ATOM-PROBE STUDY OF PHASE-DECOMPOSITION OF FERRITE DURING AGING IN FERRITIC AUSTENITIC DUPLEX STAINLESS-STEEL

The ferritic phase decomposition of thermally aged ferritic/austenitic duplex stainless steels has been examined layer by layer by a pulsed laser-atom probe. In this study Fe-25% Cr-6%Ni-3%Mo duplex steel (SUS329J2L) was aged at 450-degrees-C for up to 5000 h. The results obtained are as follows: (1) The initial fluctuation of Cr concentration increases by ageing at 450-degrees-C for 100 h. At the ageing time of 1000 h the fluctuation further increases and the alpha' phase with Cr concentration of 65% precipitates. (2) The alpha' phase is believed to be formed by spinodal decomposition. (3) In the material aged for 1000 h, the average particle size of alpha' particles ranges from 0.6 to 1.1 nm and the wave length from 1,8 to 4.0 nm. (4) Ni tends to segregate into the Cr-depleted region between the alpha' phases. (5) Ni-Si-Mo clustering is recognized in the ferritic phase prior to formation of the alpha' phase. (6) At 5000 h the particle size and the wave length increase due to the aggregation of alpha' phase. This results in decrease of hardness.