Sub- temperature Quenching Process -You Must Know Part II

Adding one or more sub-temperature quenching with heating temperature between Ac1~Ac3 between normal complete quenching and tempering can significantly improve the toughness of steel, reduce the brittleness temperature, and reduce high temperature tempering brittleness.

1. Sub-temperature quenching refines the grains, obtains an appropriate amount of uniformly distributed fine ferrite structure, suppresses crack expansion, and the ak (impact) value increases in direct proportion to the tempering temperature. Compared with conventional quenching processes, equal hardness is obtained Lower tempering temperature can be used, with higher toughness, and can inhibit stress concentration and hinder crack initiation and propagation;

2. There is undissolved ferrite in the sub-temperature quenching structure, which increases the content of carbon and alloy elements in the austenite, and there is a small amount of stable residual austenite after quenching, which can also prevent the initiation and expansion of cracks;

3. Sub-temperature quenching can reduce the segregation of harmful impurity elements in the austenite grain boundary and play a role in purifying the grain boundary;

4. The sub-temperature quenching temperature is low, and the undissolved ferrite refined by deformation quenching can prevent grain growth, and can form extremely fine austenite grains along the original coarse austenite grain boundary before quenching. Refinement increases the grain boundaries, so the content of harmful impurity elements on the unit interface decreases, which is beneficial to increase the strength and toughness and reduce the notch sensitivity.

5. Enrich P, Si, Sb and other impurities that cause reversible temper brittleness in ferrite, reduce their chances of segregation on austenite grain boundaries, and reduce grain boundary brittleness.