WPS for ASTM A572 Grade 42 Structural Steel

ASTM A572 Grade 42 sits at the bottom of the Grade 42/50/60/65 family, yet it appears on structural drawings more often than most CWIs expect — particularly in exposed architectural steel, miscellaneous framing, grating supports, and legacy renovation projects where the mill supplied what was on hand. Because every published example, training course, and WPS template seems to default to Grade 50, Grade 42 can catch a QC manager off guard when the MTR arrives and the yield strength reads 42 ksi instead of 50.

This article walks through WPS development for A572 Grade 42 under AWS D1.1:2025: prequalified qualification routes, filler metal selection, minimum preheat, and the essential variables that differentiate a Grade 42 WPS from a Grade 50 WPS.

What Is ASTM A572 Grade 42?

ASTM A572 (Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel) covers five strength grades: 42, 50, 55, 60, and 65. Grade 42 has:

• Minimum yield strength: 42 ksi [290 MPa]
• Minimum tensile strength: 60 ksi [415 MPa]
• Maximum carbon content: 0.21% (shapes and plates ≤ 4 in thick)

The carbon equivalent (CE) for Grade 42 runs slightly lower than Grade 50 because the lower strength target allows the mill to back off the alloying additions. That lower CE translates directly into a reduced preheat requirement and easier weldability — one of the few practical advantages of specifying the lower grade.

Grade 42 is available in all standard structural shapes (W, S, M, HP, C, MC, L, WT), plates up to 6 in thick, and bars. It is not, however, the most common stock item at steel service centers in 2025; if your order specifies Grade 42, confirm the mill will certify to it and not substitute Grade 50 without a change order.

Prequalified Status Under AWS D1.1:2025

AWS D1.1:2025 Table 5.3 lists A572 Grades 42, 50, 55, 60, and 65 as prequalified base metals for joints welded with prequalified processes (SMAW, SAW, GMAW, FCAW-G, GTAW) and prequalified joint geometries from Annex B. This means you can write a compliant WPS for Grade 42 without conducting a PQR test program, provided every prequalified parameter — joint geometry, fit-up tolerance, filler metal, preheat, interpass temperature, and pass parameters — is fully documented.

The prequalified route is the default for fab shops working with standard structural shapes. If you need a groove weld configuration outside Annex B, or if the contract specifies a tested WPS, you will need a PQR per Clause 6 of AWS D1.1:2025.

Filler Metal Selection for Grade 42

AWS D1.1:2025 Clause 5.7 requires that filler metals meet AWS classifications listed in Table 5.4. For carbon and HSLA steels including A572 Grade 42, standard choices are:

SMAW: E7018-H4 or E7016-H4 are the workhorses for structural fillet and groove welds. The 70 ksi nominal tensile class overmatches the 60 ksi minimum tensile of Grade 42, satisfying the matching filler metal philosophy. The H4 diffusible hydrogen designation (≤ 4 mL/100 g) is strongly recommended when welding at or near minimum preheat to guard against hydrogen-assisted cracking. E6018 is technically permissible for lower-strength applications but is rarely stocked; E7018 is the correct default.

GMAW: ER70S-3 is the AWS D1.1 minimum for matching Grade 42, but ER70S-6 is the industry norm — it tolerates moderate mill scale and handles slight porosity-promoting surface conditions better than -3. Use a shielding gas of 75% Ar/25% CO₂ (C25) or 100% CO₂ depending on your WPS parameters.

FCAW-G: E71T-1C or E71T-1M (CO₂ or mixed-gas shielded respectively) are prequalified and widely available. Confirm the -H designation (e.g., E71T-1C-H8) when low hydrogen is needed. Note that A5.36 designator electrodes were removed from the prequalified filler metal list in AWS D1.1:2025 Table 6.6 row 4 (a change from the 2020 edition) — use the legacy A5.20 or A5.29 classification.

SAW: F7A2-EM12K or equivalent flux-wire combination. SAW is less common on lightweight framing but correct for deep groove welds in plate work.

Preheat Requirements

AWS D1.1:2025 Table 4.5 gives minimum preheat and interpass temperature by base metal group and thickness. A572 Grade 42 falls in Group I (along with A36, A53 Grade B, and other common carbon steels). Minimum preheat values for Group I:

Rule library based on AWS D1.1:2025; verify against your governing edition (the AHJ or contract may specify 2020 or earlier).

Compared to Grade 50, Grade 42 has the same Table 4.5 preheat thresholds because both are in Group I. The real-world difference appears when you calculate CE using the IIW formula: Grade 42 heats at lower CE values and typically hits the critical CE threshold at a slightly heavier thickness than Grade 50, though for practical structural shapes the difference is small.

For thicker plates (> 1-1/2 in) and cold ambient conditions (below 50°F), document your preheat method (torch, induction blanket, or resistance) on the WPS. The minimum must be maintained throughout the weld — including tack welds — and cannot be let to drop below the minimum between passes.

Essential Variables Under AWS D1.1:2025 Table 6.6

If you are writing a tested WPS (not prequalified), these changes to the WPS require requalification per Table 6.6:

• Base metal P-number / group change. A WPS qualified on Grade 42 does not automatically cover Grade 50 or higher if the essential variable rules create a new grouping combination.
• Filler metal F-number change. Switching from SMAW F5 (E7018) to GMAW F6 (ER70S-6) requires requalification.
• Increase in base metal thickness beyond qualified range. The thickness range is typically the test coupon thickness to 2× the test coupon thickness for groove welds, or unlimited for fillet welds if qualified via groove weld PQR.
• Change in welding position beyond the qualified range per Table 6.6 row 3.
• Filler metal A5.36 was removed from the prequalified list in the 2025 edition (Table 6.6 row 4) — this is a 2025-specific rule change versus 2020.

For a prequalified WPS, essential variable changes under Clause 5 trigger the need for a new prequalified WPS or a tested WPS; they do not require a PQR test unless you exit the prequalified envelope entirely.

Documenting the WPS: What the Form Needs

A WPS for A572 Grade 42 using AWS D1.1:2025 Annex M format must include:

1. Base metal: ASTM A572 Grade 42, with thickness range and applicable joint type (groove or fillet)
2. Filler metal: AWS classification, F-number, A-number, diameter, lot/heat (or "per manufacturer certificate")
3. Joint design: Reference to Annex B prequalified detail (e.g., B-U2a for a butt joint with steel backing) or a dimensioned sketch
4. Preheat: Minimum preheat per Table 4.5, plus the maximum interpass temperature (typically 550°F [290°C] for carbon steel unless otherwise specified)
5. Shielding gas / flux: Type, composition, flow rate range
6. Electrical parameters: Current type and polarity, amperage range, voltage range, wire feed speed (GMAW/FCAW), travel speed range
7. Position: All positions for which the WPS is qualified
8. Technique: Stringer or weave bead, max weave width (usually ≤ 3× electrode diameter for prequalified)
9. Edition disclaimer: "Rule library based on AWS D1.1:2025; verify against your governing edition"

Common Errors on Grade 42 WPS Documents

Substituting Grade 50 parameters without review. Because Grade 42 and Grade 50 share Group I, the preheat tables look identical — but if a test-based WPS was qualified on Grade 50 tensile specimens, the recorded mechanical results may not map to a Grade 42 application without re-examining the essential variable scope.

Over-preheat on thin architecturally exposed sections. Over-preheating can distort thin exposed members. For architecturally exposed structural steel (AESS), document both a minimum and a maximum preheat (or maximum interpass) on the WPS to give the welder a target band, not just a floor.

Missing diffusible hydrogen designation. Low-hydrogen electrodes are mandatory for A572 Grade 42 in thicknesses over 1 in under most prequalified conditions. Failing to record the H4 or H8 designation on the WPS leaves the procedure open to challenge during audit.

Filler metal A5.36 entries from old templates. If you copied a template from before the 2025 edition update, check whether any A5.36 classifiers appear in your filler metal column. Under AWS D1.1:2025 Table 6.6 row 4, those are no longer on the prequalified list for GMAW and FCAW; update to A5.20 or A5.29 classifications.

When a PQR Is Warranted Despite Prequalified Status

Certain project conditions push even a straightforward Grade 42 WPS outside the prequalified envelope:

• CVN toughness requirements. If the contract or AHJ imposes Charpy impact values, the supplementary essential variables of Table 6.8 apply. Preheat interpass changes, filler metal changes, and heat input shifts can all trigger requalification under 6.8.
• AISC Seismic Demand Critical welds. AWS D1.8 rules apply; prequalified alone is insufficient — the WPS must be supported by CVN-verified PQR data.
• High-restraint thick joints. For weld deposits in joints with high triaxial restraint (e.g., column splices over 2 in thick), many owners and engineers of record require a tested WPS even when prequalified is technically permissible.

For most standard framing and plate work in Grade 42, the prequalified route using AWS D1.1:2025 Annex B joint details and Table 5 filler metals is entirely appropriate. The key is accurate documentation on the WPS form and confirmation that every prequalified condition is in fact met on the shop floor.

Need a WPS drafted for A572 Grade 42 or a full-service procedure library? See our pricing page to learn how WPS Welding generates code-compliant WPS documents in minutes. For related reading, see WPS for ASTM A572 Grade 50 Structural Steel and AWS D1.1:2025 Table 6.6 Essential Variables Explained.