The quench process of large size components is always problematic owing to non-uniform phase formation. In this context, the influence of cooling rate on phase transformation, for different cooling rates, in 300M steel is studied. A combination of high resolution dilatometry, optical microscopy, Vickers micro hardness measurements were employed to accurately determine the critical transformation temperature, and to identify microstructure constituents (bainite, martensite, and retained austenite).

A mixture of bainite, martensite and some retained austenite phases were observed to appear at cooling rate of 0.25°C/sec whereas for a cooling rate of 50°C/sec the microstructure is totally martensitic.

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