What is P91 steel?
P91 is a 9 % chromium, 1 % molybdenum steel with small, controlled additions of vanadium, niobium and nitrogen. It is also known as modified 9Cr-1Mo steel. After normalizing and tempering, its structure is tempered martensite with fine vanadium and niobium carbonitrides, called MX precipitates. These precipitates give the steel its creep strength at 550-600 °C.
Power plants use P91 instead of P22 for main steam and hot reheat lines. At the same pressure and temperature, a P91 pipe or header can have a much thinner wall. The part is lighter and has lower thermal stress during start-up and load changes.
Is P91 stainless steel?
No. P91 contains 8.0-9.5 % chromium, which is below the level of about 10.5 % normally needed for stainless steel. Some material databases list it with the martensitic stainless steels, but ASTM covers it in its ferritic alloy steel specifications, such as A335 for pipe and A336 for forgings.
P91 or F91 for a forging?
In ASTM, the letter in front of 91 shows the product form. P91 is seamless pipe to A335, T91 is tube to A213, F91 is a forging to A182 or A336, FP91 is forged and bored pipe to A369, and WP91 is a butt-weld fitting to A234. The chemical ranges are almost the same.
Many drawings still call for P91 on forged parts, and ASTM A336 allows this. Paragraph 1.2 of A336 permits forgings in steel grades listed in A335 to be ordered under A336, using the A335 chemical, tensile, heat treatment and marking requirements. This is what SA-336 P91 means on a drawing. If a drawing only says "P91 forging", please confirm the specification, because A182 F91, A336 F91 and A335 P91 have different tensile and hardness limits.
| Product form | Standard | Grade | European equivalent |
|---|---|---|---|
| Forgings for pressure and high-temperature parts | ASTM A336 / ASME SA-336 | F91 | EN 10222-2, X10CrMoVNb9-1 |
| Forged flanges, fittings, valves and parts | ASTM A182 / ASME SA-182 | F91 Type 1 or Type 2 | EN 10222-2, X10CrMoVNb9-1 |
| Seamless pipe | ASTM A335 / ASME SA-335 | P91 Type 1 or Type 2 | EN 10216-2, X10CrMoVNb9-1 |
| Forged and bored pipe | ASTM A369 / ASME SA-369 | FP91 | - |
| Seamless boiler and superheater tube | ASTM A213 / ASME SA-213 | T91 | EN 10216-2, X10CrMoVNb9-1 |
| Butt-weld fittings | ASTM A234 / ASME SA-234 | WP91 | - |
| Plate | ASTM A387 / ASME SA-387 | Grade 91 | EN 10028-2, X10CrMoVNb9-1 |
| Castings | ASTM A217 / ASME SA-217 | C12A | - |
| Boiler tube and pipe, China | GB/T 5310 | 10Cr9Mo1VNbN | - |
This steel is also listed on our site as F91, 1.4903 and X10CrMoVNb9-1.
P91 chemical composition
The creep strength of Grade 91 depends on vanadium, niobium and nitrogen staying within narrow ranges, since these elements form the MX precipitates. Aluminium is limited because it combines with nitrogen to form aluminium nitride. European forgings are ordered to EN 10222-2 under the name X10CrMoVNb9-1.
| Element | A182 F91 and A335 P91, Type 1 | A336 F91 | EN 10216-2 X10CrMoVNb9-1 | Effect |
|---|---|---|---|---|
| Carbon (C) | 0.08-0.12 | 0.08-0.12 | 0.08-0.12 | Hardenability, carbide formation |
| Manganese (Mn) | 0.30-0.60 | 0.30-0.60 | 0.30-0.60 | Deoxidizer; lowers Ac1 together with Ni |
| Phosphorus (P), max | 0.020 | 0.025 | 0.020 | Impurity |
| Sulfur (S), max | 0.010 | 0.025 | 0.010 | Impurity |
| Silicon (Si) | 0.20-0.50 | 0.20-0.50 | 0.20-0.50 | Deoxidizer, oxidation resistance |
| Nickel (Ni), max | 0.40 | 0.40 | 0.40 | Limited because it lowers Ac1 |
| Chromium (Cr) | 8.00-9.50 | 8.0-9.5 | 8.00-9.50 | Steam oxidation resistance |
| Molybdenum (Mo) | 0.85-1.05 | 0.85-1.05 | 0.85-1.05 | Creep strength |
| Vanadium (V) | 0.18-0.25 | 0.18-0.25 | 0.18-0.25 | Forms MX carbonitrides |
| Niobium (Nb) | 0.06-0.10 | 0.06-0.10 | 0.06-0.10 | Forms MX carbonitrides, limits grain growth |
| Nitrogen (N) | 0.030-0.070 | 0.03-0.07 | 0.030-0.070 | Needed for V and Nb carbonitrides |
| Aluminium (Al), max | 0.02 | 0.02 | 0.040 | Limited because it forms AlN with nitrogen |
| Titanium (Ti), max | 0.01 | 0.01 | - | Residual element |
| Zirconium (Zr), max | 0.01 | 0.01 | - | Residual element |
| Copper (Cu), max | - | - | 0.30 | Residual element |
Type 2 material to A182 or A335 has tighter limits on several elements, including Mn, S, Si and N, and adds limits for tin and antimony. Phosphorus and sulfur limits for A336 can differ between editions. Please check the edition and type on your purchase order.
P91 mechanical properties
Room temperature requirements are not the same in every standard. The main differences are the tensile range, the hardness range and whether impact tests are required.
| Property | ASTM A336 F91 | ASTM A182 F91 | ASTM A335 P91 | EN 10222-2, tR ≤ 130 mm | EN 10216-2, T ≤ 100 mm |
|---|---|---|---|---|---|
| Tensile strength | 620-760 MPa 90-110 ksi | 620 MPa 90 ksi | 585 MPa 85 ksi | 630-830 MPa | 630-830 MPa |
| Yield strength | 415 MPa 60 ksi | 415 MPa 60 ksi | 415 MPa 60 ksi | 450 MPa (ReH) | 450 MPa (ReH) |
| Elongation | 20 % | 20 % | Varies with wall thickness | 19 % (l) 17 % (t) | 19 % (l) 17 % (t) |
| Reduction of area | 40 % | 40 % | - | - | - |
| Hardness | - | 190-248 HBW | 190-250 HBW | - | - |
| Impact energy KV, room temperature | - | - | - | 40 J (l) | 40 J (l) 27 J (t) |
l = longitudinal, t = transverse. The EN standards specify ReH; Rp0.2 may be used instead with a slightly lower minimum. For A336 F91, transverse tests require 19 % elongation and 40 % reduction of area.
P91 heat treatment
ASTM A182, A335 and A336 require P91 / F91 to be normalized at 1040-1080 °C and tempered at 730-800 °C. Normalizing dissolves the carbides and forms martensite on cooling. Tempering then produces tempered martensite with fine MX precipitates. For F91 forgings with any section thicker than 75 mm, A336 also allows liquid quenching and tempering, at the manufacturer's option.
Hardness testing is a quick check of the heat treatment. ASTM A182 requires 190-248 HBW for F91 forgings, and ASTM A335 requires 190-250 HBW for P91 pipe. Low hardness is usually caused by over-tempering or incorrect normalizing. High hardness is usually caused by under-tempering. A part can pass the tensile test and still have poor creep strength or toughness, so the standards set both a minimum and a maximum hardness.
P91 forgings we make
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory at No.1 Chengxiqiao Road, Zhouzhuang Town, Jiangyin City, Jiangsu Province, China. We forge Grade 91 parts for P91 piping and pressure equipment, mostly in sizes or quantities that do not suit closed-die forging.
| Product form | Typical parts | Usual specification |
|---|---|---|
| Hollow bars, sleeves and forged pipe | Header and pipe sections, barrels, shells, casings, hubs, housings, bushings | A336 F91; A369 FP91 for forged and bored pipe |
| Seamless rolled rings | Flange blanks, valve seat rings, ring sections for pressure parts | A182 F91 for flanges; A336 F91 for pressure parts |
| Discs and blocks | Blind flanges, tube sheets, end caps, valve body blocks | A182 F91 for flanges; A336 F91 for pressure parts |
| Round, square, flat and hex bars | Valve stems, shafts, machined fittings | A182 F91 or A336 F91 |
| Valve parts | Bodies, bonnets, stems, seats, seat rings | A182 F91 |
| Branch fittings | Equal and eccentric tees, laterals, Y-pieces, piggable wyes | A182 F91 |
| Item | Range | Note |
|---|---|---|
| Diameter | 100-6000 mm | All grades |
| Length | 100-12000 mm | Shafts and bars |
| Unit weight | 10-15000 kg | Per piece |
| Hydraulic press | 4500 t | Open-die press |
| Forging hammers | 1 t, 3 t, 6 t and 9 t | Smaller sections |
| Ring rolling mills | 3 m and 6 m | Seamless rolled rings |
| Quality system | ISO 9001:2015 | - |
| Certificates | EN 10204 3.1 / 3.2 | 3.2 with third-party inspection on request |
These figures apply to the whole plant. For P91, the size limit of a part also depends on our heat treatment furnaces and on the section thickness that can be fully hardened, so we confirm it for each enquiry. Parts are supplied normalized and tempered, rough machined for UT, or finish machined to drawing.
See also: seamless rolled rings, forged sleeves, forged discs, forged tube sheets and forged shafts.
How we produce P91 forgings
Our production steps for Grade 91 forgings follow ASTM A182 and A336.
Material check
We check the heat analysis against the order, including V, Nb, N and Al, and confirm the grade of incoming billets by PMI.
Heating and forging
Billets are heated through and forged by upsetting and drawing. Rings and hollow parts are punched, then ring rolled or forged on a mandrel.
Cooling after forging
ASTM A182 requires forgings to cool below 538 °C (1000 °F) before heat treatment.
Normalizing
Heating to 1040-1080 °C, then cooling. For sections over 75 mm, A336 permits liquid quenching.
Tempering
Tempering at 730-800 °C.
Machining and UT
Rough machining, then ultrasonic testing to ASTM A388. For F91 hollow forgings, A336 requires UT to A788 supplementary requirement S20 or S22.
Testing
Tensile, elongation, reduction of area and hardness tests to the order specification. Impact tests are added when the order or EN 10222-2 requires them.
Certificates
Test results, heat treatment records and NDT reports are given on an EN 10204 3.1 certificate, or on a 3.2 certificate when a third-party inspector witnesses the tests.
P91 compared with F22, F9 and F92
The table shows ASTM A182 limits for F91 and the forging grades it is most often compared with.
| Property | F22 Class 1 | F9 | F91 | F92 |
|---|---|---|---|---|
| Chromium, % | 2.00-2.50 | 8.0-10.0 | 8.0-9.5 | 8.50-9.50 |
| Molybdenum, % | 0.87-1.13 | 0.90-1.10 | 0.85-1.05 | 0.30-0.60 |
| Other alloying elements | - | - | V, Nb, N | W 1.50-2.00, V, Nb, N, B |
| Tensile strength, min | 415 MPa | 585 MPa | 620 MPa | 620 MPa |
| Yield strength, min | 205 MPa | 380 MPa | 415 MPa | 440 MPa |
F9 has almost the same chromium and molybdenum as F91 but no vanadium, niobium or nitrogen additions, and its minimum yield strength is 35 MPa lower. F92 has less molybdenum and 1.50-2.00 % tungsten, which gives higher creep strength. It is used for the hottest steam lines in ultra-supercritical plants. See P92.
P91 forging applications
| Industry | Service | Forged parts |
|---|---|---|
| Supercritical and ultra-supercritical power plants | Main steam and hot reheat piping, superheater and reheater headers | Tees, laterals, wyes, header forgings, valve bodies |
| Combined-cycle power plants (HRSG) | High-pressure superheater and reheater headers, steam lines | Hollow forgings, nozzles, flanges |
| Steam valves | Isolation, control and bypass valves | Bodies, bonnets, stems, seat rings |
| Refineries and petrochemical plants | High-temperature process piping | Flanges, rings, fittings |
How to order P91 forgings
Please include the following in your enquiry.
| Item | Note |
|---|---|
| Drawing or dimensions | Sets the forging weight and the section to be heat treated |
| Specification and edition | A336 F91, A182 F91 and A335 P91 have different tensile and hardness limits |
| Type 1 or Type 2 | Type 2 has tighter chemistry limits |
| Quantity and delivery date | Affects heat treatment batches and lead time |
| Supply condition | Normalized and tempered, rough machined or finish machined |
| Tests and certificate | UT acceptance level, impact tests, hardness, PMI, third-party inspection, EN 10204 3.1 or 3.2 |
| Service temperature and welding | Helps confirm the grade and the PWHT |
P91 forging FAQ
What is P91 steel used for?
P91 is used for high-temperature, high-pressure steam: main steam and hot reheat piping, superheater and reheater headers, HRSGs and steam valves in power plants, and high-temperature piping in refineries and petrochemical plants.
Is F91 the same as P91?
They are the same steel grade in different product forms. P91 is seamless pipe to ASTM A335, and F91 is a forging to ASTM A182 or A336. The chemistry is almost the same, but the property limits differ. A182 F91 requires at least 620 MPa tensile strength and 190-248 HBW. A335 P91 requires at least 585 MPa and 190-250 HBW.
What does SA-336 P91 mean?
It means a forging ordered to ASTM A336 / ASME SA-336 in grade P91. Paragraph 1.2 of A336 allows forgings in A335 grades to be ordered under A336, using the A335 chemical, tensile, heat treatment and marking requirements.
What is the European equivalent of P91?
The European equivalent is X10CrMoVNb9-1, steel number 1.4903. It is covered by EN 10216-2 for seamless tubes, EN 10222-2 for forgings and EN 10028-2 for plate. The Chinese tube and pipe grade is 10Cr9Mo1VNbN to GB/T 5310. F91 is UNS K90901, and A335 P91 is often listed as UNS K91560.
What heat treatment is required for P91 forgings?
Normalizing at 1040-1080 °C and tempering at 730-800 °C, as required by ASTM A182, A335 and A336. For F91 forgings with a section over 75 mm, A336 also allows liquid quenching and tempering. Over-tempering, or tempering too close to Ac1, reduces hardness and creep strength.
What is the hardness of P91?
ASTM A182 requires 190-248 HBW for F91 forgings. ASTM A335 requires 190-250 HBW for P91 pipe. Low hardness usually means over-tempering or incorrect normalizing, and high hardness usually means under-tempering.
What is the difference between P91 Type 1 and Type 2?
Type 2 has tighter chemistry limits than Type 1, including manganese, sulfur, silicon and nitrogen, plus limits on tramp elements such as tin and antimony. The aim is more consistent creep properties. The type must be stated on the order.
What is the maximum service temperature of P91?
P91 is normally used in steam service up to about 600 °C. ASME design data for Grade 91 go up to 650 °C (1200 °F). The limit for a given part depends on creep strength, steam oxidation and design life under the applicable code.
Does P91 need PWHT after welding?
Yes. Grade 91 is ASME Section IX P-No. 15E Group 1. Welding normally needs preheat, interpass temperature control and PWHT, typically around 730-775 °C, with the holding time set by the code. Nickel and manganese lower Ac1, so codes limit the PWHT temperature when the Ni + Mn content of the filler metal is high.
Who makes P91 forgings in China?
Jiangyin Jiangnan Metal Co., Ltd. is an open-die forging factory in Jiangyin, Jiangsu Province, China. We make P91 / F91 hollow bars, seamless rolled rings, discs, flanges, fittings and valve parts to ASTM A336 and ASTM A182. Contact sales@steelforgepieces.com or +86 189 2135 9659.
What certificate do you supply with P91 forgings?
We supply an EN 10204 3.1 certificate with chemical analysis, heat treatment records, tensile and hardness results and UT results. An EN 10204 3.2 certificate is available when a third-party inspector or your representative witnesses the inspection.
Standards referenced
- ASTM A336/A336M, Alloy Steel Forgings for Pressure and High-Temperature Parts
- ASTM A182/A182M, Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service
- ASTM A335/A335M, Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service
- ASTM A369/A369M, Carbon and Ferritic Alloy Steel Forged and Bored Pipe for High-Temperature Service
- ASTM A388/A388M, Ultrasonic Examination of Steel Forgings
- ASTM A788/A788M, Steel Forgings, General Requirements
- EN 10222-2, Steel forgings for pressure purposes, Part 2: Ferritic and martensitic steels with specified elevated temperature properties
- EN 10216-2, Seamless steel tubes for pressure purposes, Part 2: Non-alloy and alloy steel tubes with specified elevated temperature properties
- EN 10204, Metallic products, Types of inspection documents
- ASME BPVC Section IX, Welding, Brazing, and Fusing Qualifications
Values on this page are taken from the standards above. The purchase specification always takes precedence.