Crack resistance and micromechanisms of fracture of thermomechanically treated high-strength steel

Abstract

We study the influence of thermomechanical treatment with deformation by the method of hydrostatic extrusion on the parameters of crack resistance of 45KhN2MFSh high-strength steel and plot the dependences of the critical stress intensity factorK Ic and critical crack opening displacements δc on temperature. It is shown that these curves have the threshold character. The results of microfractographic analysis demonstrate that changes in crack resistance observed as temperature decreases are accompanied by changes in the micromechanisms of fracture in the regions of the onset of crack propagation, which may take place under the condition of changes in the second-order stress-strain state. We show that the temperature curves of the parameters of crack resistance can be efficiently used in determining the temperature of brittle-ductile transition. In the considered case, this temperature does not depend on the size of the specimen and the loading mode and characterizes the structural state of the cracked material. As compared to conventional modes of thermal treatment, thermomechanical treatment guarantees much higher values of crack resistance, especially at low temperatures, and decreases the threshold of cold brittleness for 45KhN2MFSh steel by 20°C. The indicated increase in crack resistance is explained by the hereditary influence of the deformational substructure on the structural and morphological parameters of martensite.