Phase distributions in rapidly solidified stainless steels

Abstract

Phase distributions and the internal magnetic fields have been determined in rapidly solidified stainless steels (Fe-nCr-8Ni-0.05C, Fe-nCr-5Ni, and Fe-nCr withn in the range of 10 to 24) by transmission and conversion electron Mossbauer spectroscopy (TMS and CEMS). Based on these results, a modification of the phase boundaries in the Schaeffler diagram is suggested to account, in particular, for rapidly solidified stainless steels. The suggested modification is primarily an expansion of the austenite field toward higher Cr and lower Ni equivalent contents. Combining CEMS and TMS makes it possible to determine the phase distributions both in the near surface region (outmost 300 nm) and in the bulk of the ribbons. For the low-Cr alloys, the content of the bcc phase (martensite) in the surface region is higher than in the sample as a whole. In the high-Cr alloys, the content of the bcc phase (ferrite) is lower in the surface than in the bulk. This disparity is ascribed to the different mechanisms of formation of martensite (diffusionless) and ferrite (nucleation and growth) in relation to the higher cooling rates of the surface layers. The determinations of the internal magnetic field are in good agreement with earlier investigations on conventionally processed Fe-Cr steels, where it was found that the internal magnetic field decreases with increasing Cr content.