GCr15 Bearing Steel Effect of Tempering Temperature On Microstructure and Properties

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1.The quenched structure of chromium bearing steel is quenched martensite, undissolved secondary carbide and retained austenite. Among them, martensite is a supersaturated solid solution of C in a-Fe, which is a metastable phase. Supersaturated carbon atoms have a tendency to spontaneously precipitate from the solid solution; austenite is a high temperature stable phase, which is unstable at room temperature and has spontaneous Precipitated carbides tend to transform into a-Fe solid solution; the carbide phase tends to aggregate and grow, and the dispersity tends to decrease spontaneously; the internal stress and subcrystalline structure formed during the quenching process are also unstable in energy and spontaneously The tendency to transition to a steady state, these instabilities, and the tendency to spontaneously transition to a steady state, are all thermodynamically determined, and the transitions are all closely related to diffusion.

The higher the tempering temperature is, the faster and more thorough this process will be. The retained austenite is a metastable phase that exists in a small amount after the bearing material is quenched and tempered, and will continue to occur during the storage, transportation and use of the bearing. The transformation to martensite structure, due to the difference in specific volume between austenite structure and martensite structure, causes the size of bearing parts to change and affects the service life of the bearing.

JB/T 1255- 2001 “Technical Conditions for Heat Treatment of High-Carbon Chromium Bearing Steel Rolling Bearing Parts” For the detection of martensite, troostite, retained austenite, etc., only the standard map is used to judge whether it is qualified or not under an optical microscope. In order to improve the dimensional stability and comprehensive performance of bearings in my country, JB/T 1255–2014 “Technical Conditions for Heat Treatment of Rolling Bearing Parts of High Carbon Chromium Bearing Steel” on the basis of the original standard, increases the content of residual austenite after quenching and tempering of bearing parts. , For parts treated by conventional quenching and tempering process, the content of retained austenite is generally less than 15%.

For this reason, this study took the GCr15 material of the outer ring of the deep groove ball bearing as the test object, and by changing the tempering temperature, the changes in the hardness, retained austenite content and surface residual stress of the material after tempering at the same time were studied. Provide a reference for the formulation of the process.

2.Test materials and methods

The quenching oil is bright quenching oil with a flash point of 186 ℃, a viscosity of 23.76 mm2/s, a maximum cooling rate V…=98.4 ℃/s, and a characteristic temperature of 593.7 ℃. The deep groove ball bearings with an outer diameter of φ160mm, a width of 37 mm, and an effective thickness of 9.6 mm made of GCr15 steel were put into the RJCD-240 roll bar quenching and tempering production furnace, and heated to 850 ℃ for heat preservation. After 30 minutes of oil cooling and quenching, 5 pieces were taken out, and 1 piece was cut along the axial direction with a wire cutting machine to cut 4 samples, and then these 4 samples and the remaining 4 outer rings were added according to 1 sample. One outer ring was tempered at 165, 200, 250, and 300 °C, respectively, for a holding time of 3.5 hours, and air-cooled to room temperature after being released.

According to the requirements of JB/T1255-2014, the hardness of the bearing outer ring tempered at different temperatures is measured with a Rockwell hardness tester of type HR150-A, and the residual austenite tester of type AMI-21 is used to measure its residual austenite. The surface residual stress was measured with an X-ray diffractometer with a model of dmax2500pc. The tempered wire-cut samples were ground and polished, and then etched with 4% (mass fraction) nitric acid alcohol solution to make metallographic samples. The microstructure and morphology of the materials at different tempering temperatures were observed under an optical microscope.

2.1. Influence of tempering temperature on hardness and surface residual stress

Measure the hardness values of 5 points evenly along the end face of the outer ring around the four samples tempered at different temperatures, with the increase of tempering temperature, the hardness decreased from HRC 61.7 to HRC 56.2, with a decreasing range of 9.78%. It can be seen from Figure 2 that with the increase of tempering temperature, the surface residual stress generally shows a downward trend, from 706.8 MPa at 165 °C to 382.2 MPa at 300 °C, with a decrease of 84.9%.

The decrease in hardness is due to the decomposition of martensite during the tempering process to form an eutectoid structure. At the same time, the precipitated carbides aggregate and coarsen during the tempering and heat preservation process, and are separated from the coherent relationship with the matrix. Error density is reduced. And with the increase of tempering temperature, the martensite decomposes completely, the coherent relationship between the precipitated carbon and the matrix disappears, and the stress field disappears, resulting in the softening of the material. In addition, the higher the tempering temperature, the quenching stress is released, so the surface stress also decreases significantly. The surface residual stress at 250℃ is higher than 200℃, which may be related to the higher machining stress of the test surface.

2.2. Effect of tempering temperature on retained austenite content

Measure the retained austenite content at 5 points evenly along the end face and circumference of the outer ring for the samples tempered at different temperatures

With the increase of tempering temperature, the retained austenite content decreased from 9.88% to 3.26%. The wire-cut samples are mechanically ground and polished according to the requirements of JB/T 1255-2014, and then corroded by 4% nitric acid alcohol. The microstructure of the oil-cooled samples in the quenched state is mainly acicular martensite, granular carbides and a small amount of metastable retained austenite. It can be seen from Figure 4 that with the increase of tempering temperature, carbides gradually aggregate and grow up. This is because with the increase of tempering temperature, carbides are precipitated from martensite and transformed into tempered martensite, which is in the shape of dark gray needles. When the tempering temperature reaches 300C, supersaturated carbon is precipitated from martensite. Gather and grow gradually.

3.Conclusion

1) With the increase of tempering temperature, the hardness of CCr15 steel decreases from HRC61.7 at 165℃ to HRC56.2 at 300℃; the surface residual stress decreases gradually.

2) As the tempering temperature increases, the retained austenite content of GCr15 steel decreases from 9.88% at 165°C to 3.26% at 300°C, which is less than the standard requirement of less than 15%; Carbides gradually aggregated and grew up in the microstructure.

Editor: Frank Lee

Copyright: SIMUWU Vacuum Furnace