A pressure vessel fabricator's WPS for a SA-516-70 vessel lists the base material as "SA-516-70, P-No. 1, Group No. 2." The MTC for the plate that arrived reads "SA-516 Grade 70N." The welding engineer sees "SA-516-70" and approves the cert. The "N" suffix — indicating normalized delivery condition — goes unnoticed.
Is this the same material?
It's a legitimate question. SA-516-70 and SA-516-70N are both ASME Section II materials, both P-No. 1 Group No. 2, both with the same minimum mechanical properties. The normalized condition is a heat treatment applied to the as-rolled plate; it doesn't change the alloy composition or the code material assignment. For most welding procedure qualification purposes, SA-516-70 and SA-516-70N are the same material.
But the "N" suffix on the cert created a question that required resolution time. The welding engineer had to confirm the P-number and Group Number assignment, verify the delivery condition was within the WPS base material coverage, and document the determination. That took two hours. Multiply by the number of times this kind of question comes up on a vessel job, and you're measuring days, not hours.
How ASME Section IX Links MTC to WPS
ASME Section IX qualifies welding procedures by grouping base materials into P-numbers based on weldability characteristics. Within each P-number, Group Numbers provide finer classification. A WPS that qualifies on P-No. 1 Group No. 1 doesn't automatically qualify on P-No. 1 Group No. 2 — you need to check QW-403 and QW-420 to understand the group coverage rules.
The base material listing in a WPS and PQR must reference the actual ASME material spec and grade used during qualification testing. When you weld in production using materials not covered by your WPS base material list, one of two things must be true: the production material is within the essential variable range of the existing WPS, or you need a new WPS qualified on the production material.
The MTC is the document that establishes what the production material actually is. The spec and grade on the MTC must match the WPS coverage. If the WPS says SA-516-70 and the MTC says SA-516-70N, the determination of whether that's an essential variable change — requiring re-qualification — depends on the WPS scope and ASME Section IX rules.
Common MTC-to-WPS Mismatches That Trigger NCRs
The normalized suffix issue (SA-516-70 vs. SA-516-70N) is the most common mismatch for carbon steel pressure vessel plate. As discussed above, this is usually resolvable without re-qualification, but it requires documented engineering review.
ASTM vs. ASME designation. ASME material specs are the SA-prefixed versions of ASTM A-prefixed specs. SA-516 is the ASME equivalent of ASTM A516. For work under the ASME B&PV Code, the material must be procured to the SA spec, and the MTC must show the SA designation. An MTC that shows "ASTM A516-70" for a Code vessel is not the same as one showing "ASME SA-516-70." The ASTM spec and the ASME spec have the same requirements, but the Code requires ASME-designated material. The MTC showing the ASTM designation needs a supplemental cert or reissuance showing the ASME designation before the cert satisfies the Code requirement.
P-number assignment discrepancy. Less common, but when it occurs it's serious: a material cert that shows a spec and grade that maps to a different P-number than what the WPS covers. For example, a high-strength low-alloy plate that due to its chemistry composition actually falls under P-No. 3 rather than P-No. 1. If the WPS only qualifies P-No. 1 base materials, welding this plate without a P-No. 3 qualified WPS is a Code violation. The inspector will find this during the material-to-WPS cross-reference review.
Group Number boundary. Within P-No. 1, carbon steel plate materials are split between Group No. 1 (low-carbon, lower-strength grades) and Group No. 2 (medium-carbon, higher-strength grades). SA-516-70 is P-No. 1 Group No. 2. Some WPS qualifications cover both groups under P-No. 1; others are qualified specifically on Group No. 1. An operation that has WPS coverage on P-No. 1 Gr. 1 only and receives SA-516-70 (P-No. 1 Gr. 2) for a vessel job needs to check their WPS before proceeding.
The Cross-Reference Process
A formal MTC-to-WPS cross-reference at material release to fabrication catches these issues before any weld is made. The cross-reference requires three steps:
Step 1: Determine the P-number and Group Number from the MTC. The MTC shows the material spec and grade. ASME Section IX QW/QB-422 (the P-number table) maps each ASME material spec and grade to its P-number and Group Number. Some large fabricators have this table integrated into their incoming inspection system so the P-number is assigned automatically when the MTC is processed.
Step 2: Verify that the job's WPS covers that P-number and Group Number. The WPS base material section lists the P-number and Group Number the procedure qualifies. Confirm the MTC material falls within that range. If there are multiple WPS documents for the job, confirm each one has appropriate coverage for the materials it will be used to weld.
Step 3: Document the cross-reference. The traveler or job quality record should show that the MTC material was confirmed against the WPS base material coverage before fabrication began. This closes the loop for the ASME inspector — they don't need to work through the cross-reference themselves because your documentation already shows you did it.