Effect of microstructural change on the fracture characteristics of ferrite-martensite-bainite steels

Abstract

The effect of microstructural change on the fracture characteristics of ferrite-martensite-bainite steels was investigated with Fe-0.11C-1.64Mn-0.78Si steel. 1 inch compact tension specimens (1T-CTSs) were machined from hot-rolled plates. The microstructures composed of ferrite-martensite-bainite (tri-phase) were introduced to the specimens by suitable heat treating cycles. The volume fraction of martensite and bainite was controlled by various holding time at a lower bainitic transformation temperature (350℃). Volume fraction of bainite increased, while that of martensite decreased, and martensite particles became finer as increase in holding time at 350℃. Macro-hardness was inversely proportional to the holding time. Single specimen unloading compliance method was used to obtain the J-Resistance (J-R) curve in fracture test. Introduction of bainite to the ferrite-martensite steels (dual-phase steels) was able to improve the fracture toughness ($J_{IC}$) of the materials. Macro-hardness variation had good correlation with toughness increase. Fractographies in a scanning electron microscope (SEM) revealed that catastrophic cleavage fracture without stable crack growth occurred in the specimens containing small amount of bainite. Dimpled ductile fracture mode became dominant as increase in volume fraction of bainite. Transgranular cleavage initiated at each grain boundary. Dimple size was large inside the grains and small near the grain boundaries such as the distribution of the second phase particles in optical micrographs. The results suggest that improvement in fracture characteristics was attributed to the softening of ferrite matrix by carbon depletion, deformation of bainite, and refining of martensite particles.