| Bearing is one of the important mechanical parts. It has been deeply researched since developing of higher carbon and chromium bearing steels in more than a century. Spheroidizing annealing is very important heat treatment process for bearings made of GCr15 steels, as has important affecting on the service life of bearings resulting from the mecrostructure after quenching and tempering. The traditional spheroidizing annealing process is a lengthy process which costs a lot of energy. So, it is continually pursuit of goals for heat treatment engineers to shorten the time of spheroidizing annealing of bearing steel. In this paper, the starting and ending eutectoid transformation temperature of GCr15 steel were measured by using of Gleeble-3500 thermal simulation testing machine. The influence of austenitizing time,remained carbide morphology, as well as isothermal temperature and time on the microstructure and hardness of GCr15 are examined by means of analysis of metallographic microscope and scanning electron microscopy, hardness testing, etc. after heat treatment through box heat treatment furnace. The results are as follow:(1) The starting and ending temperature of ferrite transforming to austenite is 756℃ and 807℃ respectively as GCr15 steel is heated at 1℃/s. While cooling at same rating, the starting and ending eutectoid transformation ones is 743℃ and 644℃ respectively.(2)Considering the range of ferrite transforming to austenite and the characteristics of the box heat treatment furnace, the 805℃ is selected for austenitizing. When it is austenitized for 30 minutes, the average size of residual carbides of GCr15 steel is about 0.49μm,volume fraction about 10.89%,while for 60 minutes, the average size and volume fraction 0.42μm and 2.64%.(3)If the sample is austenitized at 805℃ for 30 minutes in box heat treatment furnace, then cooled to 745℃ with furnace and held for 90 minutes, finally cooled in water to room temperature, the Rockwell hardness of the sample could be about 75 HRB. But it is cooled to 715℃ with furnace and then held for the certain time, finally cooled to room temperature in water, lamellar carbides couldn’t be seen under scanning electron microscopy. The size of the spheroidal carbides will become bigger with the longer of isothermal time. For instance, when sample is held at 715℃ for 30 minutes, the average size of carbide is about 0.6524μm,the Rockwell hardness is about 18HRB.If the isothermal time increase to 150 minutes, the average size of carbide particles is about 0.9114μm, the Rockwell hardness is 15HRB,Brinell hardness is about 196HBW.When the isothermal temperature is 645℃ for 30 minutes, the lamellar carbides can be seen easily under scanning electron microscopy. The Rockwell hardness is about 22HRB at the same time. If the isothermal time is longer to 150 minutes, the lamellar carbides will be more. The Rockwell hardness will be the value of 27HRB.(4)According to the experiment of researching, the spheroidizing process is suggested that GCr15 steel is austenitized at 805℃ for 30 minutes, and then cooled to 715℃ with furnace and held for 150 minutes, again further cooled to 650℃ with the furnace, finally cooled to room temperature in air. The microstructure of spheroidal pearlite will be obtained through above process. The average size of carbide is about 1.35 μm, and average brinell hardness is 181HBW. The spheroidizing time will be shorten to 6.5 hours about. |
PDF Full Text Request