In this diploma thesis the microstructure development during the continuous cooling of PROTAC 600 steel was analyzed in order to determine the optimum austenitization temperature and cooling rates. The two parameters are crucial for meeting the required hardness of PROTAC 600 steel.
Experimental part consisted of dilatometer tests, hardness measurements and metallographic analysis. Dilatometer tests were performed, using specimens in the form of a cylinder with a length of 10 mm and a diameter of 3 mm, to determine the austenitization and phase transformation temperatures. Based on the obtained results, a continuous cooling diagram (CCT) was drawn from which the heat treatment parameters of the armor plate made of PROTAC 600 steel were determined.
The temperatures determined with dilatometric analysis are: Ac1 = 720 °C, Ac3 = 780 °C, Ms = 260 °C and Mf = 154 °C. These values were compared with the transformation temperatures calculated with the empirical equations. The evolution of the microstructure was monitored during the cooling from austenitization temperature (870 °C) with cooling rates between 0.05 °C/s and 100 °C/s. At a cooling rate of up to 0.1 °C/s, the microstructure consists of ferrite, perlite, bainite and martensite. At cooling rates between 0.3 °C/s and 1 °C/s, bainite and martensite are present in the microstructure. When upper critical cooling rate of 3 °C/s is applied, only martensite is present in the microstructure. Increased cooling rate while affecting the microstructure also causes the hardness increment. The maximum hardness measured is 656 HV0.5 at a cooling rate of 20 °C/s and the lowest is 449 HV0.5 at a cooling rate of 0.1 °C/s. The minimum required Brinell hardness of 570 HB (converted to Vickers 600 HV) is achieved at a cooling rate of 0.3 °C/s.
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