The ASME authorized inspector walks into the final inspection with a checklist. On that checklist is the PWHT documentation package: the base material certs, the WPS, the heat treatment chart with time at temperature and cooling rate data, the traveler showing the actual PWHT applied to each weld seam. The inspector starts at the MTC.
Not at the PWHT chart. Not at the WPS. At the mill test certificate for the base material.
This surprises fabricators who think of PWHT documentation as something that happens after welding — the furnace charts, the thermocouple records, the sign-off on the traveler. Those documents are necessary but they're the end of the chain. The beginning of the chain is the chemistry and P-number assignment on the MTC, and if that foundation is incomplete, the inspector's review stops before the furnace charts are opened.
Why the MTC Is the Foundation of the PWHT Package
PWHT requirements in ASME Section VIII Div. 1 are driven by three things: P-number of the base material, thickness, and whether the design or the welding procedure requires PWHT for reasons beyond code minimum. All three originate in or are verified against the MTC.
P-number assignment. ASME Section IX organizes base materials into P-numbers based on composition and properties. SA-516-70 is P-1. 2-1/4 Cr-1 Mo plate (SA-387 Gr 22) is P-5A. The P-number determines which welding procedure qualifications apply and whether PWHT is mandatory under the code. The P-number comes from the material spec and grade on the MTC — the inspector needs to confirm that the cert shows the material that was assigned to the design, not a different material that happens to have similar mechanical properties.
Carbon equivalent for preheat and PWHT temperature selection. ASME and AWS D1.1 reference carbon equivalent formulas to establish minimum preheat requirements and to inform PWHT temperature selection for specific applications. The IIW carbon equivalent formula is:
CE = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15
All values come from the heat analysis chemistry on the MTC. If the MTC doesn't report all of the required elements, the carbon equivalent cannot be calculated. For P-1 material (carbon steel), many shops calculate CE as part of the incoming review and document it in the job file. For alloy materials (P-4, P-5, P-9 groups), the CE calculation is more critical and is typically included in the WPS or welding traveler.
Heat treatment condition as-delivered. The MTC must document the delivery condition of the material: as-rolled, normalized, normalized and tempered, or quenched and tempered. For SA-516-70 ordered in normalized condition (required by many ASME specifications above certain thickness), the cert must show normalization was performed. This matters for PWHT because the material's mechanical properties and microstructure were established by the mill's heat treatment. The fabrication PWHT must return the material to an acceptable microstructural condition after welding — and the starting condition matters for setting PWHT parameters. A simulated PWHT coupon test (ASTM A20 S9, or supplementary requirement 9) is sometimes required to demonstrate that PWHT won't degrade properties below code minimums.
The Three-Document PWHT Chain
The ASME inspector reads the PWHT package as a chain of three documents:
Document 1: MTC (base material). Confirms material spec and grade, P-number assignment, heat analysis chemistry (for CE calculation), delivery condition. If the inspector finds the MTC incomplete or inconsistent with the design document, the review stops here.
Document 2: WPS (Welding Procedure Specification). Lists the base material spec and grade that the procedure qualifies, the P-number group, preheat and interpass temperature requirements, and PWHT requirements (temperature range, holding time, heating and cooling rate limits). The WPS PWHT requirements must comply with ASME Section VIII UCS-56 or UHA-32 (as applicable) for the base material P-number and thickness. The inspector verifies that the WPS PWHT parameters are within code-required limits.
Document 3: PWHT record (traveler and chart). Documents the actual PWHT applied: furnace identification, thermocouple locations, chart showing temperature vs. time, actual soak temperature, actual soak time, actual heating rate, and cooling rate to the temperature at which the vessel was removed from the furnace (or allowed to air-cool). The actual parameters must fall within the WPS requirements, which must fall within code requirements. Every step in the chain must be traceable.
If the MTC doesn't support the P-number assignment in the WPS, the chain is broken. If the WPS PWHT parameters don't match the code requirements for the P-number and thickness, the chain is broken. If the furnace chart shows the actual temperature didn't reach the WPS minimum, the chain is broken.
What Happens When the MTC Is Incomplete
Two common MTC gaps that break the PWHT chain at document one:
Missing chemistry elements for CE calculation. A cert that doesn't report chromium, molybdenum, and vanadium for a SA-387 Gr 22 (2-1/4 Cr-1 Mo) plate makes CE calculation impossible. The inspector will require a supplemental cert from the mill with the complete analysis before the PWHT documentation can be considered complete.
Delivery condition not stated. A cert for SA-516-70 that doesn't indicate delivery condition (as-rolled, normalized) cannot confirm that the normalizing requirement was met. For thick plate applications where normalization is required, an undocumented delivery condition means the PWHT package cannot demonstrate that the starting material microstructure was correct.
Both gaps are fixable — the mill can issue a supplemental cert. The time cost is measured in days or weeks depending on how far back the heat traceability goes. The earlier this is caught (at receiving rather than at final inspection), the less schedule impact it creates.