What is P91 Steel?
P91 or commonly known as 9Cr-1Mo-V is a creep strength enhanced ferritic (CSEF) alloy steel. These alloy steels are designed to retain strength at extremely high temperatures. The P91 material means 9 percent chrome, 1 percent molybdenum & Vanadium or commonly known as 9Cr-1Mo-V steel in oil & gas/ power plants.
P91 is used in high temperature, high-pressure steam piping not only because it retains strength at elevated temperature but also because it resists failure due to “creep,” which is the tendency of a solid material to move slowly or deform permanently under the influence of mechanical stresses. P91 also resists corrosion better than steel alloys used previously in these applications. Because of these advantages, P91 is used extensively in power generation applications, such as for high-pressure steam lines.
Material is designated as P91 for pipe, T91 for the tube, F91 for forging, C12 A for casting, etc.
CSEF’s are a family of ferritic steels whose creep strength is enhanced by the creation of a precise condition of microstructure, specifically martensite or bainite, which is stabilized during tempering by the controlled precipitation of tamper-resistant carbides, carbo-nitrides, or other stable phases & its different from other Cr-Mo steels.
Material Specification for supply
ASME P Number of P91 material: 15E, Group 1
ASTM–182 F91, ASTM-213 T91, ASTM-217 C12A, ASTM-234 WP91,
ASTM-335 P91, ASTM-336 F91, ASTM-356 12A, ASTM-369 FP91,ASTM-387 91 Cl.2, ASTM-691 91 etc.
Chemical properties of P91
Mechanical Properties of P91
|Standard||Grade||Yield Strength min.|
|Tensile Strength min.|
|ASTM A213/ A335||T91/ P91||415||585||20||250|
How to weld P91 Material
Grade 91(also known as P91) is a specially modified 9% Chromium, 1% Molybdenum, Vanadium enhanced (9Cr-1MoV) material. Since P91 is modified with vanadium, nickel, aluminum, niobium, and nitrogen, it develops very high hardness.
Tramp residual elements in this steel, such as phosphorous, sulfur, lead, tin, copper, antimony, and other elements will segregate to the grain boundaries during solidification of the weld, and, since the weld metal is very hard, it will crack quite easily. Welding of P91 material is a tedious task. Welding P91 material generally requires preheating in the joint, maintaining inter pass temperature, hydrogen bakes, and post-weld heat treatment. If material cools below minimum preheat temperature between passes, restraint issues may arise or hydrogen may contaminate the weld. Hydrogen embrittlement can lead to cold cracking of the finished weld.
1. Selection of Welding Consumables for P91 welding:
Another key to success when welding P91 is choosing the right filler metal for the application. The performance of P91 welds depends on having the correct chemical composition in the weld metal; therefore, it is highly recommended that filler metals be purchased with test reports showing actual chemical analysis for the specific heat/lot combination that one has purchased.
Also, controlling hydrogen in the weld to avoid hydrogen-induced cracking is important with any high-strength alloy, and P91 is no exception. Look for low-hydrogen filler metals designed for use with high-strength Chrome-moly materials.
Datasheets for all chromium-molybdenum filler metals should provide a typical X-factor designation for the product. This formula measures a weldment’s resistance to temper embrittlement. The number is calculated from the amounts of four key contaminants (elements) in steel: phosphorous, antimony, tin, and arsenic, which together have the greatest impact on a weldment’s susceptibility to temper embrittlement. Here is the formula: X = (10P + 5Sb + 4Sn + As)/100.
It’s especially important to know the X-factor of a filler metal when welding certain chrome-moly steels, such as P91. Look for a filler metal with an X-factor below 15 for P91 welding applications.
2. Preheating & interpass for P91 material:
Preheat temperature of a minimum of 204°C shall be applied & maintained during the welding process. Heating shall be by electric resistance method/induction heaters to allow uniform heating. same preheat temperature shall be applied for tack welding. Interpass temperature shall be 600 °F (315 °C) maximum for P91 material welding.
Recommended preheat temperature for Grade 91 material as per various codes and standards:
205°C (Non-Mandatory appendix “R”, ASME Section VIII Div.1)
200°C (Table 131.4.1-1, ASME B31.1 & Table 330.1.1 of ASME B31.3)
150°C for thickness ≤13 mm & 205°C for thickness > 13 mm (Table PW-38-1, ASME Section 1)
You can use our Online Welding Preheat Calculator to find the right preheat temperature for the variety of materials based on alloy chemistry, plate thickness, and hydrogen level.
Watch our YouTube video to Learn what is Preheat, Inter-pass & post heating.
3. Post Weld heat Treatment:
PWHT temperature as per various codes and standards:
1. 705-785°C (as per Table PW-39-5, ASME Section 1)
2. 705°C Minimum and for Max. temp. refer below (as per Table UCS 56-11, ASME Section VIII Div.1)
For welds made with matching Grade 91 filler metal (e.g., AWS A.5.5 E90xx-B91, ISO EN CrMo91), the maximum holding temperature shall be determined as follows:
1. If the Ni+Mn content of the filler metal≤1.0%, the maximum PWHT temperature shall be 790°C.
2. If the Ni+Mn content of the filler metal>1.0% but ≤ 1.2%, the max. PWHT temperature shall be 780°C.
3. If the Ni+Mn content of the filler metal>1.2%, the maximum PWHT temperature shall be at least 10°C below the lower critical transformation temperature (Ac1).
4. 705-775°C (Table 132.1.1-1, ASME B31.1 & Table 331.1.1 of ASME B31.3).
Recommended PWHT temperature for P91 Material are 740 to 760°C (1375 to 1425°F) range with a minimum holding time of 2 hours.