Grade selection
What is the pipe equivalent of SA387 Grade 22 plate?
A335 P22 is the seamless pipe equivalent of SA387 Grade 22 Class 2 plate. Both are 2.25Cr-1Mo, normalised and tempered, for elevated temperature and hydrogen service. The matching fittings are A234 WP22 and the matching flanges are A182 F22, so a complete 2.25Cr-1Mo circuit can be built from one chemistry.
Cr-Mo alloy steels are written into two different product standards depending on form, which is why the same alloy appears under two names:
- Plate — ASTM/ASME A387 / SA387, by grade and class (Grade 11, 12, 22, 5, 9, 91; Class 2 is the higher strength heat treatment).
- Seamless pipe — ASTM/ASME A335 / SA335, by P number (P11, P12, P22, P5, P9, P91).
- Fittings — A234 WP11 / WP22 / WP91. Flanges and forgings — A182 F11 / F22 / F91.
The pairings run straight across: SA387 Gr 11 with P11 (1.25Cr-0.5Mo-Si), Gr 12 with P12 (1Cr-0.5Mo), Gr 22 with P22 (2.25Cr-1Mo), Gr 5 with P5 (5Cr-0.5Mo), Gr 9 with P9 (9Cr-1Mo) and Gr 91 with P91 (modified 9Cr-1Mo-V).
Note that the mechanical minima differ between plate and pipe even at the same chemistry — SA387 Gr 22 Cl 2 is 310 MPa yield minimum, A335 P22 is 205 MPa — because the product forms are rolled and heat treated differently. Design to the standard for the form you are buying, not to the chemistry.
For service above about 550 to 580 C, Grade 91 / P91 carries roughly twice the creep rupture strength of Grade 22 / P22, which is what allows the thinner walls used on supercritical steam lines. P91 is also far less forgiving on heat treatment: hardness after PWHT must be controlled and recorded.
Grades referenced
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