Thermal and Mechanical Characterization of Non-isothermal Tempering of an Experimental Medium-Carbon Steel

A medium-carbon steel alloyed with vanadium was analyzed by differential dilatometry, microhardness and nanoindentation to characterize the fourth tempering stage or secondary hardening zone. From isochronal heating paths at different tempering temperatures, the secondary hardening zone preceding the martensite decomposition initiation temperature was delimited. A progressive increase in microhardness was observed proportional to continuous tempering temperature increase with respect to start temperature of fourth tempering stage at the lowest heating rate. This increase was related to secondary hardening zone by alloy carbides precipitation. Likewise, isothermal tempering was carried out at the start of the fourth tempering stage to verify secondary hardening. After the thermal cycles, nanoindentation tests were performed to determine the nanohardness and prove the precipitates presence by scanning probe microscopy. A decrease in the nanohardness of the steel was observed, as well as different size and distribution precipitates depending on the heating rate.