Effects of transformation temperature and austenitization temperature on the transformation kinetics and microstructure of a Nb microalloyed high-carbon steel
In order to optimize the heat treatment process of a Nb microalloyed high-carbon steel,the effects of reheating temperature and transformation temperature on pearlite transformation kinetics and microstructure and properties were studied by dilatometry,optical microscope,scanning electron micro-scope,electron backscattering diffraction and hardness tests.The results show that the pearlite trans-formation kinetics is reduced by increasing the reheating temperature.One reason is that the number of pearlite nucleation sites decreases with the increase of prior austenite grain size.On the other hand,the solubility of Nb increases with the reheating temperature,which decreases the carbon diffusion coeffi-cient and thus decreases the pearlite growth rate.The pearlite transformation kinetics decreases as the transformation temperature decreases from 625 ℃ to 575 ℃.In addition,when the transformation tem-perature decreases from 625 ℃ to 575 ℃,the pearlite nucleation mechanism changes from nucleation mainly at the corners and edges of prior austenite to nucleation mainly at the surfaces and edges.Moreover,the decrease in transformation temperature can refine the pearlite lamellar spacing and im-prove the hardness of the pearlitic steel.The pearlite nodule size is also significantly refined with the de-crease of transformation temperature and reheating temperature.Among the three studied processes,900 ℃ reheating with 575 ℃ transformation technology provides the highest hardness and the smallest nod-ule size.Therefore,in order to obtain the best combination of strength and toughness,it is recommen-ded to reduce the reheating temperature and transformation temperature.
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