Article
Most stainless piping uses the low-carbon L grades because they weld well and resist intergranular corrosion. But at high temperature, low carbon becomes a weakness. That is where the H grades come in.
What the H means
The H grades have a controlled, higher carbon content and a coarser grain size. For 304H and 316H, carbon is held between 0.04% and 0.10%, and ASTM A312 requires a grain size of ASTM No. 7 or coarser. Both features increase strength at high temperature, particularly creep strength.
Comparison
| Grade | Carbon % | Main strength |
|---|---|---|
| 304L / 316L | 0.035 max (A312) | Weldability, corrosion resistance |
| 304 / 316 | 0.08 max | General purpose |
| 304H / 316H | 0.04–0.10 | Strength and creep resistance above about 500 °C |
Where H grades are used
- Boiler superheater and reheater piping
- Refinery heater tubes and transfer lines
- High-temperature process lines in petrochemical plants
- Exhaust and hot gas ducting
Trade-offs
Higher carbon makes H grades more susceptible to sensitisation if they sit in the 450–850 °C range for long periods, which can reduce resistance to intergranular corrosion in wet, corrosive conditions. For high-temperature service where aqueous corrosion is also a concern, stabilised grades such as 321H or 347H are often selected.
Dual certification
Some mills supply 304/304H or 316/316H dual-certified pipe, meeting both sets of requirements. This simplifies stock but should be confirmed on the test certificate, including grain size.
Ordering
Pipe 4" SCH 80S, ASTM A312 TP316H, seamless, grain size ASTM 7 or coarser, EN 10204 3.1
Frequently asked questions
What is the difference between 304 and 304H?
304H has a controlled carbon content of 0.04–0.10% and a coarser grain, giving better strength at high temperature.
Why not use 316L at high temperature?
Its low carbon gives lower allowable stress at high temperature; 316H is stronger in creep conditions.
Is grain size checked for H grades?
Yes. ASTM A312 requires H grades to have a grain size of ASTM No. 7 or coarser.
Key takeaways
- H grades have controlled higher carbon and coarser grain.
- Use them for strength above roughly 500 °C.
- For hot and wet corrosive service, consider 321H or 347H.
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