Article
As temperature rises, carbon steel loses strength and, over long periods, slowly deforms under load, a process called creep. It also oxidises faster. Above roughly 400 to 425 °C, chromium-molybdenum alloy steels take over. ASTM A335 covers seamless ferritic alloy steel pipe for high-temperature service, and it is the standard behind most boiler, steam and refinery heater piping.
The common grades
| Grade | UNS | Nominal composition | Typical use |
|---|---|---|---|
| P11 | K11597 | 1.25% Cr, 0.5% Mo | Steam lines and refinery service at moderate high temperature |
| P22 | K21590 | 2.25% Cr, 1% Mo | Higher temperatures, hydrogen service, headers |
| P5 | K41545 | 5% Cr, 0.5% Mo | Refinery heaters and sulphur-bearing service |
| P9 | K90941 | 9% Cr, 1% Mo | Heater tubes, corrosive high-temperature service |
| P91 | K91560 | 9% Cr, 1% Mo, with vanadium and niobium | Main steam and hot reheat lines in modern power plants |
What each alloying element does
- Chromium improves resistance to oxidation and to corrosion by sulphur compounds at high temperature.
- Molybdenum improves creep strength.
- In P91, small additions of vanadium, niobium and nitrogen give much higher creep strength, which lets thinner walls be used in high-pressure steam lines.
Heat treatment matters
These grades depend on correct heat treatment for their properties. A335 pipe is supplied normalised and tempered (or annealed for some grades), and welds generally need preheat and post-weld heat treatment. P91 is particularly sensitive: its strength relies on a tempered martensitic structure, and poor control of welding or heat treatment can leave a joint with a fraction of the intended creep life. Hardness checks on P91 welds are common for this reason.
Matching fittings and flanges
- Buttweld fittings: ASTM A234 WP11, WP22, WP91.
- Forged flanges and fittings: ASTM A182 F11, F22, F91.
- Plate: ASTM A387 Grades 11, 22 and 91.
What to put on the enquiry
The grade, the size and schedule, the heat treatment condition, and any additional testing such as hardness or PMI. See why PMI matters for alloy steel, and browse pipes and tubes.
Frequently asked questions
What is ASTM A335 used for?
Seamless ferritic alloy steel pipe for high-temperature service, mainly in power plants and refineries.
Why is P91 so widely used?
Its high creep strength allows thinner walls at high temperature, reducing weight and thermal stress.
Does P91 need special welding?
Yes. P91 requires controlled preheat, interpass temperature and post-weld heat treatment.
Key takeaways
- A335 covers seamless Cr-Mo pipe for high temperature.
- P11 and P22 are common; P91 offers higher creep strength.
- Cr-Mo grades need careful welding and PWHT.
Need these products?
Sawariya Metal Industries supplies stainless steel pipes, flanges and buttweld pipe fittings from Mumbai to buyers in India and overseas. Send your size, grade, standard and quantity through our enquiry form and we will reply with a clear quotation and the test documents you need. You can also browse our stainless steel pipes, flanges and buttweld fittings.