Taking into account the importance of the amount of bainite phase on the microstructure of cast irons and its influence on the improvement of mechanical properties, this research selected an alloy of gray cast iron containing Nickel-Molybdenum and conducted the austenitising and austempering processes at 900°C and 400°C for 60 minutes, respectively. The way of bainite phase formation and the effect of sample thickness, that is, cooling rate, were examined by selecting a standard staircase sample. The results indicated that, by increasing the cross sections of samples, the martensite percentage decreases and the phase proportion of bainitic ferrite increases.
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