The thesis investigates the precipitation hardened steel UTOPNiCu properties and behaviour in various heat-treated conditions. First, samples were cut out from various parts of the forging with transverse dimensions of 405 x 185 mm, which were then austenitized at 900 °C (for 1 hour) and aged at 550 °C (for 1, 2, 4, 8, 16, 24 and 48 hours). The focal point of the research was the influence of heat treatment on microstructure and mechanical properties at various durations of precipitation hardening. Therefore, in the experimental part, the following procedures were carried out: tensile material testing, impact toughness and hardness measuring, microstructure observing using an optical microscope, determining the retained austenite volume fraction using the X-ray phase analysis as well as dilatometry with a CCT (Continuous-Cooling-Transformation) diagram construction. It was discovered that both the appropriate austenite grain size and combination of mechanical properties are achieved after 1 hour of austenitizing at 900 °C and between 4 and 8 hours of precipitation hardening at 550 °C. There was no trace of retained austenite found in steel. The observed dimensional changes during dilatometry indicated the key processes' outcome during the first hour of aging at 550 °C as well as the interaction between precipitation and tempering of martensitic base. The construction of a CCT diagram enabled a broader understanding of steel UTOPNiCu properties, phase transformations and heat treatment.
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