The influences of different austenitisation temperatures on the matrix microstructure and mechanical properties (K1C and hardness) of one cast have been investigated. 300M steel has been austenitised at 925 °C for increasing holding times from 1h to 4h and 1025 °C for 2h holding time. Microstructures were examined by optical and scanning electron microscopy. It has been found out that higher austenitisation temperature alter the microstructure, the fine (<600 nm) precipitate population and subsequently the fracture toughness. The reduced volume fraction of fine particles increased the hardenability of the steel, resulting in an increased percentage of upper bainite, that led to a lower temper response compared to 925 austenitisation temperature. The improved K1C value was attributed to increased dissolution of fine particles due to higher austenitisation temperature
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