Influence of Bainite Morphology on Ductile Fracture Behavior in a 0.4C-CrMoNi Steel

In this study, lower and upper bainite microstructures were produced by austempering process of a NiCrMo low alloy steel containing 0.4% C and their mechanical properties were studied by tensile testing at room temperature. Fracture mechanisms involved were investigated on special samples cut from the necked region of the tensile specimens. The specimens were metallographically prepared and examined by scanning electron microscopy. Analysis of both microstructures revealed that at lower strains, voids were nucleated by cracking or interface decohesion of Al2O3 inclusions. However, at higher strains, especially near the fracture strain, voids were mainly initiated by decohesion of cementite/ferrite matrix interfaces. Lower bainite showed an elongation of 14% in tensile testing compared with 17% for the upper bainite, but its strength was about 20% higher. In order to present the void growth kinetics behavior of these steels, applicability of the three known models; Rice-Tracey, Chae, and Huang, was examined and it was found that in both bainitic steels, void growth kinetics closely obeyed the Rice-Tracey model. It was concluded that the main fracture mechanism in both steels was creation of micro-cracks by coalescence of voids, initiated by cementite/matrix interface decohesion, and growth of these cracks by a shear mechanism.X1111sciescopu