Effect of Al on Pearlite Transformation and Spheroidization of High Carbon Steel

Abstract

Normalizing and isothermal spheroidizing annealing tests were carried out on two types of high-carbon steel specimens with different aluminum (Al) contents. The effects of Al content on the phase transformation and spheroidization of high-carbon steel were analyzed. It was found that addition of Al element can increase the pearlitic eutectoid transition temperature of high-carbon steel, making the eutectoid point move toward the high-carbon direction, and inhibiting the precipitation of proeutectoid cementite during pearlitic transformation. The higher the Al content, the greater the undercooling degree during pearlitic transformation and the finer the pearlite lamellar spacing would be after transformation. In addition, the finer pearlite lamellar spacing allowed the test steel to achieve more phase interfaces in the same crystal range and more dislocations and other defects on the interface, thus providing more nucleation centers for lamellar cementite during isothermal spheroidizing annealing. In addition, more defects provided more diffusion channels for carbon during spheroidizing annealing and smaller interlamellar spacing to shortening the diffusion distance of carbon atoms, thus accelerating carbon diffusion rate and spheroidizing the lamellar cementite into granular one. The addition Al element can improve the machinability of steel and facilitate the commercial application of pearlitic steel.