Effect of cooling rate on carbide precipitation behavior of M50 steel

The microstructure and properties of M50 steel were examined using techniques such as SEM, TEM, XRD, and hardness testing, aiming to investigate the influence of cooling rate on the precipitation of carbide in M50 steel, providing theoretical and data support for the regulation of carbide in M50 steel used in aerospace bearings. Results show that the content of precipitated carbide and bainite increases with the lower limit of the furnace cooling temperature decreases. When the lower limit of the furnace cooling temperature is 800 °C, the bainite content is about 64.2% higher than that in the direct salt bath without furnace cooling. The hardness after austempering and tempering appears downtrend with the decrease of the lower limit of the furnace cooling, and the lowest values are 59.7 HRC and 55.8 HRC respectively at the lower limit of 800 °C, which is mainly related to the decomposition of martensite and the precipitation of carbide. With the extension of the tooling controlled cooling time, the content of precipitated carbide increases, and the content of residual austenite decreases. The hardness of the steel decreases with the increase of the tooling controlled cooling time. The hardness after austempering and tempering corresponding to 12 min is the lowest, which are 59.9 HRC and 56.9 HRC, respectively. This is mainly due to the gradual increase of precipitated carbide during the tooling controlled cooling process, which has a hindrance effect on the diffusion and precipitation of secondary carbide during the tempering process.