A study aimed at identifying the cause of crack formation after heat treatment of alloy steel parts with high hardenability is presented. A CCT diagram was constructed by the dilatometric method, and the temperature-time intervals for the austenite transformation were established. The hydrogen content in different heats of the studied steel was determined. An analysis of the microstructure and hardness distribution over the cross-section of the part was carried out. The computational modeling of the processes occurring during the heat treatment of parts was carried out. The significant carbon segregation in the axial zone of the part was determined using optical emission spectroscopy. This segregation led to a decrease in the martensite transformation start temperature in the core of the part.