Submerged arc welding consumables selection

Unlike GMAW, SMAW, and FCAW consumables, for which the classification required is easily determined and the product can be selected with minimal consideration, submerged arc welding (SAW) flux and wire combinations require a multiple-step process to determine the optimal choice.

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Factors that must be considered include:

• Is this a single-pass or multipass application?
• What are the required Charpy V-notch (CVN) properties of the weld metal?
• Will the weldment be post-weld heat-treated? Time of stress relief?
• Is the weld a two-run or single-run weld?
• Is the weld for sour service applications?

Some welding consumable manufacturers have a comprehensive flux and wire portfolio. When you consider all the possible permutations and combinations of flux and wire, it may be possible your current combination is not optimal for your application.

Welding Electrode Selection

For a particular application, the AWS/CSA electrode classification is often dictated by the engineer of record, the particular welding code, or perhaps simply the need to match the base material chemistry.

A common example is an AWS E-1 or CSA E-1 SMAW electrode, or the AWS E71T-1 or CSA E491T1-C1A3-CS1 gas-shielded flux-cored electrodes.

SAW Flux Wire Classification

A SAW electrode normally has an AWS classification, which is determined by the electrode composition, so you can have an equivalent electrode. SAW flux alone cannot have an AWS/CSA classification, so there is no “equivalent flux.”

A SAW flux/wire combination, however, does have an AWS/CSA classification. You can have an equivalent flux/wire classification.

Caution must be used when you select a flux/wire solely by classification.

For purposes of discussion, open arc electrodes of similar classification will have comparable performance and mechanical properties. For example, different brand/trade name ER70S-6 and B-G 49A 3 C1 S6 GMAW wires generally can be used for the same application.

This is, however, not the case with SAW fluxes and wires.

For example, Lincoln Electric has more than eight flux and wire combinations that all meet the same F7A2-EM12K classification. The electrode in this example is the same (Lincolnweld L-61), but multiple fluxes yield this classification.

Unlike our example of the GMAW electrode, the performance of different combinations for SAW may vary tremendously, despite all having the same flux/wire classification.

Active or Neutral Flux

Active fluxes add a certain level of silicon (Si) and manganese (Mn) into the weld deposit.

Neutral fluxes, as the name implies, contribute relatively low amounts of Si and Mn.

A flux is deemed “active” or “neutral” depending on the Wall Neutrality Number. This number is determined by the flux manufacturers through a series of weld deposit chemistry tests.

WN# = 100 (|Δ|Si + |Δ|Mn)

As per AWS A5.17, a flux is deemed neutral if its WN# is equal to or lower than 35.

Why does this matter? It is generally accepted that an active flux should be used only for single-pass applications. The reason for this is that in multiple passes, the Mn content can increase to a level where strength and hardness levels become excessive. Elongation properties can also decrease. These conditions can lead to weld failure. This situation can be exacerbated by excessive voltage levels, since higher voltage can cause greater flux melt off into the weld.

Back to our example of AWS F7A2-EM12K / CSA F49A3-EM12K:

Lincolnweld 761/L- 61 has a high Wall Neutrality Number and is considered an active flux. It therefore excels, due to the relatively high deoxidizer content (Si/Mn), at welding over light contaminants such as rust and mill scale. This combination, however, is not a good choice for multiple pass heavy plate welds.

Lincolnweld 960/L-61 has the same classification of F7A2-EM12K as 761/L-61 but it is a neutral flux. This makes it a much better choice for multiple-pass welding. Performance on mill scale and other contaminants, however, will not be as good.

Whether the application is single or multiple pass is a critical factor in selecting a flux/wire combination.

Charpy V-Notch Requirement

The testing temperature of the impact properties (test temperature of CVN) can be seen in a flux wire classification.

AWS F7A2-EM12K denotes a -20-degree-F CVN test temperature.

A similar CSA F49A3-EM12K denotes a -30-degree-C CVN test temperature.

In selecting a flux/wire combination, consider the required CVN properties.

For example, if a pressure vessel application requires a CVN value of 20 ft.-lbs. at -60 degrees F as welded, we need to ensure that classification is at least an F7A6.

The commonly used Lincolnweld L-61 (EM12K) electrode yields classifications ranging from F7A0 (0 degrees F CVN) to F7A8 (-80 degrees F CVN). This is a significant range of toughness just by changing the flux it is paired with.

Flux Basicity

The flux basicity index (BI) is calculated using various formulas that quantify the ratio between basic and acidic components of the flux.

Lincoln Electric uses the Boniszewski basicity index formula:

BI = 0.5(FeO + MnO) + CaO + MgO + Na2O + K2O + CaF2 / SiO2 + 0.5(TiO2 + ZrO2 + Al2O3)

Generally, a higher BI yields a microstructure that is more conducive to robust CVN properties. The notable exception is single-pass or two-run welds.

Flux manufacturers typically publish the B.I. of their fluxes on the material datasheet.

It should be noted, however, that fluxes should not be chosen based solely on BI, nor should fluxes be cross-referenced based on this number.

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Post-weld Heat Treat (PWHT)

Generally speaking, for a carbon steel weld deposit, the ultimate tensile strength (UTS) and yield strength (YS) will drop after PWHT.

This must also be taken into consideration when selecting flux/wire combinations since not all of them will meet the same strength level as welded and after stress relief.

The strength can be determined by the flux/wire classification. The A in the classification denotes the “as welded” condition, and the P denotes a PWHT condition.

Please note that typically PWHT results are for one hour at a specific temperature, usually 1,150 degrees F/650 degrees C. For longer hold times and higher temperatures, strength levels will be further reduced.

For example, the Lincolnweld 882/Lincolnweld LA-71 electrode classification is F7A6-EH11K/F7P6-EH11K. This indicates that this combination will meet the requirements for an F7, or 70-KSI UTS.

Two-run Versus Multi-run: Grain Refinement

Most classifications are determined with multiple-pass test plates as required by CSA and AWS.

Grain refinement from reheating occurs when a subsequent weld pass is made over a previous pass. This darker zone of finer grains can resist impact better than the coarse, unrefined grain structure.

As you can see in the photo (Figure 2), a two-run weld (one pass each side) has much less refined weld metal. As a result, the CVN properties of the two-run weld tend to be less robust than the multiple-pass weld. Furthermore, the base material dilution is typically much greater in a two-run weld.

Rule of Two-run Welds

Do not use multiple-pass test results (particularly impact toughness) to predict the performance of a flux/electrode combination in a two-run application, and vice versa.

AWS A5.23 Two-run Classification

Two-run welds are typically found in pipe mills, shipyard panel welding, and wind towers.

If your application is two-run or single-run, you should review if the combination has a two-run certificate of conformance (COC). Two-run COCs are denoted by the addition of the letter T after the strength designator.

For example, Lincolnweld WTX-TR flux with Lincolnweld L-61 has both a multi-run COC (F7A6-EM12K) and two-run (F7TA4-EM12K) COC.

Flux/wire combinations must be carefully selected for single- and two-run welds since the vast majority of flux/wire combinations in the market may not be suitable for this application.

NACE Sour Service Requirement

The National Association of Corrosion Engineers (NACE) MR /ISO for Sour Service (H2S) limits the weld deposit to 1 per cent nickel (Ni). Not all Ni low-alloy electrodes will meet this limit.

Due to the complex nature of flux and wire selection, it is suggested that you engage your welding consumable manufacturer to discuss in detail your particular requirements.

Job Knowledge 89

Part 1
Part 2

As with the BS EN specifications for submerged arc welding consumables, the American Welding Society (AWS) system also uses a dual flux type/wire composition designation to identify the flux/wire combination that will provide the required properties.

The AWS system is somewhat simpler than the BS EN method, particularly if the full flux descriptor is used. There are, however, only two specifications that deal with both wire composition and the flux but an additional two specifications that cover bare wires for stainless steels and the nickel based alloys. These are A5.17 - Carbon Steel Electrodes and Fluxes for Submerged Arc Welding and A5.23 Low-Alloy Steel Electrodes and Fluxes for Submerged Arc Welding. The bare wire specifications are A5.9 Wire Electrodes, Strip Electrodes, Wires, and Rods for Arc Welding of Stainless and Heat Resisting Steels-Classification and A5.11/A5.11M Nickel and Nickel-Alloy Bare Welding Electrodes and Rods for Shielded Metal Arc Welding.

In AWS A.5.17 and AWS A5.23 the first part of the designation describes the flux type and may comprise up to six digits depending upon whether the flux is supplied with the tensile strength expressed in increments of 10 megapascals (two numbers where 43 represents 430MPa) or in pounds per square inch (1 digit ie 6 represents 60,000psi or two digits ie 12 represents 120,000psi).

The first digit, the letter 'F', identifies the consumable as a submerged arc welding flux, the next letter 'S' is only included if the flux is made from or includes crushed slag. Omission of this letter 'S' indicates that the flux is unused and contains no crushed used flux introduced either by the flux manufacturer or the welding fabricator.

The next one or two digits specify the minimum tensile strength as explained above and this is followed by 'A' or 'P' for whether the test results were obtained in the as-welded,(A condition) or post-weld heat treated, (P condition). The last digit identifies the minimum temperature at which a Charpy-V impact value of 27J can be achieved as in Table 1 below.

Table 1 Impact Test Requirements

DigitTest TemperatureImpact value
(Joules)Impact value
(ft.lbf) °C°F Z no impact requirements 27 20 0 -18 0 27 20 2 -29 -20 27 20 4 -40 -40 27 20 5 -46 -50 27 20 6 -51 -60 27 20 7 -70 - 27 - 8 - -80 - 20 10 -100 -100 27 20 15 - -150 - 20

In AWS A5.17 wiresare split into three groups of low, medium and high manganese. The first digit, 'E', identifies the consumable as a bare wire electrode. If supplemented by 'C' the wire is a composite (cored) electrode. The composition of the solid wire is obtained from an analysis of the wire. However, since the composition of a cored wire may be different from that of its weld deposit the composition must be determined from a low dilution weld deposit made using a specific, named flux.

The next letter, 'L', 'M' or 'H' indicates a low (0.6% max), medium (1.4% max) or high (2.2% max) manganese content. This is followed by one or two digits that give the specific composition. An optional letter 'K' indicates a silicon killed steel. There are a final two or three optional digits identifying the diffusible hydrogen in ml/100g weld metal, H16, H8, H4 or H2.

A full designation for a carbon steel flux/wire combination could therefore be F6P5-EM12K-H8. This identifies this as being a solid wire with a nominal 0.12% carbon, 1% manganese and 0.1 to 0.35% silicon capable of achieving an ultimate tensile strength of 60 k.p.i. (415MPa), a Charpy-V impact strength of 27J at -50°F (-46°C) in the post weld heat treated condition.

The classification in AWS A5.23 is, of necessity, rather more complicated as this specification covers a wide range of low alloy steels, a total of forty six solid wires and thirty two composite wire weld metal compositions. Within the confines of this brief article it will not be possible to cover in full the entire classification of the wires.

The flux designation is almost identical to that of AWS A5.17, except that a four, five or six digit identifier may be used. Why this additional sixth digit? Because some of the electrodes in the specification are capable of providing tensile strengths above 100,000 psi - in these cases the designation may be, for example, F11, identifying the flux as providing 110 ksi (760MPa) minimum tensile strength.

The classification of the wire comprises two parts - the first that of the wire, solid wires being prefixed 'E' and composite wires 'EC', the second part specifies the composition of the weld deposit. The wire classification commences with 'E' to identify a bare wire, the next letter places the wire in a 'family' of wires. 'L' or 'M' identifies the wires as being alloyed with copper, 0.35% max; 'A'as containing molybdenum, 0.65% max; 'B' as the creep resisting steels containing chromium and molybdenum; 'Ni' for those wires containing nickel. 'F comprises the Ni-Mo or Cr-Ni-Mo wires; 'M' triple de-oxidised Ni-Mo wires; 'W' aNi-Cu wire and 'G' not specified.

This use of wires to this latter 'G' designation may lead to problems as quite large changes can be made to the composition to achieve the required mechanical properties - a good example of this is where the NACE requirements for sour service of 248BHN or 1% nickel maximum are required. To achieve the required tensile or impact strength the consumable manufacturer may increase the carbon or nickel contents above those used in the procedure qualification test and still supply to the same designation.

Table 2 in AWS A5.23 classifies both solid wire and composite wire/flux combinations by means of weld metal compositions but still using the identifying letters as for the solid wires described above. The prefix 'E' is, however, omitted thus a carbon/molybdenum deposit may be classified, for example, as A3, a Cr-Mo deposit as B4, Ni-Mo as F5 etc.

Thus a full designation for a flux/wire combination for an as welded 1% Ni/0.25% Mo weld deposit with an ultimate tensile strength of 80ksi and an impact strength of 27J at -60°F (-51°C) may therefore be F8A6-ENi1-Ni1 and for a similar deposit using a cored wire in the PWHT'd condition F8P6-ECNi1-Ni1.

As mentioned in earlier articles on the topic of consumable specifications, it must be remembered that the mechanical properties and compositions are determined from test pieces taken from absolutely minimal dilution welds made on specified parent plates with a standard set of welding parameters - heat input, preheat, interpass temperature, post weld heat treatment temperature and time. They may therefore NOT reflect the results obtained in a production weld and the designation cannot be relied upon to guarantee the properties required by the application.

Where these properties are important it is therefore essential that mechanical testing, chemical analysis etc are determined from test specimens made using parent materials and parameters representative of production welding.

This article was written by Gene Mathers.

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