F91 is one of the branches of 91 steel for various purposes. In American ASTM and ASME standards, the steel series distinguished by use are as follows:

• A213 T91 Ferritic and austenitic seamless steel tubes for boiler superheater and heat exchanger
• A182 F91 forged alloy steel tubes, flanges, forged fittings and valves and other components for high temperature service
• A234 wp91 carbon and alloy steel forged fittings for medium and high temperature service
• A200 P91 ferritic alloy steel seamless steel pipe for petrochemical industry
• A336 F91 steel forgings, high temperature and high pressure alloy steel parts
• Cold drawn alloy seamless steel tubes for a199 T91 heat exchanger and condenser tubes
• A369 fp91 hollow pipe forged with carbon steel and ferritic alloy steel for high temperature service
• Ferritic seamless steel pipe for A335 P91 high temperature pipeline

ASTM A182 F91 Pipe Fittings

All of these steels are basically the same in composition, but there are some differences in performance assessment due to different purposes of use. For example, the thermal strength of T91 is ≥ 585mpa, while that of wp91 is σ B = 590-760mpa. If there is a requirement for reduction of area (> 40%) in forgings but not in pipe fittings, these differences are very small and easy to achieve. All 91 series steels are actually the original 9Cr-1Mo A modified steel grade formed by adding strengthening elements V, Nb, N and so on. It is usually called martensitic heat-resistant steel, but in American Standard, the content of Cr less than 10% is listed as ferrite series, so the name in the standard is still ferritic type (in fact, different phases can be obtained by different heat treatment processes, so it is not necessary to stick to the name).