Cooling Behavior of Heat Treated Carburizing Steel 23CrNi3Mo with Proper Hardness Gradient and Microstructure Distribution
JIANG Bo1, DAI Guangyong2, YAN Yongming1, LIU Guanglei2, WANG Zhilin2,
WANG Guocun2, LIU Yazheng1
1 School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083; 2 Xining Special Steel Co. Ltd, Xining 810005
Abstract: The continuous cooling transformation and isothermal transformation were investigated by thermal simulation machine Gleeble-1500, optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM). Based on the investigation, a new heat treatment cooling process was designed. Results show that the microstructures at the surface, the transition zone and the matrix of the steel after carburizing at the cooling rates of 0.05 ℃/s and 0.1 ℃/s are high carbon martensite, a mixed microstructure of lower bainite and high carbon martensite and lower bainite, respectively. The network carbide was formed along the boundary in the microstructure at the surface when the carburizing steel was cooled from the quenching temperature with a lower cooling rate from 0.05 ℃/s to 3 ℃/s. The temperature of bainite transformation of steel without carburizing ranges from 375 ℃ to 450 ℃. The new heat treatment cooling process was set as follow: the carburizing steel was cooled from 880 ℃ with a fast cooling rate of 5 ℃/s to the bainite transformation zone, and put into the salt bath furnace soaking for 5-10 minutes at 450 ℃; after that, the steel was slowly cooled at the cooling rate lower than 0.1 ℃/s. Microstructural examination indicates that the depth of carburizing case of the specimen after the new process is 1.4 mm, which is nearly the same with the drill bit from Atlas abroad (1.2 mm). The hardness distribution of the specimen after the new process is smoother. The obtained microstructures at the surface, the transition zone and the matrix of the specimen after the new process and the drill bit from Atlas abroad are both high carbon martensite, a mixed microstructure of lower bainite and high carbon martensite and lower bainite, respectively. The carburizing steel which has a same level as the drill bit abroad was obtained by designing the new heat treatment cooling process.
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2017, Vol. 31 
