Alloy steel · 9Cr-1Mo-V · creep-strength-enhanced ferritic
P91: the 9-chrome steel that lives or dies by heat treatment
In short: P91 (T91 for tube, Grade 91 for plate, F91 for forgings) is a modified 9% chromium, 1% molybdenum steel with vanadium, niobium and nitrogen. Normalised and tempered, it forms a tempered-martensite structure with much higher creep strength than P22, which is why it is the standard choice for supercritical steam piping, headers and HRSG components.
Overview
P91 in brief
- Creep-strength-enhanced ferritic (CSEF) steel
- Much higher creep strength than P22: thinner walls
- Strict normalise-and-temper and PWHT control
- Type 1 / Type 2 designations in recent A335/A213 editions
Grade 91 is a creep-strength-enhanced ferritic (CSEF) steel. Its tempered-martensite microstructure, strengthened by fine vanadium and niobium carbonitrides, allows much thinner walls than P22 at the same temperature and pressure. That reduces weight and thermal stresses in main steam and hot-reheat lines, headers and HRSG components.
Its properties depend entirely on correct heat treatment: normalising, then tempering at about 730–800 °C. Fabrication needs strict control of preheat, interpass temperature and PWHT, and hardness testing of welds. Recent editions of A335/A213 split P91/T91 into Type 1 and Type 2 with tighter residual-element limits, so specify the type required.
Available as
P91 product forms
Select a form for its specification, sizes and weights.
Where it's used
Industries that specify P91
Microstructure
Strength comes from the process, not just the chemistry
Two pipes with identical chemistry can have very different creep lives if one was tempered wrongly. That is why P91 orders and site checks focus on heat-treatment records, hardness and PMI.
Chemistry
Composition of P91
- Density
- 7.77 g/cm³
- Type
- 9Cr-1Mo-V (CSEF)
| Element | Limit, % | Why it matters |
|---|---|---|
| C | 0.08 – 0.12 | |
| Mn | 0.30 – 0.60 | |
| P | 0.020 max | |
| S | 0.010 max | |
| Si | 0.20 – 0.50 | |
| Cr | 8.00 – 9.50 | 8–9.5% |
| Mo | 0.85 – 1.05 | 0.85–1.05% |
| V | 0.18 – 0.25 | 0.18–0.25%: carbonitride strengthening |
| Nb | 0.06 – 0.10 | 0.06–0.10%: carbonitride strengthening |
| N | 0.030 – 0.070 | Combines with V and Nb |
| Ni | 0.40 max | |
| Al | 0.02 max |
Limits per ASTM A335 (P91); other product forms and specifications differ slightly. Your order's specification and edition govern.
Mechanical properties
Room-temperature minimums
ASTM A335 (P91): room-temperature specification values for the annealed condition.
Supply
Grade 91 by product form
Each designation links to sizes, weights and supply details.
| Designation | Product |
|---|---|
| ASTM A335 P91 | Seamless pipe |
| ASTM A213 T91 | Boiler and superheater tube |
| ASTM A387 Grade 91 | Pressure-vessel plate |
| ASTM A234 WP91 | Butt-weld fittings |
| ASTM A182 F91 | Forged flanges and fittings |
- Designation per form: A335 P91, A213 T91, A387 Gr 91, A182 F91, A234 WP91
- Type 1 or Type 2
- Normalise and temper temperatures on the certificate
- Hardness results within the specification limits
- IBR certification for Indian boilers
Ordering
What to put on an order for P91
Getting these on the enquiry avoids the usual back-and-forth and makes sure the certificate matches your specification.
P91 questions
Answers about P91 / T91
What is P91 steel?
A creep-strength-enhanced ferritic steel: about 9% chromium and 1% molybdenum, with vanadium, niobium and nitrogen. Normalised and tempered, it has much higher creep strength than P22, allowing thinner walls in main-steam and hot-reheat piping.
What heat treatment does P91 get?
Normalising at about 1040–1080 °C, then tempering at about 730–800 °C, per ASTM A335.[1] Its strength depends on getting both right; a wrong temper can leave pipe that looks fine on a tensile test but creeps early.
What are P91 Type 1 and Type 2?
Recent editions of A335 and A213 split P91/T91 into Type 1 and Type 2, with Type 2 adding tighter limits on residual elements. Specify the type your design code or owner requires.
Is there a hardness requirement for P91?
Yes. Current A335 editions set hardness limits for P91 as well as tensile properties; commissioning teams check hardness on site, together with PMI, to catch heat-treatment errors.[1] See P91 fabrication pitfalls.
P91 or P22?
P22 is easier to fabricate and fine at lower steam conditions; P91's higher creep strength wins for supercritical and high-temperature lines. Compare P22 vs P91.
Sources
References
Heat-treatment ranges per ASTM A335; confirm Type 1 / Type 2 and limits against the edition on your order.
Need P91 or T91?
Send size, schedule, quantity and Type 1 or Type 2. IBR certification available where required.