Structural steel fabrication rarely involves a single grade of steel throughout a project. A typical commercial building frame mixes ASTM A36, A572 Gr 50, A992, and possibly one of the newer ASTM high-performance grades. Column splice plates get welded to heavy W14 sections. Gussets cut from A36 plate get welded to A572 Gr 50 bracing. Connection plates are frequently one grade; the member they connect to is another.
Welding across a strength difference is routine — but it has requirements that the WPS must address. Getting the filler selection wrong is the most common error. Getting the PQR qualification scope wrong is the second most common. Both are audit findings, and both have structural consequences.
Rule library based on AWS D1.1:2025; verify against your governing edition.
The core principle: match or overmatch the lower-strength base metal
AWS D1.1:2025 follows a well-established metallurgical principle for filler metal selection: the deposited weld metal must meet or exceed the minimum mechanical properties of the weaker base metal. You do not need to match the stronger material.
In the A36-to-A572 Gr 50 example:
- A36 minimum yield: 36 ksi, minimum tensile: 58 ksi
- A572 Gr 50 minimum yield: 50 ksi, minimum tensile: 65 ksi
- The governing base metal for filler selection is A36
A 70-ksi tensile class filler (E7018 for SMAW, ER70S-6 for GMAW, E71T-1 for FCAW) satisfies this requirement. The weld metal at 70-ksi tensile overmatches A36 and undermatches A572 Gr 50 — which is acceptable under AWS D1.1:2025 because the structural design assumes the weld metal is at least as strong as the weaker member. The joint is not weaker than its weaker connected part.
This principle breaks down if someone selects a 60-ksi filler for the same joint thinking "it's just an A36 piece." A 60-ksi tensile filler technically meets A36 minimum tensile (58 ksi) but with only 2 ksi margin, and AWS D1.1:2025 specifies the matching filler requirements explicitly. Using a 60-ksi filler where a 70-ksi is required is an essential variable change under Table 6.6 and, if it appears in production, is a nonconformance.
Base metal groups and PQR qualification scope
AWS D1.1:2025 organizes base metals into groups in Table 6.9 (formerly Table 4.9 in older editions). The groups capture materials with similar weldability, carbon equivalent, and mechanical property ranges. PQR qualification using a base metal in one group qualifies the WPS for that group — it does not automatically qualify combinations across group lines.
For common carbon and low-alloy structural steels, many routine grades fall within the same base metal groups. A572 Gr 42, Gr 50, and Gr 55 are in the same group. A36 and A572 Gr 50 are frequently covered by the same base metal group qualification. However, the CWI and welding engineer must verify this for each combination — do not assume a single PQR covers every possible base metal pairing without checking Table 6.9.
Where dissimilar materials span group lines — for example, a carbon steel base metal joined to a quenched-and-tempered high-strength alloy steel — the qualification scope narrows. The PQR may need to test the specific combination, and the welding engineer should review whether Clause 5 prequalified provisions or Clause 4 qualification-by-test provisions apply. See WPS essential variables vs. nonessential variables for how base metal group is treated as an essential variable.
Prequalified WPS provisions for mixed grades
Clause 5 of AWS D1.1:2025 allows fabricators to use a prequalified WPS — one that does not require a supporting PQR with destructive testing — provided all of the Clause 5 conditions are met. For dissimilar base metal combinations, both base metals must appear in Table 5.3 (the prequalified base metal list), and the joint geometry, groove dimensions, and filler must conform to Clause 5 requirements.
The practical upside of a prequalified WPS is speed and cost: no coupon testing, no external lab, no waiting for results. The constraint is that you must stay within the letter of Clause 5. If either base metal in a dissimilar combination is not in Table 5.3, or if the application involves impact-tested CVN requirements, the prequalified path closes and Clause 4 qualification-by-test is required.
High-yield-strength grades — A913, A1085, certain A709 grades — are not in the prequalified base metal list. If those appear in a dissimilar weld combination, budget for a PQR.
PQR testing for dissimilar combinations
When qualification-by-test is required, the PQR test weld must use base metals representative of the production combination. The test weld uses the two dissimilar materials (or approved substitutes within the qualification range), the production WPS parameters, and is subjected to the required destructive tests: reduced-section tensile, guided-bend, and in some cases CVN impact testing.
The tensile test acceptance criterion is failure at or above the minimum specified tensile strength of the weaker base metal, or at or above the minimum specified for the filler metal, whichever is higher. This is where the filler selection matters mechanically: a properly matched filler means the tensile fracture should occur in the base metal, not in the weld metal — which is evidence that the weld is not the weak link.
For CVN supplementary essential variables, see AWS D1.1:2025 Table 6.8 CVN toughness requirements. CVN testing on dissimilar joints requires impact specimens taken in a representative location — the decision on specimen location depends on the joint configuration and the temperature requirements.