Effect of Carbon Concentration Gradient on Multi-Level Composite Microstructure and Performance of M50NiL Steel

By means of the measurement of carbon concentration gradient in the carburized layer, the microstructure observation after heat treatment, and the detection of hardness distribution, the effect of carbon concentration gradient on the multi-level composite microstructure and hardness of M50NiL steel after vacuum isothermal quenching was studied. Results show that from the surface to the core of the steel, the carbon concentration gradually decreased from 1.42 to 0.13%. Referring to the definition of carbon steel, the carburized layer was divided into ultra-high carbon zone, high carbon zone, medium carbon zone, and medium-low carbon zone. As the depth from the surface increased, the morphology of residual austenite after isothermal quenching changed from blocky to film. In the ultra-high carbon zone, the lower bainite carbides did not spheroidize after growing up during tempering at 545 °C and still maintained the parallel arrangement. Due to the influence of carbon concentration, the hardness of the surface layer after isothermal quenching mainly depends on the transformation degree of residual austenite and the amount of pre-formed martensite. Through improving the microstructure of the carburized layer, the impact property of M50NiL steel increased significantly.