The goal of the diploma thesis was to optimize the heat treatment of PROTAC 600 steel. This steel must have the appropriate combination of the mechanical properties to provide proper ballistic protection. The focus was on the measurements of hardness, tensile strength, yield strength, and elongation.
The temperatures of phase transformations temperatures, i.e. Ac1, Ac3, Ms, and Mf, were determined by dilatometry analysis and compared with the temperatures calculated from the empirical equations. For the determination of the optimal austenitization temperature, a hardenability test (sensitivity-to-grain growth) was performed. The optimal austenitization temperature was chosen based on hardness after quenching and the sizes of the prior austenite crystal grains. Rolled state samples were then quenched from the optimal austenitization temperature and tempered at temperatures from 150 °C to 400 °C. On the tempered samples, hardness measurements and microstructure characterization were carried out. Tensile strength, yield strength, and elongation were determined by tensile testing. Microstructural analysis was made to describe the changes in the microstructure of the quenched and tempered steel at various tempering conditions.
Results from the hardenability test (sensitivity-to-grain growth) show that the optimal austenitization temperature is 870 °C. After quenching from this temperature, the steel has a hardness of 677 HV10 and the size of the prior austenite crystal grains is 10 μm. The most suitable mechanical properties were achieved after tempering for one hour at a temperature of 150 °C. After this heat treatment, the steel PROTAC 600 has a hardness of 642 HV10, yield strength of 1640 MPa, the tensile strength of 2134 MPa, and elongation of 9,8 %. The ratio between yield strength and tensile strength is 0,768.
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