Strengthening and Phase Transformations in 304 Stainless Steel

摘要

The alpha' martensite transformation in 304 stainless steel was induced only by deformation which resulted in very considerable increases in both yield and tensile strengths. Further increase in these properties was achieved by aging the material deformed at -196C at temperatures from 200C to 400C. The latter increase in strength depended on the amount of prior deformation. Magneto-, x-ray and transmission electron metallographic techniques revealed that the most probable cause of further strengthening upon aging is the formation of thermally-nucleated martensite. Detailed transmission electron microscopy revealed rather convincingly that the eta phase is indeed a phase and that the sequence in the transformation is gamma to eta to alpha'. Formation of alpha' is favored at intersections of two eta bands. The orientation relationship between gamma and alpha' was found to be predominantly the Nishiyama relationship at the start which changes to the Kurdjumov-Sachs relationship on further deformation. A mechanism based on the most probable atom movements from the eta - hcp structure to the bcc structure is proposed which predicts the habit plane, shear direction and amount of shear. (Author)