The marks on the rim of a flange are not random. The standard defines what has to be stamped, and reading those lines tells you almost everything about the part.
What the stamps say
| Mark | Meaning | Example |
|---|---|---|
| Manufacturer mark | Maker's name or registered abbreviation | SDY, ABC |
| Material grade | ASTM grade | A105, F316/F316L, F51 |
| Pressure class | Class or PN value | 300, 150, PN16 |
| Standard | Dimensional standard | B16.5, EN 1092-1 |
| Nominal size | Inches or DN | 4", DN100 |
| Heat number | Traceability of the melt | H260815 |
| Extra marks | Special service or inspection | NACE, PMI, X |
A dual grade stamp such as F316/F316L means the part meets the chemical and mechanical requirements of both grades, which is accepted practice on most projects.
Why the heat number matters
The heat number identifies the melt the part came from. If the number on the certificate matches the number on the part, traceability is established. It is the first thing inspection checks; if they do not match, the certificate does not belong to that part.
How the marks are applied
Parts made for low temperature and sour service are usually marked with low stress stamps or electrolytically. Hard stamping can create stress concentration and is avoided in those services.
When the marking is gone
Paint, rust or wear may have erased the stamp. The part is then identified by measurement: outside diameter, bolt circle, bolt count and thickness match a single line in the standard tables, and PMI is needed for the material. See how to identify an unmarked flange for the full method.
An undocumented part should not go into a critical line; even with dimensions confirmed, inspection rejects it without traceability.
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FAQ
What does "F316/F316L" mean on a flange?
The part meets both F316 and F316L requirements; the lower carbon content preserves corrosion resistance after welding.
Why is the heat number asked for?
It links the part to its material certificate and is the first checkpoint of inspection on projects that require traceability.
Can a flange with erased markings be used?
Not in critical service. The class can be confirmed by measurement, but a part without documented material and traceability is rejected.
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