Cause analysis and improvement measures of cracks in extra large bearing sleeves made of high carbon chromium steel

Cause analysis and improvement measures of cracks in extra large bearing sleeves made of high carbon chromium steel

Cracks were found in the non-destructive testing process of a super large bearing sleeve after grinding, which led to the failure of normal production. The bearing sleeve is made of GCr15SiMn steel with outer diameter of 800mm and height of 80mm. The processing procedure is: raw material inspection → sawing → forging → spheroidizing annealing → turning → heat treatment → grinding → magnetic particle testing → assembly. In this paper, through the use of macro examination, metallographic examination and other analysis methods, from the bearing sleeve material quality and heat treatment quality of bearing sleeve crack comprehensive detection and analysis, and from the heat treatment process to study the measures should be taken, so as to avoid the recurrence of cracks.

Physical and chemical examination

Macroscopic examination

Cracks were found in the fluorescent magnetic particle testing process of the oversize bearing sleeve. The cracks are located on the end face of the bearing sleeve, in a linear shape, roughly distributed along the circumference, with a length of about 3cm. The appearance is shown in Figure 1.

Figure 1 crack appearance

Chemical composition analysis

The sample size is 15mm at the crack of bearing sleeve × 15mm × 15mm. ARL-4460 direct reading spectrometer was used to detect the chemical components. The results are shown in Table 1. According to the test results, the bearing sleeve material is GCr15SiMn steel, which meets the requirements of high carbon chromium bearing steel (GB / T 18254-2016).

Table 1 chemical composition of bearing sleeve (mass fraction) (%)

Project	C	Mn	Si	Cr	Mo	P	S	Ni	Cu
Detection result	1.01	1.11	0.55	1.43	0.08	0.017	0.001	0.14	0.10
Standard requirements	0.95~1.05	0.90~1.25	0.45~0.75	1.40~1.65	≤0.10	≤0.025	≤0.025	≤0.23	≤00.25

Non metallic inclusion inspection

After making metallographic sample at crack position, non-metallic inclusions were detected. The results are shown in Table 2.

Table 2 non metallic inclusion test results (grade)

Non metallic inclusion	A				B			C			D
	Coarse series		Fine line	Coarse series		Fine line	Coarse series		Fine line	Coarse series	Fine line
Measurement result	0		0.5	0		0.5	0		0.5	0	0.5
Standard requirements	≤2.0		≤1.5	≤1.5		≤0.5	≤0.5		≤0.5	≤1.0	≤0.5

Metallographic examination

The metallographic specimen was observed under gx51 optical microscope. It can be seen that the crack is approximately perpendicular to the surface, with tearing shape on both sides, sharp tail and about 6 mm depth. No defects such as oxide scale and material inclusions were found, and the local micro morphology is shown in Fig. 2.

Figure 2 local micro morphology of crack section

No decarburization and grinding burn were found on both sides of the crack. The microstructure was tested, and the results were evaluated according to JB / T 1255-2014 technical conditions for heat treatment of high carbon chromium bearing steel parts for rolling bearings. The microstructure was grade 3 martensite, which was qualified (see Fig. 3). According to the crack morphology and the above analysis, the surface crack of bearing sleeve is quenching crack.

Fig. 3 microstructure

Cause analysis

There are many reasons for quenching cracks. The common influencing factors are metallurgical defects of materials and poor quenching process, such as too high quenching temperature, poor cooling, improper quenching repair process, etc.
1) From the above inspection results, it can be seen that the quality of raw materials is qualified, thus eliminating the possibility of quenching cracks caused by the quality problems of raw materials.
2) The results of metallographic examination show that the microstructure of the bearing sleeve after quenching and tempesleeve is martensite grade 3, thus eliminating the possibility of quenching cracks caused by too high quenching temperature or too long quenching heating time.
3) Because the crack sleeve is not repaired by quenching, the possibility of quenching crack caused by improper quenching repair process is eliminated.
4) The quenching process of bearing sleeves was analyzed. When the bearing sleeve is quenched, the quenching medium is fast quenching oil, and the cooling method is to use quenching machine to cool in the quenching groove. Because the cooling speed is too fast, the cooling speed difference between the surface and the center of the bearing sleeve is too big, resulting in a large temperature difference between the surface and the center of the bearing sleeve; At the same time, the cooling speed is too fast, which makes the structural transformation of the surface and the center of the bearing sleeve inconsistent. When the center of the bearing sleeve drops below the MS point (the martensite starting point) and starts the martensite transformation, the surface has already completely transformed into martensite, and the martensite on the surface will seriously hinder the volume expansion caused by the martensite transformation in the center, resulting in large structural stress, These stresses cause the bearing sleeves to crack due to quenching.
To sum up, the cracks on the end face of bearing sleeve are produced in the process of quenching and cooling.

Improvement measures

Improve the cooling mode of bearing sleeve

The cooling mode of bearing sleeve before improvement is quenching machine cooling; The cooling mode of the improved bearing sleeve is that after the sleeve is mounted, the belt frame moves up and down in the cooling groove.

Control the oil outlet temperature of bearing sleeve

Infrared thermometer is used to control the oil temperature of bearing sleeve quenching cooling at 100 ~ 110 ℃. According to the improvement measures, more than 200 extra large bearing sleeves were heat treated. After quenching and tempesleeve, the hardness of bearing sleeve was 59.5 ~ 60.5hrc. According to JB / T 1255-2014 “technical conditions for heat treatment of high carbon chromium bearing steel parts for rolling bearings”, the bearing microstructure was examined by portable metallographic microscope, and it was evaluated as grade 3 martensite. The bearing sleeves were processed to finished products and tested with fluorescent magnetic powder 100%. No quenching cracks were found.

Concluding remarks

• 1) The reason of crack on the end face of oversize bearing sleeve is improper cooling mode and too fast cooling speed.
• 2) The improvement measures are to improve the existing cooling mode of bearing sleeve and control the oil outlet temperature of bearing sleeve.
• 3) The extra large bearing sleeve processed according to the improvement measures can avoid the recurrence of quenching crack of bearing sleeve, and the product meets the technical requirements.

Authors: Hao Aoxuan, Wang Yunguang, Li Guifang, Liu Zhiqiang

Source: China Straight Sleeve Manufacturer – Yaang Pipe Industry (www.pipelinedubai.com)

Reference:

• [1] Liu Jinyi. Heat treatment crack analysis [J]. Dongfang Electric machinery, 2019 (1): 44-51
• [2] Cao Jingwei, Li Tao, Ma Yongqiang, et al. Cause analysis of bearing sleeve defects of GCr15SiMn steel [J]. Physical and chemical inspection physical volume, 2015, 51 (12): 905-907
• [3] Cheng Bing, Ma Shuang, Liu Chuanming. Analysis and prevention of common quenching cracks in rolling bearing parts [J]. Metal processing (hot processing), 2019 (5): 36-39
• [4] Sun Qinhe. Defect analysis of high carbon chromium bearing sleeves during heat treatment [J]. Metal processing (heat processing), 2015 (11): 48-49
• [5] Wang Guangsheng. Defects analysis and cases of metal heat treatment [M]. Beijing: China Machine Press, 2007:3-5