P91 is a chrome moly alloy metal that contains excellent strength and temperature resistance. It is designed for enhanced creep strength, making it a creep strength enhanced ferritic (CSEF). This metal is made by normalizing at 1050 °C, air cooling to 200 °C, and then tempered through heating to 760 °C. This process is the main reason behind P91’s creep strength and durability. P91 stands for the metal’s composition of 9% chrome and 1% molybdenum plus vanadium. The chrome increases temperature strength and oxidation resistance. The molybdenum increases the elasticity, resistance to wear, and high temperature creep strength.
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.