A stainless steel SUS329J2L which has generally been in commercial use in the form of a duplex phase was heat-treated to ensure a single ferrite phase. The ferritic steel was aged at 723 K for times up to 10.8 Ms (3000 h) to clearify the mechanism of the phase decompositon of ferrite. Precipitation of Cr-rich α′ phase was investigated by difference Mössbauer spectra and TEM observation. A time evolution of the compositional fluctuation was evaluated on the basis of the probabilities of the hyperfine magnetic field distribution computed from the Mössbauer spectra. The phase decomposition proceeded phenomenologically in three stages to yield finally a mixture of Fe-rich α and Cr-rich α′ phases. In the initial stage of the decomposition, the formation of α′ phase and spinodal-like compositional fluctuation occurred simultaneously. In the following stage, the precipitation of α′ phase proceeded with increasing aging time, whereas the compositional fluctuations grew slower. The final stage was characterized by coalescence of α′ phases.
