Abstract
A study was performed of the effect of various cooling rates from austenitizing temperature of 950 °C and tempering on microstructure, strength properties, transition behaviour of Charpy-notch toughness and of fracture toughness, respectively. Furthermore, the step-cooling heat treatment (SCHT) procedure was used to find out the susceptibility of microstructures generated to the embrittlement which can occur due to long-term operation at elevated temperature. It was proved that the tempered bainitic structure exhibited the best properties, the lowest transition temperatures evaluated from notch-toughness transition curve as well as from fracture toughness temperature curve. On the contrary, the poor fracture properties were exhibited by mix ferrite-bainite structure. The SCHT had practically no influence on transition behaviour of notch and fracture toughness. The transition fracture behaviour of a range of structures examined was explained by using the concept of critical tensile stress for cleavage initiated brittle fracture.

Key words
austenitizing, tempering, microstructure, strength properties, transition behaviour, Cr-Mo-V cast steels, fracture toughness

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