Effect of austempering processing parameters on microstructure and hardness of nodular cast iron dense bar

In order to find the optimum austempering process parameters for QT500-7 nodular cast iron dense bar produced by horizontal continuous casting(HCC), the orthogonal experiment was used to optimize the parameters, i. e austenitizing temperature, austenitizing time, austempering temperature and austempering time, according to the microstructure and hardness of the dense bar. The results show that the major factors influencing hardness of the austempered nodular cast iron dense bar are austempering temperature and austenitizing temperature. As the temperatures increases, the fraction of retained austenite increases and the hardness discreases. The highest hardness of the austempered QT500-7 dense bar is obtained by using following process parameters: austenitizing temperature and austempering temperature are 840°C and 280°C, and austenitizing time and austempering time are 60 min and 100 min, respectively. The microstructure of the dense bar under the optimum austempering process is composed of thin acicular ferrite and retained austenite, and the hardness is 476 HBW.