Effect of carbon content on toughness of tempered martensitic steels analyzed by toughness prediction model

Abstract

To evaluate the effect of carbon content on the toughness of tempered martensite, the critical crack tip opening displacement (CTOD) was evaluated for 0.1/0.3/0.5C-1.5Mn-1.0Mo (mass%) steels. The critical CTOD was the highest for 0.5C steel because of grain refinement rather than strengthening and cementite coarsening. The results were analyzed by a toughness prediction model that considered the strength, grain size, and cementite size. This model incorporated the microstructure information, stress distribution calculated using the finite element method (FEM), and fracture process criteria. It calculated the fracture point at which the local stress and local strength of the material correspond. The fracture process was divided into the following three stages: Stage I, cementite cracking; Stage II, microcrack propagation into the cementite and ferrite boundary by stress concentration caused by dislocation pileup along the major axis of the martensite block; and Stage III, crack propagation into the first intersecting 15°-oriented boundary with the crack length of the minor axis of the martensite block. The model calculation reflected experimental trends, revealing that the bottleneck in the fracture process was Stage III. Therefore, the refinement of the minor axis of the block was effective for toughness improvement.