Structure effect on fracture resistance of large-sized articles of 50KhN3M steel

To improve the quality of heat treatment, a quantitative correlation of structures in different cross-sections of an article made of the 50KhN3M steel and the alteration of characteristics of resistance to plastic and brittle fracture are determined. Apart from determining standard mechanical properties and impact strength, optical and electron microscopy of the luminescent and raster type and microfractography are applied. It is shown that during quenching at the rate lower rhat 2 deg C/s in articles made of the 50KhN3M steel, a mixed structure is formed which consists of martensite, lower and upper bainite and residual austenite. The formation of the upper bainite during quenching reduces plasticity and characteristics of resistance to the fracture of steel. To considerably increase parameters of fracture resistance, quenching techniques which increase the cooling rate should be used. Tempering parameters should also be increased