Effect of the Normalizing Temperature on the Size and Homogeneity of Austenite Grain in 20CrMnTi Carburized Gear Steel

This study investigated the effects of normalizing temperature, carburizing temperature, and time on the grain size and homogeneity of parent austenite grain (PAG) in 20CrMnTi gear steel. The grain coarsening mechanism was revealed to clarify the significance of normalizing temperature. The results manifested two different critical carburizing parameters that showed inverse grain evolutions at 940 and 800 degrees C normalizing temperatures, respectively. Compared to PAG without normalizing, the PAG size was smaller when the normalizing temperature was 940 degrees C for the carburizing temperature <= 970 degrees C and the holding time <= 4 h. Additionally, PAG with normalizing had a lower inhomogeneity factor than that without normalizing. When the carburizing process went beyond the critical carburizing parameters, grain coarsening and mixed grain appeared. Another critical carburizing parameter was transformed to a low temperature and a short time at 800 degrees C normalizing temperature. Additionally, under the same carburizing, the average grain size and mixed grain degree after normalizing at 940 degrees C were smaller than those at 800 degrees C. The redissolution and maturation of the (Nb, Ti, Mo)(C, N) particles during carburizing yielded grain coarsening and mixed grain phenomenon. The normalizing temperatures of 940 and 800 degrees C had little effect on the distribution of the (Nb, Ti, and Mo)(C, N) particles. However, the high-temperature normalizing at 940 degrees C generated fine and uniform austenite grains due to the microstructure heredity by reaustenitizing. Nevertheless, ferrite + austenite duple-phase microstructure was obtained at 800 degrees C, where the coarse ferrite grains increased the inhomogeneity of the austenite during carburizing.