The project was a subsea manifold for an oil field operator. The customer's purchase order ran 22 pages. Pages 14 through 25 contained supplementary material requirements: maximum carbon equivalent of 0.43, mandatory Charpy impact testing at -46°C with a minimum absorbed energy of 27J, normalized delivery condition, and MTC certification to ASME Section II SA-516 with specific supplementary requirements S1 and S5 invoked.
The fabricator received the PO, acknowledged it, and placed a purchase order with their plate service center. That PO said: "SA-516 Grade 70, as per customer requirements." No supplementary requirements listed. No mention of Charpy testing. No carbon equivalent limit. No delivery condition specified.
The service center sourced the plate from a mill that had it in stock. Standard SA-516-70, quenched and tempered, carbon equivalent of 0.47. No Charpy testing performed. No S1, no S5.
The MTC was real, accurate, and completely wrong for the job.
Why the Gap Exists
Most distributors and service centers process orders against stated product spec and grade. If your PO says SA-516-70, they find SA-516-70. The supplementary requirements in the customer's 12-page document existed in the fabricator's files and nowhere else in the supply chain.
This is the spec relay problem. The information exists at the top of the chain — the customer has been explicit about what they need. But the mechanism for transmitting those requirements downstream to the mill doesn't exist in most mid-size fabrication and distribution operations. The specifications travel as PDF attachments to emails, get acknowledged with a signature, and then sit in a folder while the actual procurement proceeds on abbreviated PO descriptions.
The consequences appear at delivery or inspection, not at order placement.
What Supplementary Requirements Actually Look Like on an MTC
ASTM and ASME product standards include optional supplementary requirements that buyers can invoke. For pressure vessel plate under ASTM A20, those include:
- S1: Vacuum treatment — reduces hydrogen content in the plate
- S2: Additional tension tests — testing from each end of each plate rather than one test per heat
- S4: Charpy V-notch impact testing — specifies test temperature and energy requirements
- S5: Ultrasonic examination — per the referenced UT standard (ASTM A578)
- S7, S8: Additional chemical requirements including carbon equivalent formulas
When a customer invokes these supplementary requirements, the resulting MTC must document the test results for each one. An MTC that doesn't reference these requirements doesn't prove they were met — it proves they weren't performed.
Beyond ASTM supplementary requirements, customers often impose their own requirements: proprietary carbon equivalent formulas, minimum and maximum chemistry windows tighter than the standard, specific normalizing temperature ranges, documentation of cooling rates. These don't appear in any published ASTM table. They live in the customer's procurement specification and nowhere else unless you explicitly pass them to the mill.
Building a Spec Relay Process
A functioning spec relay process has three checkpoints:
Checkpoint 1: PO receipt and supplementary spec extraction. When a customer PO arrives with supplementary requirements — whether in the PO body, an attached spec, or a referenced document — those requirements need to be extracted into a structured format. A simple checklist works: product spec, grade, delivery condition, chemistry limits (standard + any tightened ranges), mechanical property requirements (standard + any tightened values), impact testing (temp, absorbed energy, transverse vs. longitudinal), NDT requirements, heat treatment requirements, documentation requirements (including which supplementary requirements must appear on the MTC). This isn't a lengthy process. It takes 20–30 minutes for a typical PO. The point is to make the requirements explicit and structured rather than buried in paragraph 14 of a 22-page document.
Checkpoint 2: Supplier PO generation with explicit requirements. The supplier PO must list the requirements in full — not by reference. "As per customer PO" is not sufficient. The mill or service center that receives your PO may be sourcing from stock or placing a mill order. They need the specific requirements stated so they can confirm available material or order to spec. Every supplementary requirement that needs to appear on the MTC must be on your PO to the supplier.
Checkpoint 3: MTC review against the extraction checklist. When the MTC arrives, it should be compared against the extraction checklist from Checkpoint 1. Each required element should be present and within the required limits. If a supplementary requirement was invoked, the corresponding test results should appear on the cert. If they don't, the material needs to be quarantined and the gap resolved before it enters the shop.
What Happens When You Skip It
In the subsea manifold case, the fabricator received the plate, cut it, and began fit-up before anyone compared the MTC to the customer's supplementary spec. When the customer's inspector arrived and reviewed the cert package, the absence of Charpy test results and the carbon equivalent overage created an immediate hold.
The fabricator had to reorder compliant material — 8-week lead time for normalized SA-516-70 with S4 and S5. The plate already cut was scrapped. The project slipped past its critical path.
The customer invoked the late delivery penalty clause.
All of that started with a PO that said "as per customer requirements" rather than listing the requirements.