Zurnadzhy V. Improvement of mechanical properties of сost-saving steels using heat treatment according to «Quenching and Partitioning» technology

The work’s aim is increasing the nickel-free medium carbon structural steels mechanical properties complex by creating a multiphase microstructure by optimizing the technological scheme and temperature and time parameters of the Q-n-P hardening heat treatment mode. The research’s object is the structural steels’ structure and properties formation during heat treatment. The research’s subject is the regularities of Q-n-P heat treatment effect on the microstructure and mechanical properties of medium-carbon nickel-free steels with high silicon content. The research methods: optical microscopy, scanning electron and transmission electron microscopy; X-ray diffraction, dilatometric, magnetometric and durometric analysis methods; test on mechanical properties and abrasive wear. The thesis presents a theoretical generalization and a new solution of relevant scientific and technical problem of creating high-strength products producing technologies made of cost-saving nickel-free steels by developing heat treatment modes based on the Q-n-P principle. The prospects of Q-n-P heat treatment technology using to standard steel 60Si2CrVА and experimental 55Si3Mn2CrVMoNbA in order to create a high-strength state (UTS=1500-2374 МПа) with increased ductility (UE up to 22 %) and impact toughness are shown. The fact of interfacial carbon partitioning during Q-n-P treatment is established. It leads to an increase in carbon content in γ phase (up to 1,32 %) and ensures the heterogeneous martensite/austenite/bainite structure formation, containing up to 25% retained austenite. For the first time, the thermal strengthening mode of steel grinding balls based on the Q-n-P principle was developed and introduced into production. The mode application provides uniform cross section hardness 54-58 HRC of the balls over with high impact resistance, the cracks absence.