FORMATION OF PRECIPITATED AUSTENITE IN 9%Ni STEEL AND ITS PERFORMANCE AT CRYOGENIC TEMPERATURE

The formation of precipitated austenite in 9% Ni steel exposed at the temperature of α+γ re-gion and its influence on impact tonghness at cryogenic temperature have been studied.Austenite-rich and ferrite-rich bands are formed during soaking because of the re-distribu-tion of elements of C,N and Ni.The former phase is enriched of Ni,Mn,C and N,while thelatter one is relatively pure.Part of the austenite formed at intermediate temperatures trans-forms into martensite when the steel is cooled down to room temperature.The complex struc-ture which consists of fine martensite and austenite exhibits a moderate strength and highenough cryogenic toughness.The austenite enriched of C,N and Ni is still stable at thecryogenic temperature.The tearing ridges on the impact fracture surface is densely occupiedby the precipitated austenite,elongated along the tearing direction.One of the importantcause of the excellent eryogenic properties is that the precipitated austenite absorbs theimpurities and thus purifies the matrix of the steel.