Effects of Ni Content and Grain Size on the Martensitic Trans-formation and Deformation Behavior in 18Cr-8Ni Stainless Steels

Abstract

The effects of Ni content and grain size on the athermal and deformation-induced martensitic transformations in 18-8 austenitic stainless steels have been examined. Furthermore, their deformation characteristics, especially 0.2% proof stress (0.2PS) and flow stress at sub-zero temperatures have also been examined in terms of austenite stability. The results are as follow:
(1) Ms and Md temperatures are depressed with increasing Ni content. Ms temperatures of commercial alloys are about 130K lower than those of high-purity alloys with low interstitials. The effect of grain size on Md is larger than that of Ms.
(2) The amount of athermal martensite formed at 77°K is analysed according to Magee's equation. There seems a good agreement between observed and calculated values when grain size is 115μ. In case of smaller grain size with 36μ, however, calculated values are larger than observed ones. This disagreement may comes from the differences of Ms and the nucleation frequency at Ms.
(3) The content of deformation-induced χ' and ε' martensite depends on Ni content and tensile temperature.
(4) The change of transformation mode from γ→ε'→α'to γ→α' is observed with increasing Ni content and tensile temperature, which can be understood in terms of stacking fault energy and chemical driving force (ΔG_γ→α').
(5) Inverse dependence on temperature of 0.2% proof stress (0.2PS) does not occur in this study. The temperature dependence of 0.2PS of untransformed stable austenite is about 2.7 times larger than that of of 0.2PS of unstable austenite in which martensitic transformation intervenes before yielding of austenite.
(6) The flow stress of unstable austenite e. g. 18Cr-9Ni alloys at 93K does not follow the simple law of mixture of austenite and martensite.