The purpose of the graduation thesis was to research the effect of heat treatment on toughness transition temperature of martensitic precipitation hardening stainless steel PK346. The steel was homogenized, quenched and aged at different temperatures and time periods, and impact toughness was measured using the Charpy impact test. ASTM standard was used to visually assess the fractured surface, i.e. the share of brittle and ductile fracturing. A more precise fractographic analysis was carried out with a scanning electron microscope (SEM). A dilatometer was used to determine the transformation points: AC1, AC3, Ms and Mf, and to detect the coarsening of copper precipitates, which affect the stability of austenite. The austenite was measured with the X-ray diffraction analysis (XRD) before and after the aging process. Metallographic analysis on the samples was performed using a light microscope and we also measured the hardness using the Brinell test. It was established that the content of the austenite lowers the toughness transition temperature and the hardness of steel. We also determined the optimum heat treatment for the PK346 steel to achieve the lowest toughness transition temperature (-153 °C) with impact toughness of 75 J. It consists of homogenizing at 1040 °C, air quenching and double aging at 760 °C/2 h and 621 °C/4 h.