Tensile fracture of tempered martensitic steel

The work is devoted to the study of development regularities of plastic deformation and destruction of medium-alloy steel with a tempered martensite structure (0.34C–Cr–Ni–3Mo–V–Fe steel) under active tension. This steel is characterized by a multi-scale defect structure and contains cementite precipitates and special carbides. An experimental study of the evolution of defect and carbide subsystems during plastic deformation required the use of different methods: optical and electron (scanning and transmission) microscopy; X-ray structural analysis; measurements of quantitative characteristics of the microstructure and pattern of microcracks and their statistical processing. The research revealed that the places of significant localization of plastic deformation at the pre-destruction stage are the boundary areas of grains (former austenitic and real marten­sitic); all structural components of tempered martensite (plates, packets, blocks of laths, laths). A comparison of nature of the deformation relief and the fine structure formed before destruction with the pattern of fractures at various structural-scale levels indicates that the destruction of the steel under study, as well as the plastic deformation preceding it, bears the features of heredity of the original internal structure. Thus, the fracture of the studied steel has a multi-level nature, caused by: hierarchy of the initial internal microstructure; evolution of carbide phases; localization of plastic deformation developing at all stages of plastic deformation and, as a consequence, preparing the paths for microcracks propagation.