Two 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 in form 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 results from 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 modified law of mixtures, fracture profiles, and true stress-strain analysis.
