Abstract
The reverse transformation α′ → γ within two austenitic stainless steels of nominal composition Fe–18Cr–8Ni and Fe–18Cr–12Ni has been studied. A magnetic balance apparatus was used to determine the phase contents after rapid, salt bath heating. Information concerning the structures produced and the mechanisms of the transformation was obtained using transmission electron microscopical examination, including in situ hot stage experiments and X-ray microanalysis. The results obtained show that the α′ → γ transformation proceeds by both diffusional and shear processes depending upon the heating time and temperature, initially producing austenite having a twinned substructure which recovers into fine subgrains. Diffusion occurs at temperatures between the A s and A f to produce the equilibrium austenite and ferrite duplex structure. Further observations on the recrystallization behaviour, the effect of transformation cycling, and the phenomenon of a magnetic phase ‘peak’ at ∼ 400°C are also reported, and possible mechanisms operating during these processes are discussed.
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