When a specified grade is unavailable or slow to source, a supplier may offer an alternative. Accepting it without a documented review can leave the plant with material that does not meet the design code – and with no record of who approved it. A short, structured substitution request avoids both.
When a Substitution Request Is Needed
- A different grade (for example 316 offered for 316L, or WP11 offered for WP12).
- A different product standard or edition.
- A different form or manufacturing route (welded instead of seamless, plate instead of forging).
- A heavier wall or higher class than specified.
- Dual-certified material offered against a single-grade requirement.
What the Request Should Contain
| Section | Content |
|---|---|
| Original requirement | Line item, specified standard, grade, form, size and wall/class |
| Proposed alternative | Exact standard, grade, form, size and wall/class offered |
| Comparison | Chemistry, mechanical properties, heat treatment and corrosion behaviour side by side |
| Code check | Whether the alternative is listed and allowed by the design code at the design temperature and pressure |
| Impact | Effect on welding procedures, NDT, PWHT, mating components and delivery |
| Evidence | Supplier certificates or data sheets for the offered material |
| Approval | Engineering sign-off, procurement acceptance and client approval where required |
Approval and Records
Engineering decides whether the alternative is technically acceptable; procurement should only accept it after that decision is recorded. Keep the approved request with the purchase order and update the line item, so receiving inspection checks the delivered material against the approved alternative, not the original specification.
Related: PMI, MTC and heat traceability and inspection and test plan for fittings.
Frequently Asked Questions
Can 316 be substituted for 316L?
Only if engineering confirms it is acceptable for the service and code. The difference in carbon content affects weldability and sensitisation. Dual-certified 316/316L material is often used to meet both.
Who should approve a material substitution?
Engineering should approve the technical case, procurement should record acceptance, and the client should approve it where the contract requires.
Is a heavier wall always an acceptable substitute?
Not automatically. A heavier wall can affect weld fit-up, mating components, weight and stress analysis, so it should go through the same review.
