Static CCT Curve Measurement and Microstructure Analysis of 18CrNiMo7-6 Steel

The continuous cooling expansion curve of 18CrNiMo7-6 wind power steel was tested by using a Gleeble-3800 thermal simulator at a cooling rate of 0. 1-30 ℃/s. The static continuous cooling transformation curve （CCT curve） of 18CrNiMo7-6 wind power steel was plotted by using metallographic hardness method. The microstructure transformation law of 18CrNiMo7-6 wind power steel under different cooling rate conditions was analyzed. The experimental results show that the Ac1 phase transition temperature of 18CrNiMo7-6 wind power steel is 765 ℃， and the Ac3 phase transition tempera ture is 843 ℃. When the cooling rate is less than 0. 5 ℃/s， the microstructure of the experimental steel is ferrite and pearlite， and high-temperature phase transformation occurs； At a cooling rate of 0. 5-1 ℃/s， ferrite disappears and plate-like martensite is initially formed； Starting from 2 ℃/s， the microstructure is mainly bainite and martensite， with both medium temperature and low temperature phase transformations occurring simultaneously； After 10 ℃/s， the bainite content de creases and the martensite content increases with the increase of cooling rate； When the cooling rate is greater than 20 ℃/s， the microstructure is entirely martensitic with only low-temperature phase transformation occurring； During the process of increasing the cooling rate from 0. 1 ℃/s to 30 ℃/s， the hardness of the wind power steel shows an upward trend.