CORRELATION BETWEEN HARDNESS AND MICROSTRUCTURAL EVOLUTION OF P22 STEEL SUBJECTED TO ANNEALING

Abstract

This study investigates the correlation between hardness and microstructural evolution in P22 carbon steel subjected to annealing. Brinell hardness testing revealed a reduction from 161.3 BHN in the as-received condition to 121.3 BHN after annealing, corresponding to an approximate 25% decrease. Optical microscopy of the as-received sample showed irregularly shaped grains with elongated, clustered carbide precipitates concentrated along grain boundaries, which contributed to higher hardness by impeding dislocation motion. In contrast, the annealed microstructure exhibited coarser, more homogeneous grains with spheroidized and uniformly dispersed carbides, consistent with thermal stability achieved through recovery and recrystallization. Complementary SEM analysis further revealed that machining induced grooves and work-hardened surface features present in the as received specimen disappeared after annealing, leaving a uniform recrystallized surface morphology. The combined hardness and microstructural observations demonstrate that annealing promotes softening by reducing dislocation density and redistributing carbides, thereby enhancing ductility at the expense of strength. These findings provide insight into the role of annealing in balancing mechanical performance and microstructural stability of P22 steel for high-temperature service applications.