Effect of Cooling Rate on Microstructure Evolution and Hardness of Steel Balls

The objective of this study was to find the effect of cooling rate on microstructure and hardness of steel ball. Steel samples of phi 3 mm x 10 mm have been used under the different cooling experiments to study microstructure evolution. The microstructure is observed with optical microscopy and scanning electron microscope, respectively. The transformation temperature is determined by dilatometric curves. The results show that faster the cooling rate is, the lower is the starting and finishing temperatures of austenite-to-pearlite transformation. However, faster cooling rate has no effect on the starting temperature of martensite transformation. The prior austenite grain size increases rapidly with a higher temperature and almost remains invariant with a longer austenitization time. The hardness y of steel and cooling rates x accord with a relationship: y = 11.885x(4 )- 131.06x(3) + 443.75x(2 )- 320.21x + 387.45.