A pre-formed martensitic quenching plus inverted multi-step austempering process is performed on a medium-carbon alloy steel: after austenitization, the specimen is cooled to 175 °C below the martensite start temperature (Ms), then raised to the first austempering step at 220 °C for 5 h, followed by the second step at 300 °C and held for 3 h. The conventional inverted multi-step austempering process and the one-step austempering process are carried out to form a contrast test. Through pre-formed martensitic quenching plus an inverted multi-step austempering process, the martensite/austenite (M/A) and bainitic ferrite in the microstructure are refined. The total time used for heat treatment is reduced markedly due to accelerating bainitic ferrite formation. This study provides theoretical support for the process design of ultra-fine bainitic steel.
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