Contents
A335 Grade P91 Alloy Steel Seamless Pipe Guide
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As a leader in high-performance alloy solutions, SteelPro Group combines advanced engineering with stringent standards to deliver reliability under demanding conditions. This guide explores the unique properties, manufacturing specifications, and applications of P91 steel pipes.
What Is P91 Steel?
P91 steel is a high-temperature alloy steel under the ASTM A335 standard. It is a modified 9Cr-1Mo-V alloy with a martensitic-ferritic microstructure. This ensures strength, durability, and thermal stability at temperatures exceeding 1100°F (593°C). P91 offers excellent stress rupture resistance and thermal stability. It is ideal for high-pressure steam pipelines, boilers, heat exchangers, and superheater tubes.
A335 Grade P91 Steel Chemical Composition
| Element | P91 Steel Type 1 (wt%) | P91 Steel Type 2 (wt%) |
| Carbon (C) | 0.08 – 0.12 | 0.08 – 0.12 |
| Silicon (Si) | 0.20 – 0.50 | 0.20 – 0.40 |
| Manganese (Mn) | 0.30 – 0.60 | 0.30 – 0.50 |
| Phosphorus (P) | ≤ 0.02 | ≤ 0.02 |
| Sulfur (S) | ≤ 0.01 | ≤ 0.005 |
| Chromium (Cr) | 8.00 – 9.50 | 8.00 – 9.50 |
| Nickel (Ni) | ≤ 0.40 | ≤ 0.20 |
| Copper (Cu) | – | ≤ 0.10 |
| Molybdenum (Mo) | 0.85 – 1.05 | 0.85 – 1.05 |
| Arsenic (As) | – | ≤ 0.01 |
| Nitrogen (N) | 0.03 – 0.07 | 0.035 – 0.07 |
| Titanium (Ti) | ≤ 0.01 | ≤ 0.01 |
| Tungsten (W) | – | ≤ 0.05 |
| Vanadium (V) | 0.18 – 0.25 | 0.18 – 0.25 |
| Niobium (Nb) | 0.06 – 0.10 | 0.06 – 0.10 |
| Zirconium (Zr) | ≤ 0.01 | ≤ 0.01 |
| Boron (B) | – | ≤ 0.001 |
| Aluminum (Al) | ≤ 0.02 | ≤ 0.02 |
| Tin (Sn) | – | ≤ 0.01 |
| Antimony (Sb) | – | ≤ 0.003 |
- N/Al ≥ 4.0: Ensures high-temperature strength and controls AlN precipitation.
P91 Steel Mechanical Properties
| Property | Imperial Value | Metric Value |
| Tensile Strenth | ≥ 85 ksi | ≥ 585 Mpa |
| Yield Strenth | ≥ 60ksi | ≥ 415 Mpa |
P91 Steel Pipes Calculated Elongation
| Wall Thickness | Min. Elongation (2 in. or 50 mm), % | |
| in. | mm | Longitudinal |
| 5/16 | 8 | 20 |
| 9/32 | 7.2 | 19 |
| 1/4 | 6.4 | 18 |
| 7/32 | 5.6 | 17 |
| 3/16 | 4.8 | 16 |
| 5/32 | 4 | 15 |
| 7/8 | 3.2 | 14 |
| 3/32 | 2.4 | 13 |
| 1/18 | 1.6 | 12 |
P91 Steel Applications
- Power Plants – Boilers, steam lines, and reheat systems.
- Petrochemicals – Heat exchangers, refineries, and chemical plants.
- Oil & Gas – Pipelines, offshore platforms, and processing units.
- Industrial Heating – Furnaces and high-temperature systems.
- Structural & Fluid Transport – Supporting frameworks and fluid transmission.
P91 steel is valued for its strength, heat resistance, and durability in extreme conditions.
P91 Steel Specifications and Standards
P91 alloy steel is utilized across different forms of manufacturing, including pipes, pipe fittings, and steel plates. Our products correspond to the following ASTM standards:
| Material Type | Standard | Application |
| Seamless Pipe | ASTM A335 P91 / ASME SA335 P91 | High-temperature, high-pressure piping systems |
| Pipe Fittings | ASTM A234 P91 / ASME SA234 P91 | Elbows, tees, reducers, caps for piping systems |
| Forged Flanges & Fittings | ASTM A182 F91 / ASME SA182 F91 | Flanges, fittings, and valves for high-temperature use |
| Chrome Moly Steel Plate | ASTM A387 Grade 91 / ASME SA387 Grade 91 | Pressure vessels, boilers, high-temperature applications |
| Welded Chrome Moly Pipe | ASTM A691 91 / ASME SA691 91 | Welded pipes made from A387 plates for industrial use |
P91 Steel Pipe Dimensions
Our A335 P91 seamless alloy steel pipes are manufactured to meet high-temperature and high-pressure requirements. Below is a detailed specification of our P91 alloy steel pipes.
| Specification | Details |
| Pipe Sizes | ½” to 24” (DN25 to DN600) |
| Wall Thickness | SCH 40, SCH STD, SCH 80, SCH XS, SCH 160 |
| Length Options | 6 meters, 12 meters, or customized |
| Manufacturing Process | Seamless hot rolled |
| Surface Treatment | Polished, Varnished, 3LPE (Three-Layer Polyethylene), FBE (Fusion Bonded Epoxy) |
| End Finishes | PE (Plain End), BE (Beveled End) |
P91 Steel Pipe Dimensional Tolerances
Our A335 P91 seamless alloy steel pipes follow strict dimensional tolerances to guarantee precision and consistency in high-temperature and high-pressure applications.
Wall Thickness Tolerances
| Nominal Pipe Size (NPS) [DN] | Tolerance Over (+%) | Tolerance Under (-%) |
| 1/8” to 2 ½” [DN6 to DN65] (all t/D ratios) | +20% | -12.5% |
| Above 2 ½” [DN65] with t/D ≤ 5% | +22.5% | -0.125 |
| Above 2 ½” [DN65] with t/D > 5% | +15% | -0.125 |
Outside Diameter Tolerances
| NPS [DN] | Tolerance Over (+ in./mm) | Tolerance Under (- in./mm) |
| 1/8” to 1 ½” [DN6 to DN40] | 1/64” (0.015”) / 0.4 mm | 1/64” (0.015”) / 0.4 mm |
| Over 1 ½” to 4” [DN40 to DN100] | 1/32” (0.031”) / 0.79 mm | 1/32” (0.031”) / 0.79 mm |
| Over 4” to 8” [DN100 to DN200] | 1/16” (0.062”) / 1.59 mm | 1/32” (0.031”) / 0.79 mm |
| Over 8” to 12” [DN200 to DN300] | 3/32” (0.093”) / 2.38 mm | 1/32” (0.031”) / 0.79 mm |
| Over 12” [DN300+] | ±1% of the specified OD | ±1% of the specified OD |
Inside Diameter (ID) Tolerances
| Pipe Outer Diameter (OD) | Tolerance (+/- in.) | Tolerance (+/- mm) |
| ¼” to 1 ½” (6mm to 40mm) | ±0.016” | ±0.40 mm |
| 1 ½” to 4” (40mm to 100mm) | ±0.031” | ±0.79 mm |
| 4” to 8” (100mm to 200mm) | +1/16” (0.062”) / -1/32” (0.031”) | +1.59 mm / -0.79 mm |
| 8” to 12” (200mm to 300mm) | +3/32” (0.093”) / -1/32” (0.031”) | +2.38 mm / -0.79 mm |
| 12”+ (Over 300mm) | ±1% of specified OD | ±1% of specified OD |
P91 Steel Heat Treatment
At SteelPro Group, we specialize in high-quality ASTM A335 P91 seamless pipes and ensure precision heat treatment. The process involves normalizing and tempering or quenching and tempering.
Normalization and Tempering
- Heated to 1900-1975°F (1040-1080°C)
- Air-cooled to about 200°C
- Tempered at 1350-1470°F (730-800°C) to relieve stress and improve toughness
Quenching and Tempering
- Heated to 1900-1975°F (1040-1080°C)
- Rapidly cooled (quenched) to retain strength
- Tempered at 1350-1470°F (730-800°C) to enhance ductility
P91 Steel Welding
P91 alloy steel allows for at least a 2-to-1 wall thickness reduction, leading to:
- Lighter hanger loads
- Reduced welding time
- Lower filler metal consumption
For thick-wall P91 pipes, induction heating is preferred for preheating and inter-pass temperature control.
About Process
- Preheating: 200-300°C (392-572°F)—prevents cracking and thermal shock.
- Post-welding baking: Optional but improves weld toughness.
- Cooling: Pipes must cool below 100°C (212°F) before use to ensure proper microstructural transformation.
- Post-weld heat treatment (PWHT) is typically required for stress relief and microstructure refinement.
Upgrade to High-Performance P91 Steel
SteelPro Group delivers premium A335 Grade P91 alloy steel seamless pipes designed for maximum durability and efficiency. Whether you need custom sizes, expert welding recommendations, or guidance for your application, our team is ready to assist.
Get a customized quote today and take your high-temperature piping systems to the next level with SteelPro Group.
