As a part of this thesis, we studied the influence of different combinations of heat treatments on the mechanical properties of structural steel SIQUAL 7765.
The steel was in the form of bars with diameter of approx. 30 – 40 mm, which were made by two hot forming processes – rolling and forging. With the help of dilatometer analysis, we determined the variable temperatures Ac1 = 770 °C, Ac3 = 843 °C, Ms = 370 °C and Mf = 200 °C, by which the optimal methods of heat treatment was determined. The heat treatment was carried out in three ways: one pair of samples was pearlitizated, quenched and tempered, the second pair was spheroidizational annealed, quenched and tempered, and the third pair was only quenched and tempered. The temperatures of austenitisation (Tγ) and tempering (Tp) were the same for all samples, namely 880 °C and 640 °C, respectively. The hardness measurements were performed on the steel in its delivered state, before / after quenching and tempering. The impact toughness tests were done at room temperature and at -40 °C. We also determined the yield stress, tensile strength, elongation, contraction and analyzed the microstructure. The best results, i. e. the highest strength properties and toughness, were achieved on the rolled sample, where a combination of spheroidization annealing, hardening and tempering was carried out. In addition to the highest agreed yield stress and tensile strenght, we also obtained the highest toughness at a temperature of -40 °C
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