ANS:C - decomposition of retained austenite upon heat treatment.

Secondary hardening in steels typically arises from the precipitation of fine alloy carbides at high temperatures, but it can also occur due to the decomposition of retained austenite upon heat treatment. Retained austenite is a metastable phase of steel that remains in the microstructure after quenching from the austenitic region. During subsequent heat treatment, such as tempering, retained austenite may decompose into other phases, such as martensite or secondary carbides. The decomposition of retained austenite contributes to secondary hardening by transforming the metastable phase into more stable phases with higher hardness. This transformation involves the redistribution of carbon atoms within the microstructure, leading to the formation of additional strengthening phases. As a result, the decomposition of retained austenite upon heat treatment can lead to an increase in hardness and strength, contributing to secondary hardening in certain types of steels.