Relationship between Amount of Retained Austenite and Mechanical Properties in Martensitic Precipitation Hardening Stainless Steel

Abstract

The existence of small amount of retained austenite in precipitation hardening stainless steels (PHSS) is known to be effective to improve the ductility caused by transformation-induced plasticity. However, the detailed mechanism is yet inadequately understood. In the present study, two types of PHSS are prepared and solution heat treated with different cooling rate to systematically change the amount of retained austenite in the PHSS. The amount of retained austenite was investigated by XRD analysis, microstructure was observed by TEM and EBSD, and micro Vickers hardness test and tensile test were carried out in order to elucidate the relationship between the amount of retained austenite and mechanical properties. As a result, retained austenite is finely distributed with a block or film shape along the martensite lath boundaries. The excess amount of retained austenite forms large blocks in several ten μm in the vicinity of grain boundaries and hence inhomogeneous microstructure and hardness. 0.2% proof stress decreases and rupture elongation increases with increasing the amount of retained austenite. On the other hand, tensile properties of the 3Co steel with a large amount of retained austenite do not change in spite of the precipitation in the martensite matrix by the aging treatment. It is considered by the effect of percolation phenomenon of retained austenite with low mechanical strength. Further improvement of mechanical properties of PHSS will be achieved by controlling not only amount of retained austenite but also microstructure especially focused on the connectedness of retained austenite.