Effects of Shape and Distribution of Retained Austenite on Toughness of Two Ni-Cr-Mo Steels Containing Medium Carbon

Abstract

A study has been made of effects of the shape and distribution of retained austenite on toughness of two Ni-Cr-Mo steels containing 0.4 and 0.6 per cent. Results obtained are as follows;
(1) Retained austenite remained acicularly and lenticularly along martensite lath boundaries yielded desirable effects on toughness, but retained austenite remained massively along prior austenite grain boundaries did not necessarily yield desirable effects on toughness in the steel containing 0.4 per cent carbon.
(2) Retained austenite remained lenticularly along martensite plate boundaries did not necessarily yield desirable effects on toughness, but retained austenite remained massively along prior austenite grain boundaries yielded desirable effects on toughness in the steel containing 0.6 per cent carbon.
(3) In both steels, retained austenite was stable in the process of deformation.
(4) From these results, it is assumed that toughness is closely associated with coherency between retained austenite and the matrix when retained austenite remains in the matrix containing a less twin martensite and toughness is closely associated with relaxation of stress concentration against internal defects in the matrix when it remains in the matrix containing more twin martensite.