Chrome Moly Alloy A335/SA335 Grade P91 Pipe

for High Temperature Service

U.S. Metals is a leading supplier of the industry standard material for high temperature applications.Chrome Moly Alloy A335/SA335 grade P91 is growing in popularity in the power and other industries for its high strength properties.

Our inventory remains at the forefront of the industry, meeting and exceeding new specifications to prevent problems like creep cracking and hardness issues. U. S. Metals P91 inventory meets the requirements of Code Case 2864. For a copy go to the linc titled Code Case 2864. We work closely with our clients and agencies like EPRI to make sure our inventory is on the cutting edge of industry requirements. We have been purchasing our P91 Inventory to the EPRI enhanced chemistry recommendations since the start of this initiative by EPRI.

U. S. Metals stocks ASTM /ASME A/SA P91 Pipe, Heater Tubes, Fittings, Flanges and Valves from ½” NPS through 30” NPS with wall thickness ranging from STD through 4” Thick. We are considered to have the largest inventory of P91 High Energy Pipe, Fittings, Flanges & Valves. USM stock also consists of Minimum Wall Thickness pipe as well as material certified to ASTM/ASME A/SA 213 which is required by some power plant equipment designs. Go to the U. S. Metals comparison chart titled The Difference Between ASTM A213 and ASTM A335 for more information on the difference between P91 and T91.

About ASTM A335 P91 Pipe

Trade Name: P91
Grade: P91
UNS #:
Pipe: A335/SA335
Forged/Bored Pipe: A369/SA369
Length: Specific or random

A335/SA335 P91 is a ferritic alloy steel that has very high strength properties, which do not deteriorate with time, and are creep resistant. It is also called 9 Cr 1 Mo steel, or chrome moly pipe, based on its composition.

Compared to its predecessors, the T22 or P22 grade, grade P91 exhibits high strength, enduring temperatures up to 600 °C. The oxidation temperature limits are also higher allowing elements to be engineered with less thickness. This contributes to an almost 10 times higher thermal fatigue life and allows engineers to increase the operating temperature to a higher level, increasing efficiency.

P91 typically allows a wall thickness reduction by at least a 2-to-1 ratio. A wall that is thinner creates lighter hanger loads, less welding time and uses less filler metal.

These advantages are enabled by this steel’s high amount of chromium. Grade 91 contains 9% chromium and 1% molybdenum compared to 2.5% chromium in the next best P22 grade

P91 Pipe Chemical Requirements

  • Carbon - 0.08% - 0.12%
  • Manganese - 0.30% - 0.60%
  • Phosphorus, max - 0.020%
  • Sulfur, max - 0.010%
  • Silicon, max – 0.20% - 0.50%
  • Chromium - 8.00% - 9.50%
  • Molybdenum - 0.85% - 1.05%
  • Vanadium: 0.18% - 0.25%
  • Nitrogen: 0.030% - 0.070%
  • Nickel: 0.40% Max
  • Aluminum: 0.02% Max
  • Columbium: 0.06% - 0.10%
  • Titanium: 0.01% Max
  • Zirconium: 0.01% Max

Chromium, or chrome, improves high-temperature strength and increases oxidation resistance. It is virtually irreplaceable in resisting oxidation at elevated temperatures. Chrome also raises the tensile, yield and hardness at room temperatures.

Molybdenum increases the strength, elastic limit, resistance to wear, impact qualities and hardenability. It increases the resistance to softening, restrains grain growth and makes chromium steel less susceptible to embrittlement. Moly is also the most effective additive for increasing high temperature creep strength, or creep resistance. It also enhances the corrosion resistance of steel and inhibits pitting.

Also present are smaller quantities of nickel and manganese which increase the hardenability of the steel.

The addition of Vanadium (V) and Columbium/Niobium (Cb/Nb) as well as the control of Nitrogen (N) greatly Improves Creep Strength.

More important than the alloying elements is the formation of this alloy steel. The steel is formed by normalizing at 1050 °C, then air cooling down to 200 °C. It is then tempered by heating to 760 °C. The temperatures and cooling rates produce the microstructure that results in the high creep strength properties.

P91 Pipe Specifications

P91 Pipe Applications

A335/SA335 P91 is suitable for bending, flanging (vanstoning), and similar forming operations and for fusion welding. P91 is most commonly used in the power industry and petrochemical plants where fluids and gases are transported at extremely high temperatures and pressures.

P91’s strength and high temperature endurance makes it ideally suitable for plants that operate on a cyclic basis - like combined cycle plants. Also, the reduction in thickness suits HRSG designers, in an HRSG the temperature head is limited and locating the coils in the heat transfer path is very critical.

P91 is the correct grade for:

P91 Welding Procedure

When welding P91, preheating, maintaining inter-pass temperatures and post-weld treatment procedures are essential to maintain the P91 grade. Welding has an important effect on the microstructure and failure to follow the procedures will result in catastrophic failures.

For thick-walled pipes, the use of a modern induction heating system – where the coils do not heat up – is the ideal method. This gives better control, and uniform heating between the inner and outer diameters. Induction heating is ideal for maintaining the inter-pass temperatures and carry out the welding. This is also a more worker-friendly heating process and is ideal for complex shapes like weldolets and tees.

The composition of the welding electrodes used should always be in line with the parent material. The nickel and manganese content of P91, though in smaller percentages, has profound effects on the critical temperatures, which decides the heat treatment temperatures and the cooling rates.

Steel that has not been heat-treated has a great affinity to hydrogen which can cause stress corrosion cracking. Pre-heating must be done properly to remove any moisture. In addition, the post-weld heat treatment must be done as quickly as possible to avoid any contact with moisture from condensation, rainfall, etc. Be sure all joints are post-weld heat treated prior to hydro test.

Dissimilar weld joints – especially at complicated geometries – can result in the heat treatment not having the desired affect throughout the cross sections. This can also lead to failures.

Call U.S. Metals today to learn more about our Chrome Moly Alloy Seamless P91 Pipe inventory today.

Information provided in this page is in accordance with the 2013 Edition of the ANNUAL BOOK OF ASTM STANDARDS.
Refer to ASME B & PV Section II for non-identical A/SA requirements