Effect of martensite morphology on mechanical properties of low alloy steels having mixed structure of martensite and lower bainite

Abstract

AbstractTwo low alloy structural steels having carbon contents of 0·2 and 0·4 wt-% and similar levels of Cr, Mo, and Ni have been studied to determine the effect of martensite morphology on mechanical properties of steels having a mixed structure of martensite and lower bainite. The 0·2% and 0·4%C steels were isothermally transformed at 693 and 593 K, respectively. In the 0·2%C steel, the lower bainite was acicular inform and partitioned the prior austenite. The martensite formed on quenching was of entirely lath morphology. The transformation products of the 0·4%C steel were similar, but a small amount of twinned martensite was also present. After tempering at 473 K, the values of strength, fracture ductility, and true notch tensile strength of the 0·4%C steel were considerably superior to those of the 0·2%C steel. This difference resultsfrom plastic restraining of the lower bainite by the stronger martensite of the 0·4%C steel during plastic deformation. The details are discussed in terms of the modifie...