Dissimilar base metal welding — joining materials from different groups in AWS D1.1:2025 Table 6.9 — is common in structural fabrication and frequently under-qualified. The failure mode is predictable: a shop qualifies a WPS for A36 plate work, then begins running fillet and groove welds into A572 Grade 50 beam flanges, treating the existing PQR as sufficient. It usually is not. When a joint crosses a base metal group boundary, the qualification scope must cover both sides.

AWS D1.1 Base Metal Groups (Table 6.9)

AWS D1.1:2025 Table 6.9 organizes prequalified base metals into groups based on carbon equivalent, yield and tensile strength range, and weldability history. The groups a structural fab shop most commonly works across:

Group I — Lower-strength carbon steels: A36, A53 Grade B, A106 Grade B, A500 Grades A and B, A1011 and A1018 in lower-yield-strength grades, and similar materials.

Group II — Higher-strength carbon and HSLA steels: A572 Grades 42 through 65, A992, A913, select grades of A709, and similar higher-yield-strength structural steels.

Group III — Quenched-and-tempered and other steels with specific weldability requirements.

Different editions of D1.1 have different group boundary assignments and material entries. The 2025 Table 6.9 is the controlling reference for current work. A material that appeared in Group I in a previous edition may be reclassified in 2025.

When a Joint Crosses a Group Boundary

The most common dissimilar combination in structural fabrication is Group I to Group II: A36 material welded to A572-50 or A992. This appears in:

  • Built-up sections: A572-50 flanges with A36 webs
  • Column splices and beam connections: A36 or A572 connection plates welded to A992 wide-flange columns
  • Crane rails and bearing seats: A36 stiffeners welded to higher-strength column flanges
  • Mixed-material trusses: Lower-grade chord gussets welded to higher-strength chord members

In each case, the weld joint bridges two different base metal groups. A WPS qualified for Group I alone does not support this work. Table 6.6 lists base metal group change as an essential variable — when the production joint moves outside the qualified base metal groups, the WPS needs either a new PQR or a demonstration that the existing PQR covers both groups.

Qualifying the Procedure

There are two acceptable approaches when covering a dissimilar group combination:

Option 1 — Test with the combination. Prepare the PQR test coupon using both base metals: one side of the groove is Group I material, the other is Group II. Tensile specimens must meet the minimum tensile for the lower-strength member. Bend specimens are cut across the full joint, which samples both HAZ regions. This provides direct evidence of the procedure's performance across the actual material boundary and is rarely questioned by owner inspectors or auditors.

Option 2 — Test with the higher-group material. Some qualification provisions allow a PQR qualified on the higher-strength group material to cover joints combining that group with a lower-strength group, because the lower-strength group's qualification requirements are encompassed within the higher-strength test results. Verify whether your governing edition, owner specification, and contract documents permit this reading — some owner specifications explicitly require combination testing regardless of this provision.

For a full walkthrough of what mechanical tests a PQR requires, see PQR tensile and bend test requirements.

Filler Metal Selection

Filler metal for dissimilar group joints is typically selected to match or slightly overmatch the higher-strength member. For a Group I to Group II joint, ER70S-6 (GMAW) or E7018 (SMAW) at 70 ksi classification is commonly appropriate — it overmatches A36 at its minimum tensile level and is consistent with the strength of A572-50 and A992 structural connections under most design-basis approaches.

The filler must appear in the filler classification framework of Table 6.6 for the process used. Changing filler classification after the PQR is an essential variable change per Table 6.6 — you cannot substitute ER70S-3 for ER70S-6 after qualification without checking whether the change is covered by the existing PQR or requires a new one. For the complete picture of essential vs. nonessential filler variables, see WPS essential variables vs. nonessential variables.

Preheat for Mixed-Material Joints

When the two base metals carry different carbon equivalents, use the preheat requirement of the material with the higher carbon equivalent for the entire joint. AWS D1.1:2025 Annex I provides a formula-based preheat calculation that accounts for carbon equivalent, hydrogen level, and base metal thickness. For prequalified work under Clause 5, the preheat table is applied to the more demanding material.

High-strength materials such as A992 and A913 Grade 65 impose minimum preheat requirements that apply to the full joint even when the other member (A36, for example) would not require preheat at the same thickness under Clause 5 conditions. This is a documented deficiency in third-party audits: the WPS lists preheat for the lower-strength material only, ignoring the higher-strength member's requirement. For more on preheat documentation see preheat and interpass temperature on a WPS.

Impact on Welder Qualification

Welder qualification scope follows the WPS. A welder qualified under a WPS covering Group I base metal to Group I is not automatically qualified for Group I to Group II joints if the WPS has a more limited scope. Where the WPS lists specific base metal group coverage, the welder's performance qualification extends across the same limits. Verify that the WPS base metal scope and the welder's performance test record are aligned before assigning mixed-material joint work. For an overview of WPQ scope and traceability, see welder qualification and WPQ traceability.

Documentation on the WPS

The WPS should explicitly list the base metal combinations it covers. "A36 to A36" is not the same scope as "Group I to Group II" or "A36 to A572-50." When writing a WPS for dissimilar combinations:

  • List both base metals or the applicable Table 6.9 groups in the base metal field
  • Identify the PQR(s) that support the combination
  • Specify the filler metal and its classification
  • Document preheat applicable to the higher-requirement member
  • Note whether tensile test evidence was obtained on a combination coupon or a higher-group-only coupon, and reference the applicable qualification provision

A procedure library that specifies "Group I only" for a large portion of its WPSs is a red flag in any structural audit — especially for shops running mixed A36/A992 connection work.

AISC Certification and Dissimilar Base Metal WPSs

Shops certified under AISC Quality Certification programs are audited on WPS scope. An AISC auditor reviewing a WPS library will check whether the documented base metal coverage matches the materials specified on project drawings. A WPS listing "A36 to A36" while drawings call for A992 column material is a documented corrective action finding — requiring a procedure qualification update before the shop can continue producing the affected connection type under the certification program.

This is distinct from the underlying AWS D1.1 compliance question. AISC certification adds an independent third-party audit layer that surfaces WPS scope gaps even when the original qualification was technically correct for what was tested. Review your WPS base metal coverage against current project base metal schedules at least annually. For a broader overview of audit preparation, see AISC certification and WPS audit readiness.

ASME Section IX procedure qualification for pressure vessels and piping handles dissimilar base metal work differently — P-number combinations govern that framework — and the qualification paths do not transfer between codes. WPS qualification management for both AWS D1.1 and ASME Section IX is available on the Pro and Shop plans at wpswelding.com/pricing.

Rule library based on AWS D1.1:2025; verify against your governing edition.