On this page 11 sections
Which chrome-moly steel should be used?
Short answer
Choose the Cr-Mo grade by temperature and hydrogen service: P1 and P11 for moderate steam temperatures, P22 for higher temperatures and hydrogen service, P5 and P9 for oxidation and sulphidation in refineries, and P91 for the highest creep strength in modern power plants.
Designation map
The same chemistry appears as P (pipe, A335), T (tube, A213), Grade (plate, A387), F (forgings, A182) and WP (fittings, A234). For example, Grade 91 = P91 / T91 / Gr 91 / F91 / WP91.
Choosing a grade
Fabrication
All Cr-Mo grades need preheat and PWHT, and P91 is especially demanding: correct normalise-and-temper, controlled interpass temperature, and hardness checks after PWHT are essential.
Cr-Mo designation map
| Chemistry | Pipe A335 | Tube A213 | Plate A387 | Forging A182 | Fitting A234 |
|---|---|---|---|---|---|
| C-½Mo | P1 | T1 | — | F1 | WP1 |
| 1¼Cr-½Mo | P11 | T11 | Gr 11 | F11 | WP11 |
| 2¼Cr-1Mo | P22 | T22 | Gr 22 | F22 | WP22 |
| 5Cr-½Mo | P5 | T5 | Gr 5 | F5 | WP5 |
| 9Cr-1Mo | P9 | T9 | Gr 9 | F9 | WP9 |
| 9Cr-1Mo-V | P91 | T91 | Gr 91 | F91 | WP91 |
What each element does
Molybdenum adds creep strength and resists temper embrittlement, chromium adds oxidation, sulphidation and hydrogen resistance, and vanadium and niobium (in P91) form fine carbonitrides for creep strength. Compare pairs in P11 vs P22 and P22 vs P91.
Ordering across product forms
The same chemistry is designated differently for pipe, tube, plate, forgings and fittings. Use the Cr-Mo designation chart so every item in a line matches.
Materials in this guide
Related guides, comparisons and articles
Frequently asked questions
Is P5 used in power plants?
Less often. P5 and P9 are mainly used in refinery heaters for sulphidation resistance.
Do Cr-Mo steels need PWHT?
Yes, generally, with requirements by grade and thickness in the design code.
Is P91 the same as T91?
Same chemistry, different product specification: P91 is pipe (A335), T91 is tube (A213).
How do I choose between grades?
Start from the service environment (chlorides, acids, temperature, pressure), then check the governing code and cost. Our comparisons and grade pages list typical choices, and our team can help shortlist grades for your conditions.
Does Bright Tubes supply the materials in this guide?
Yes. P91 Alloy Steel (Modified 9% chromium-1% molybdenum-vanadium), P11 Alloy Steel (1¼% chromium-½% molybdenum), P22 Alloy Steel (2¼% chromium-1% molybdenum), P5 Alloy Steel (5% chromium-½% molybdenum), P9 Alloy Steel (9% chromium-1% molybdenum) and related products are supplied from stock or mill-sourced, with EN 10204 certificates.