Pull a flange off the rack and look at the outer edge. On a properly made one you will find a cluster of letters and numbers stamped into the steel: a logo, a few abbreviations, a long number that looks like a serial code. Most people in the field can read about half of it on instinct and guess at the rest. That guessing is where trouble starts, because every one of those marks is telling you something specific about what you are holding.
This is the companion question to a piece we ran recently. If your flange has no stamp at all, start with our field guide to identifying unmarked flanges. This post is the opposite situation: the marks are right there, and you want to know exactly what they mean. Here is how to read flange markings, line by line.
What the Standard Actually Requires
Flange marking is not left to the manufacturer's preference. ASME B16.5 ties its marking provisions to MSS SP-25, the Standard Marking System for Valves, Fittings, Flanges, and Unions. That standard spells out what has to appear on the part and roughly where it goes.
The marks you should expect to find on any legitimate forged flange are the manufacturer's name or trademark, the material designation, the pressure class, and a heat number for traceability. On larger and special-service flanges you will see more, but those four are the backbone. They almost always sit on the outside diameter, on the face opposite the raised face. On weld neck flanges they occasionally land on the hub instead.
If a flange is missing all of these, that is a red flag worth taking seriously. A blank flange has no traceable history, and no MTR to tie it back to. Decode what is present first, then decide whether what is missing is a dealbreaker for your service.
Decoding the Material Marking
The material mark is the one people misread most often, and it is the one that matters most. Start with the prefix.
You will see grades stamped as either "A105" or "SA-105." They are the same steel. ASME adopts ASTM material specs into Section II of its Boiler and Pressure Vessel Code and adds an "S" to the front. So SA-105 is chemically and mechanically identical to ASTM A105 forged carbon steel. The difference is paperwork and intent: SA-grade material is certified for ASME code pressure-vessel work, while the A-grade designation shows up on general process piping built to B31.1 or B31.3. If your job is code-stamped vessel work, that single letter is the difference between an acceptable part and a rejected one.
After the prefix comes the grade symbol. A few you will run into constantly:
A105 is forged carbon steel, the default for ambient-temperature process service. A182 covers the alloy and stainless forgings, and the grade follows as an F-number: F304, F316, F11, F22, F91. An "L" suffix means low carbon, so F316L is the low-carbon version of F316, which matters for corrosion resistance and welding. A350 LF2 is the low-temperature carbon grade, impact-tested for cold service. When you see "LF" anywhere on the stamp, think low temperature.
The grade tells you the steel. It does not tell you the heat treatment or the actual chemistry of that specific part. That is what the heat number is for.
Pressure Class, Size, and the Numbers
The pressure class is the easy one. ASME B16.5 flanges are marked with one of seven class designations: 150, 300, 400, 600, 900, 1500, or 2500. That number is the class, not a pressure in PSI. A Class 150 flange does not hold 150 PSI; its actual pressure rating depends on the material group and the temperature, which is why the class and the material grade have to be read together.
Nominal size is often stamped alongside the class, though not always, especially on smaller flanges where it is obvious. On reducing flanges you may see two sizes. On a flange with a non-standard bore, the bore may be called out separately. When the stamped size and your tape measure disagree, trust the tape and the dimensional tables. Our ASME B16.5 dimension reference lays out the OD, bolt circle, and bolt pattern for every size and class if you need to confirm.
The Marks That Trip People Up
Past the big four, a handful of marks carry real weight and get overlooked.
The heat number is the long alphanumeric string that looks like a serial number. It is not decorative. It ties the part back to a specific melt of steel and the Material Test Report that documents that melt's chemistry and mechanical properties. If you ever need to prove what a flange is made of, the heat number is the thread you pull. Lose it and you lose traceability.
On ring type joint flanges, you will see an "R" followed by a number, like R-37 or R-45. That is the ring groove number per ASME B16.20, and it tells you which RTJ gasket fits the groove. The ring number is tied to the size and class combination, so it is a useful cross-check: if the R-number does not match what the size and class should call for, something is off. We cover this in more depth in our guide to RTJ flanges.
Country of origin shows up on most imported flanges and increasingly on domestic ones too. On federally funded, municipal water, or military work, that mark feeds straight into Buy American, BABA, and AIS compliance, and an inspector will look for it. Domestic-melt-and-manufacture parts are marked to prove it.
For sour service, the relevant detail usually is not a single stamp but the grade and a hardness limit. NACE MR0175 work requires the flange body at or below 22 HRC, and that is verified by the MTR rather than read off the OD. When sour service is in play, the stamp gets you to the grade; the documentation does the rest.
Common Flange Stamp Elements, Decoded
Mark on the Flange | What It Tells You | Example |
Manufacturer name or logo | Who forged it; trademark for traceability | Stamped logo or initials |
Material spec and grade | The steel and its certification path | SA-105, A182 F316L, A350 LF2 |
"S" prefix (SA vs A) | ASME code material vs general ASTM piping | SA-105 (code) vs A105 |
Pressure class | ASME B16.5 class designation, not PSI | 150, 300, 600, 900, 1500, 2500 |
Heat number | Ties the part to its MTR and melt chemistry | Long alphanumeric string |
"R" plus number | RTJ ring groove number per ASME B16.20 | R-37, R-45 |
Country of origin | Domestic vs import; AIS and BABA compliance | USA, or country name |
The Bottom Line
Reading flange markings is not guesswork once you know what each mark is for. The manufacturer ID and heat number give you traceability, the spec and grade tell you the steel and its code path, the class and size place it in the dimensional tables, and the special marks handle ring fit, origin, and compliance. Decode all of it before a flange goes into service, and verify against the MTR when the application is critical.
If you are working through a pile of stamped flanges and something does not add up, or you need replacements with full documentation and the right marks the first time, get in touch with our team. We stock ASME B16.5 flanges across 50-plus grades and can confirm exactly what you are getting before it ships.
Texas Flange & Fitting Supply | 281-484-8325 | texasflange.com
