G20Cr2Ni4 steel is commonly used in the production of rolling mills and super large bearings that can withstand heavy load impacts. After carburizing treatment, the material surface has high hardness, wear resistance, and contact fatigue strength, while the carbon content in the core is low and has good toughness, which can withstand large impact loads. A certain steel rolling mill uses rolling mill bearings supplied by two manufacturers, model FCDP100144530. One of the manufacturers' bearings experienced roller fracture when it was put into operation. The author conducted testing and analysis on the chemical composition, hardness, and microstructure of two bearing rollers to determine their essential differences.
1. Analysis and testing
1.1 Macro analysis
Macroscopic inspection was conducted on the failed cylindrical bearings of the two manufacturers (sample 1 for non fragile bearing rollers and sample 2 for fragile bearing rollers). The rollers had a hollow cylindrical shape with a central hole, and the roller size was 50 mm × 100 mm, with an inner hole of 10 mm. Macroscopic fracture analysis was performed on the failed samples, with sample 1 showing fatigue fracture morphology and sample 2 showing instantaneous fracture morphology with the inner hole as the crack source. Upon testing the surface quality of the sample, it was found that the inner wall of sample 2 was rougher than that of sample 1, and there were visible cracks on both end faces of sample 2, as shown in Figure 1.
1.2 Chemical composition analysis
The chemical composition analysis of the failed roller was carried out using the SPECTRLAB M10 photoelectric direct reading spectrometer, and the results are shown in Table 1. According to Table 1, the materials of these two rollers meet the composition requirements of G20Cr2Ni4 steel specified in GB/T 3203-1982 "Technical Conditions for Carbonized Bearing Steel".
Fig． 1 Photos of the failure bearing roller
Table 1 Chemical composition of the bearings ( wt%)
1.3 Microstructure analysis
Cut samples with a height of 20mm from the failed samples 1 and 2, grind and polish them, and observe them using an Olympus GX51 metallographic microscope after being corroded by a 4% nitric acid alcohol solution. The microstructure of the carburized layer in samples 1 and 2 is composed of fine hidden martensite and small carbides, while the microstructure in the center is composed of lath martensite and a small amount of bainite, as shown in Figure 2.
The area with cracks on the roller end face of sample 2 was ground, and the crack depth was measured to be 0.4 ～ 0.6 mm, After corrosion, it was found that there were clustered block carbides near the surface in the quenched layer (see Figure 3 (a)) and elongated carbides (see Figure 3 (b)). The sample is corroded with a saturated picric acid solution and the original austenite grain size is observed. The austenite grain size required by GB/T 3203-1982 "Technical Conditions for Carbonized Bearing Steel" is grade 5-8. After testing, the austenite grain size of sample 1 was grade 8, and that of sample 2 was grade 9. The austenite grain size of sample 2 exceeded the requirements of GB/T 3203.
Fig． 2 Microstructure of the samples ( a) sample 1,surface; ( b) sample 1,center; ( c) sample 2,surface; ( d) sample 2,center
It is understood that the production process of G20Cr2Ni4 steel rollers by two manufacturers is the same, which involves turning, drilling, carburizing, quenching, high temperature tempering, secondary quenching and low temperature tempering. According to JB/T 8881-2001 "Technical Conditions for Carbonization Heat Treatment of Rolling Bearing Parts", the depth and hardness test results of the carburized layer on the roller are shown in Table 2. The depth of the carburized layer meets the requirements, and the surface hardness of samples 1 and 2 is relatively low, mainly because the samples have already been used. Due to the decrease in hardness caused by heating, it is not possible to determine whether the original hardness of the material is qualified.
2. Analysis results
By comparing and analyzing the samples from two manufacturers, the following results can be obtained: ① The microstructure of the two samples is the same, both are fine cryptocrystalline martensite and fine carbide, and the central structure is Flat noodles martensite and a small amount of bainite. ② The depth of the quenched layer of the sample meets the requirements, but the hardness is slightly lower than the required range. ③ The fracture morphology of the sample section is different, with sample 1 showing fatigue fracture and sample 2 showing instantaneous fracture. ④ The inner wall of sample 2 is relatively rough, with visible cracks on the end face, and large carbides in block and strip shapes near the cracks.
Fig． 3 Carbide in sample 2 near surface ( a) blocky carbide; ( b) strip carbide
Due to the fact that the manufacturer produces double-sided carburized rollers, the inner hole is carburized without mud blockage, which increases the wear resistance of the inner hole surface. However, this requires higher quality of the inner hole wall. However, the inner hole wall of sample 2 is relatively rough, and the micro grooves become the source of fatigue cracks, causing the roller to crack outward from the inner hole wall. In addition, the strip-shaped carbides generated during the carburizing process generate significant thermal and structural stresses during quenching, which accelerate the failure of the roller due to quenching cracks.
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