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Sub-zero processing is an effective method to enhance the mechanical properties of steel due to the complete removal of residual austenite, and better precipitation of secondary carbides. Especially, the sub-zero processing temperature significantly influences the mechanical properties of steel. In the present study, the impacts of shallow and deep sub-zero processing on the microstructure, hardness, wear characteristics, and toughness of 440C martensitic steel were evaluated. An attempt has been made to correlate the influences of microstructural changes on these measured properties. It was found that although shallow sub-zero processing was capable of reducing the amount of residual austenite, deep sub-zero processing was required for its complete removal. Due to the low-temperature martensite formation at −196 °C, deep sub-zero processed specimens possessed a twinned martensite microstructure that significantly improved their strength. Furthermore, deep sub-zero processing increased the volume of micro-carbides by driving alloying carbides to the nearby defects. Hence, deep sub-zero processing led to a 15% improvement in hardness and up to a 60% improvement in wear resistance with an 11% drop in toughness. In contrast, shallow sub-zero processing improved the hardness and wear resistance by 7% and 23%, respectively, with a 30% drop in toughness.

Investigation on microstructural and mechanical properties of sub-zero processed AISI 440C steel