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P91 Alloy Steel – Chemical Composition, Properties and Uses

Over the past decades, technological advancements have significantly elevated the capabilities of industry-standard materials. T91 grade material, also known as P91, stands out as a premier choice for applications requiring high temperatures. Typically utilized in modern capacity boilers, the steam leaving the superheater ranges between 570 °C to 600 °C, with pressure levels from 170 bar to 230 bar.

In the intricate system, the final stage of this superheater involves transporting steam through pipes to the turbine, where it must endure extreme conditions. To meet these demands, essential materials with robust strength and enduring properties are imperative, especially those that resist deterioration over time and exhibit exceptional creep resistance.

Strength-Enhanced Ferritic Steel (CSEF) and advanced Chromium-Molybdenum Steels have gained widespread use globally, showcasing their versatility across various applications. These materials play a pivotal role in enhancing efficiency, offering superior material properties even in high-temperature environments. Their widespread adoption highlights the crucial role advanced materials play in pushing the boundaries of industrial capabilities.

What is P91 steel?

P91 steel is a specialized alloy characterized by its composition of 9% chromium, 1% molybdenum, and low carbon, further micro-alloyed with vanadium and niobium elements. It also features controlled nitrogen content. This alloy exhibits exceptional strength at high temperatures and possesses notable creep properties within the temperature range of 560℃ to 610℃. ASTM A335 P91 steel is renowned for its corrosion resistance, oxidation resistance, and high-pressure hydrogen resistance.

Widely utilized in the construction of high-temperature, high-pressure, and hydrogen-proof equipment, P91 steel finds prominent applications in superheater and reheater pipes within power plant boilers. Additionally, it is instrumental in the fabrication of equipment for distillation, cracking units, and refineries. Its robust properties make it a key player in withstanding extreme conditions, contributing to the reliability and efficiency of various industrial processes.

Why P91 steel is different?

P91 steel stands out due to its advanced composition, featuring chromium-molybdenum with additional options for tubing such as 9 CrMoV, boron-enhanced materials, and tungsten. This category encompasses various Grades, including 92, 122, E911, 23, 24, among others. Notably, P(T)91, classified as a Strength-Enhanced Ferritic Steel (CSEF), brings distinctive qualities, demanding specialized attention in its usage.

What sets P91 apart is its higher chromium content, a crucial factor contributing to its widespread application across various industries. With 9% Chromium and 1% Molybdenum, P91 surpasses materials like P22 Grade, which has only 0.5% Chromium. The elevated Chromium content in P91 enhances its strength at higher temperatures and augments oxidation resistance.

Moreover, the inclusion of Molybdenum in P91 plays a key role in improving its creep resistance, making it exceptionally suitable for applications requiring prolonged exposure to high temperatures. The addition of Manganese and Nickel further enhances the hardness of the steel, making P91 a robust choice for demanding applications where strength, resistance, and durability are paramount.

What is P91 Material?

P91 is a high-temperature, high-strength alloy steel that is commonly used in the manufacturing of components for power generation plants, particularly in the construction of boilers and piping systems. The “P” in P91 stands for “pressure,” and it’s part of a group of materials that are suitable for high-temperature and high-pressure applications, such as those found in power plants.

Here are some key characteristics and features of P91 material:

  1. High Temperature Strength: P91 exhibits excellent strength and creep resistance at elevated temperatures, making it suitable for applications in high-temperature environments.
  2. Creep Resistance: Creep is the tendency of a material to deform over time under constant load and elevated temperatures. P91 has good creep resistance, allowing it to withstand prolonged exposure to high temperatures.
  3. Chromium-Molybdenum Alloy: P91 is a type of ferritic-martensitic steel alloy that contains significant amounts of chromium and molybdenum. These alloying elements contribute to the material’s high-temperature strength and corrosion resistance.
  4. Application in Power Plants: P91 is commonly used in the construction of boilers, superheaters, and piping systems in power plants. It is favored for its ability to withstand the high temperatures and pressures associated with power generation.
  5. Weldability: P91 can be challenging to weld due to its high hardenability. Proper welding procedures and post-weld heat treatment are crucial to maintaining the material’s properties after welding.
  6. Post-Weld Heat Treatment (PWHT): P91 requires a specific post-weld heat treatment to ensure optimal mechanical properties and to reduce the risk of cracking. This is an essential step in the fabrication process.

ASTM A335 P91 Alloy Steel Chemical Composition

The chemical composition of P91 alloy steel is as follows:

  • Chromium (Cr): 9%
  • Molybdenum (Mo): 1%
  • Carbon (C): Small amount
  • Manganese (Mn): Small amount
Alloy Steel P91CCrMnMoPSSi
0.08-0.128.00 – 9.500.30-0.600.85 – 1.050.020 max0.010 max0.20 -0.50

P91 Chemical Composition

The chemical properties of P91 alloy steel contribute to its impressive performance in high-temperature and corrosive environments. Here are the key chemical properties:

  • Chromium (Cr):
      • Role: Provides excellent resistance to oxidation at high temperatures.
      • Impact: Enhances the alloy’s ability to withstand the effects of high-temperature exposure, making it suitable for applications in power plants and other industries.
  • Molybdenum (Mo):
      • Role: Improves resistance to oxidation and corrosion.
      • Impact: Enhances the alloy’s overall corrosion resistance, especially in challenging environments, contributing to its durability and longevity.
  • Carbon (C):
  • Role: Increases strength.
  • Impact: The presence of carbon enhances the alloy’s strength, making it well-suited for applications where structural integrity and robustness are crucial.
  • Manganese (Mn):
    • Role: Improves ductility.
    • Impact: Contributes to the alloy’s ductile properties, enhancing its ability to deform without breaking, which is essential in various industrial processes.

P91 Alloy Steel Mechanical Properties

The mechanical properties of P91 alloy steel highlight its exceptional strength and durability, making it well-suited for high-performance applications. Here are the key mechanical properties:

  1. Yield Strength:
    • Value: 85 ksi (586 MPa)
    • Significance: Represents the amount of stress the material can withstand without undergoing plastic deformation or yielding.
  2. Tensile Strength:
    • Range: 95-110 ksi (655-758 MPa)
    • Significance: Indicates the maximum stress the material can endure while being stretched or pulled.
  3. Elongation:
    • Range: 15-20%
    • Significance: Measures the material’s ability to deform before fracture, providing insights into its ductility.
  4. Hardness:
    • Range: 280-320 HBW (Brinell Hardness)

Significance: Reflects the material’s resistance to penetration, indicating its overall hardness.

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 Steel Physical Properties

The physical properties of P91 alloy steel provide insights into its behavior under various conditions. Here are the key physical properties:

  • Density:
  • Value: 7.85 g/cm³ (0.284 lbs/in³)
  • Significance: Represents the mass per unit volume of the material, indicating its compactness.
  • Thermal Expansion Coefficient:
  • Range: 6-7 x10⁻⁶ per °C (2–4×10⁻⁶ per °F)
  • Significance: Reflects the material’s tendency to expand or contract with changes in temperature.
  • Specific Heat Capacity:
  • Value: 0.48 KJ/Kg·K (0.12 BTU/lb·°F)
  • Significance: Indicates the amount of heat energy required to raise the temperature of a unit mass of the material by one degree.
  • Electrical Conductivity:
  • Value: 11 MS/m at 20°C (68°F)
  • Significance: Reflects the material’s ability to conduct electric current, providing insights into its electrical properties.
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)
20/68460 (-)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 Grades

ASTMASMEEquivalent materialJIS G 3458UNSBSDINISOABSNKLRS
A335 P91SA335 P91T91, K90901, X10CrMoVNb9-1, 1.4903 K91560    

P91 Alloy Steel Uses

The exceptional mechanical properties and corrosion resistance of alloy steel grade P91 make it a versatile material suitable for a myriad of applications across industries. Here are some key uses:

  • Oil & Gas Pipelines:
  • Role: P91 steel’s robustness and corrosion resistance make it suitable for pipelines, ensuring reliability in the demanding conditions of the oil and gas industry.
  • Petrochemical Plants:
  • Role: P91 is utilized in various components of petrochemical plants where its strength and resistance to corrosion are crucial for long-term performance.
  • Power Plants:
  • Role: Commonly employed in power plant components such as superheater and reheater pipes due to its excellent high-temperature strength.
  • Refinery Components:
  • Role: P91 finds application in critical components within refineries, where its durability and resistance to harsh conditions are paramount.
  • Flanges, Valves, and Pumps:
  • Role: Used in the manufacturing of flanges, valves, and pumps due to its mechanical strength and corrosion resistance.
  • Fasteners:
  • Role: P91 is a reliable choice for fasteners in industrial settings, ensuring structural integrity and longevity.
  • Pressure Vessels:
  • Role: P91 steel’s strength makes it suitable for constructing pressure vessels, ensuring safety and durability.
  • Fuel Tanks:
  • Role: Due to its corrosion resistance, P91 is used in the construction of fuel tanks where exposure to various elements is a concern.
  • Tubing and Cylinders:
  • Role: P91’s mechanical properties make it suitable for applications requiring tubing and cylinders, ensuring structural stability.

P91 Steel Corrosion Resistance

Alloy steel grade 91 exhibits exceptional corrosion resistance, primarily attributed to its elevated chromium content. The presence of chromium enables the formation of a protective oxide layer, commonly known as a passive film, on the surface of the alloy. This adherent and stable film act as a robust barrier, preventing oxygen from reaching the metal’s surface and effectively thwarting further corrosion. The alloy’s ability to develop and maintain this protective layer makes it highly resilient in corrosive environments, ensuring prolonged durability and reliability in applications ranging from oil and gas pipelines to power plants and petrochemical facilities.

Heat Resistance

Alloy steel grade 91 stands as a formidable contender against extreme heat, thanks to its substantial chromium content. The high chromium levels play a pivotal role in conferring robust heat resistance to the alloy. This attribute becomes particularly significant in high-temperature environments, such as furnace components and boiler parts. The chromium content acts as a shield, preventing oxidation even under the most demanding heat conditions. This exceptional heat resistance makes alloy steel grade 91 a preferred material for applications where exposure to elevated temperatures is a constant, ensuring longevity and reliability in challenging thermal environments.

Heat Treatment for Alloy Steel Grade 91

To elevate the mechanical properties and workability of alloy steel grade 91, strategic heat treatments can be employed. Common techniques include quenching and tempering or normalizing and annealing. These processes are designed to refine the alloy’s structure, optimizing its strength, hardness, and overall performance. Careful application of heat treatments ensures that alloy steel grade 91 meets the stringent demands of diverse industrial applications.

Machining Techniques

Machining alloy steel grade 91 demands precision, and this is best achieved with sharp cutting tools. Utilizing cutting tools coated with titanium nitride is recommended, as this enhances tool life span. The use of sharp tools, coupled with advanced coatings, ensures efficient machining and contributes to the longevity of the cutting tools.

Welding Techniques

Welding alloy steel grade 91 necessitates skillful techniques to mitigate potential challenges. Oxyacetylene welding or TIG arc welding can be employed, but caution is paramount. This material is prone to cracking if not handled with care during welding processes. Therefore, special precautions must be observed to ensure a successful and robust weld, maintaining the integrity of alloy steel grade 91 in various applications.

Conclusion:

In summary, alloy steel grade 91 emerges as a formidable choice across diverse applications, from oil and gas pipelines to the intricate components of furnaces and boilers. Its commendable mechanical properties, coupled with robust corrosion and heat resistance, position it as a reliable material for demanding industrial scenarios. However, the key lies in adhering to precise machining techniques, ensuring the use of sharp tools coated with titanium nitride to harness the material’s full potential. Moreover, welding, whether through oxyacetylene or TIG arc techniques, demands meticulous attention, as mishandling can lead to potential cracking. With due care and adherence to proper procedures, alloy steel grade 91 proves to be a stalwart performer, delivering consistent and trustworthy results across a spectrum of applications.

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