ThePipingMart Blog Grades P91 Alloy Steel (UNS K91560) – Composition, Properties and Uses

P91 Alloy Steel (UNS K91560) – Composition, Properties and Uses

Alloy Steel Grade P91

Alloy steel grade P91 is an alloy steel with many benefits and features. It is widely used in various applications, including oil and gas, petrochemical plants, power plants, and other industrial applications. In this guide, we’ll take a look at the composition, chemical properties, mechanical properties, physical properties, uses, corrosion resistance, heat resistance, heat treatment methods and machining techniques for alloy steel grade P91.

P91 Alloy Steel Composition

Alloy steel grade P91 comprises 9% chromium and 1% molybdenum. This combination gives excellent steel strength and wear resistance. The alloy also contains small amounts of carbon and manganese for improved toughness.

Alloy Steel P91 C Cr Mn Mo P S Si
0.08-0.12 8.00 – 9.50 0.30-0.60 0.85 – 1.05 0.020 max 0.010 max 0.20 -0.50

P91 Alloy Steel Chemical Properties

The chromium content in alloy-grade P91 helps it resist oxidation at high temperatures. Its molybdenum content helps further improve its resistance to oxidation as well as other forms of corrosion. The carbon content helps increase its strength, while the manganese content improves its ductility.

P91 Alloy Steel Mechanical Properties

Alloy steel grade P91 has excellent mechanical properties due to its composition. It has a yield strength of 85 ksi (586 MPa), while its tensile strength ranges from 95-110 ksi (655-758 MPa). It also has an elongation rate between 15-20%. The hardness rating is generally between 280-320 HBW.

Yield strength Rp0.2 (N / mm 2 ) > = 450 Normalized
+ tempered
Yield strength Mpa / ksi 415/60 ASTM A 335 -P91
Tensile strength Rm (N / mm 2 ) 620-850 Normalized
+ tempered
Elongation at break-A5 (%) min.lengths > = 20 Normalized
+ tempered
Notched impact strength (J) across 40 transversely / normalized
+ tempered
Tensile strength Rm N / mm 2 / ksi 585/85 ASTM A 335 -P91

P91 Alloy Steel Physical Properties

Alloy steel grade P91 has a density of 7.85 g/cm3 (0.284 lbs/in3). Its thermal expansion coefficient ranges from 6-7 x10-6 per °C (2–4x10_6per°F). Its specific heat capacity is 0.48 KJ/Kg·K (0.12 BTU/lb·°F). Its electrical conductivity is 11 MS/m at 20°C (68°F).

Temperature T
° C / ° F
(° C / F)
Specific heat J / kgK
(Btu / lb ° F)
Thermal conductivity
W / mK
(Btu · in / ft 2 · h · ° F)
Electrical resistance
μΩ · cm
(Ω circ / ft)
Young’s modulus
kN / mm 2
(10 3 ksi)
Expansion coefficient from 20 ° C to T
10 -6 / K
(10 -6 / ° F)
20/68 460 (-) 26 (-) 218 (-)
200/392 207 (-) 11.3 (-)
400/752 190 (-) 12.0 (-)
500/932 30 (-) 12.3 (-)
600/1112 12.6 (-)
650/1202 162 (-) 12.7 (-)

P91 Alloy Steel Equivalent

ASTM ASME Equivalent material JIS G 3458 UNS BS DIN ISO ABS NK LRS
A335 P91 SA335 P91 T91, K90901, X10CrMoVNb9-1, 1.4903 K91560

P91 Alloy Steel Uses

Due to its excellent mechanical properties and corrosion resistance capabilities, alloy steel grade 91 can be used in numerous applications such as oil & gas pipelines; petrochemical plants; power plants; refinery components; flanges; valves; pumps; fasteners; pressure vessels; fuel tanks; tubing; cylinders; etc.

Corrosion Resistance

Alloy steel grade 91 offers excellent corrosion resistance due to its high chromium content, which helps form an adherent oxide layer on the surface known as a passive film which protects the metal from further corrosion attack by preventing oxygen from reaching the material’s surface.

Heat Resistance

Alloy steel grade 91 also offers good heat resistance due to its high chromium content, which prevents oxidation even at very high temperatures making it suitable for use in high-temperature environments such as furnace parts or boiler components.

Heat Treatment

To improve the mechanical properties or workability of alloy steel grade 91, it can be subjected to different heat treatments such as quenching and tempering or normalizing and annealing.

Machining Techniques

When machining alloy steel grade 91, it should always be done with sharp cutting tools, preferably coated with titanium nitride, since this will help improve the tool life span.

Welding Techniques

Welding can be done using oxyacetylene welding or TIG arc welding techniques; however, special precautions must be taken when working with this type of material since it tends to crack if not handled properly.


As you can see, there are many advantages to using alloy steel grade 91 in various applications ranging from oil & gas pipelines to furnace components or boiler parts thanks to its excellent mechanical properties combined with good corrosion and heat resistance capabilities. In addition, proper machining techniques must always be followed when working with this type of material since it tends to crack only if handled correctly. Finally, welding can be done using either oxyacetylene welding or TIG arc welding techniques, although extra caution should always be taken when doing so. With proper care, alloy steel grade 91 can provide reliable performance in many different types of applications.

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