being qualified with impact testing the results of the bend tests the results of the tension tests, including ultimate tensile strength the results of the impact tests, including test temperature the organization’s certification or signature on the PQR and the identification of the test laboratory report containing the mechanical test results

The test laboratory report is not required to be kept with the PQR since the organization is ultimately responsible for the accuracy of the contents of the PQR. However, it is good practice to file all original test laboratory reports with the PQR.

Writing the WPS Once the PQR has been generated, the WPS can be written. The WPS should have an effective date and it cannot be a date earlier than the latest of the dates on the supporting PQRs. The requirements for the contents of the WPS are the same as those listed earlier, with the addition of the supplementary essential variable ranges, as applicable. This, however, just ensures compliance with Section IX of the ASME BPV Code as well as the applicable construction Code. There is more to a useful WPS than this. Other factors must be addressed. These include, but are not limited to, client-specific restrictions that have been previously discussed, limitations that the organization may want to impose, and other instructions to the welder that are not addressed by Section IX rules. These issues have been discussed previously in Module 5 and will not be repeated here.

Supplemental PQR Many times, an organization will have a WPS qualified without notch-toughness only to find out later that an application for this WPS will require notch-toughness. Rather than welding an additional set of coupons and repeating all of the bend testing and tension testing, in addition to the impact testing, the organization has the option of generating a supplemental PQR to support a new, revised, WPS qualified with notch-toughness. There are two methods, which may be used to generate the required WPS:

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(a) During the welding of the original PQR coupon without impact testing, weld additional material, with adequate identification, and keep it in reserve for impact testing at a later date. i. Ensure that all essential and supplementary essential variables were recorded during the welding of the coupon. ii. Conduct the impact testing as required by the applicable construction Code. iii. Write a supplemental PQR, which must contain essential variables, supplemental essential variables, and impact test results. iv. Write a new WPS referencing both the original PQR and the supplemental PQR – this WPS is now qualified with notch-toughness. (b) Simply wait until the impact testing requirements are imposed and weld a supplemental PQR coupon at that time. i. The same essential variables as those used for the original test coupon must be used and recorded (see paragraph QW-401.3). ii. Record the applicable supplementary essential variables during the welding of the supplemental coupon. iii. Conduct impact testing of the supplemental coupon as required by the applicable construction Code (bend testing and tension testing of this coupon are n o t required). iv. Write a supplemental PQR, containing the essential variables, supplemental essential variables, and impact test results. v. Write a new WPS referencing both the original PQR (which has the bend and tension test results) and the supplemental PQR (which has the impact testing results) – this WPS would then be qualified with notch-toughness.

In both cases, the PQR generated from the impact tested coupon is referred to as a ‘supplemental PQR’ because it supplements the original PQR which lists the essential variables and the results of the bend and tension tests. The ‘supplemental’ PQR only needs to contain the following information: (a) suitable information to identify the organization (b) the date of welding and the date of testing (c) the essential variables for the specific welding process – these must be identical to those used on the test coupon for the original PQR (d) the supplementary essential variables for the specific welding process (e) the specific construction Code to which the impact testing was conducted (f) the results of the impact tests, as specified by the construction Code, including test temperature (g) the organization’s certification or signature on the PQR and (h) the identification of the test laboratory report containing the impact test results When the WPS qualified with notch-toughness is written, it should n o t be a revision of the original WPS; instead, it should be a new WPS with a different designation. It should also:

(a) be dated on or after the date on which the supplemental PQR coupon was welded (b) reference BOTH the original PQR and the supplemental PQR (c) specifically describe the construction Code to which the impact tests were conducted. So the WPS should state that the impact testing was performed in accordance with ASME B31.3 or ASME Section VIII, as appli cable (d) list the essential variable an d supplementary essential variable ranges qualified as required by the applicable table in QW-250 for the welding process used and (e) list reasonable ranges for all required nonessential variables.

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ASME-Notch-Toughness & Supplementary Essential Variables

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