The analyst has comprehensive production inspection knowledge and experience in crack analysis of rolling bearing

The crack analysis of bearing is divided into two parts: waste analysis and failure analysis: the analysis of cracks in the production process of bearing belongs to waste analysis; The analysis of cracks, fractures and large pieces falling off of bearing in the process of use belongs to failure analysis. Although these two parts are interrelated, the former part focuses on production, the latter part focuses on use, and this paper focuses on the former part.

In the production process of bearing, including smelting, rolling and upsetting of raw materials, stamping, turning, heat treatment and grinding, some surface cracks are often found during surface inspection between processes or even after the completion of the last process. How to analyze and judge these cracks has become an urgent problem to be solved in production. In this case, the nature of the crack (including the cause of the crack and its scope, etc.) must be clarified before the rejection of this batch of products can be determined. Once such a task is assigned, it is often time tight, difficult and difficult. It may involve all processes of the part production process, which requires analysts to have more comprehensive production inspection knowledge and experience. This paper only arranges and summarizes the crack analysis in the way of example analysis.

1. Cracks Caused by Raw Material Defects

Material defects include material cracks, shrinkage residues, white spots, decarburization, inclusions, micro pores and steel plate delamination. These defects become the source of crack initiation in the process of later processing and use.

There are a batch of φ12.7 mm steel balls made of GCr15 steel. No cracks are found after heat treatment, but cracks and cracks are found during grinding. Their morphology is shown in Figure 1. From the fracture morphology of the steel ball, there are bending black lines caused by the stamping deformation of the steel ball. After hot pickling, it is found that the black lines are perpendicular to the ring belt. Metallographic analysis shows that there are a large number of non-metallic inclusions and oxidative decarburization. It can be inferred that the cracking of the steel ball is due to the stress concentration in the quenching process caused by the residual defects of the material shrinkage tube, and then the grinding stress is superimposed, which finally leads to the cracking of the steel ball.

The inner ring of 51272 bearing produced by a factory is made of GCr15 steel. Small short linear cracks are found on the outer surface of the finished product. If there are many small decarburized cracks on the surface of the sample after pickling (see Fig. 3 for metallographic analysis), there are many small decarburized cracks on the surface of the sample after pickling (see Fig. 3 for metallographic analysis). Therefore, it can be considered that this defect is a "white spot", and its cause is generally considered that there is a certain amount of hydrogen in the steel during smelting, which cools quickly during forging and rolling, and it is too late to diffuse and escape and accumulate in the steel, resulting in great internal stress, forming many fine cracks and damaging the continuity of the steel. The mechanical properties of parts with white spots will be seriously reduced and easy to crack during quenching; If it is installed and used by mistake, it will also be damaged suddenly during operation, resulting in accidents, so the parts with white spots must be scrapped.

Fig. 1 crack morphology of φ 12.7 mm steel ball

2. Cracks Caused by Poor Forging Process

Due to the poor forging process, cracks or folds and other defects are formed in the ferrule or steel ball. If the depth is too large, it can not be completely removed after turning or soft grinding, but some cracks are retained or affected by the stress of heat treatment and grinding, and the cracks will further expand. After quenching, two kinds of cracks were found on the inner ring of 6210 bearing produced by a factory, both of which were located on the end face of the ferrule: one was "herringbone", and obvious decarburization layer was found around the crack after polishing and corrosion (see Fig. 4); Another circumferential crack (see Fig. 5), no decarburization layer is found around the crack. Through the investigation of the forging process, it is found that there are water traces on the ground at the forging site of this batch of ferrules, and the punching process control is not standardized. Some confirm that the crack in Figure 4 is the surface crack or wet crack caused by too fast cooling speed in the forging process. Fig. 5 the crack is caused by partial rolling deformation caused by punching deflection, and the stress is concentrated, forming a ring layered crack during quenching.

Fig. 2 crack morphology of working surface of 51272 bearing inner ring

Fig. 3 fracture crack morphology of 51272 bearing inner ring

Fig.4 cracks on the end face of 6210 bearing inner race

Fig.5 layered crack on the end face of bearing inner ring 6210

Fig.6 layered cracks on the end face of 16020 bearing outer ring

Fig. 7 metallographic structure of 16020 bearing outer ring crack section (75) ×)

The 16020 bearing produced by a unit is made of ZGCr15 steel. During the inspection of the finished product, it is found that there is a circumferential crack on the end face of the outer ring, and its length accounts for about 2/5 of the whole circumference (see Fig. 6). Take samples for metallographic analysis. The crack depth is 0.30 mm and the decarburization layer depth is 0.47 mm. The crack is about 30 ° to the surface and there is gray oxide scale inside (see Fig. 7). It can be inferred that the crack was generated in the forging process and left on the surface of the finished product, which belongs to the residual of forging folding.

3. Stamping Folding Crack

Stamping is a key process for manufacturing steel balls and rollers. If the hole diameter of the cutting die is too large, or because the hole diameter of the cutting die is too large, and the cutting knife is passivated, or because the gap between the cutting die and the cutting knife is too large, the surface defects of the steel ball or roller may be caused and scrapped.

Figure 8 shows a group of steel balls made of GCr15 steel. During the inspection of the finished products, it is found that there is an arc defect on the spherical surface. After Cold Pickling, it is found that there is decarburization. After hot pickling with 50% hydrochloric acid aqueous solution, it is found that the defect is just located at the edge of the "pole". Through metallographic observation, it is found that the defect is a folding crack and decarburization is around. Therefore, it can be determined that the surface defects of these steel balls are stamping folding cracks (Fig. 8). The reason for its formation is that when stamping, the end face of the cutting material is not neat and has burrs. Stamping the steel ball blank with this forging material will press the burrs into the edge of the "pole" and form folding cracks.

Fig.8 stamping folding crack

4. Quenching Crack Caused by Rough Machined Surface

The composition, structure and properties of bearing steel have a direct impact on its cutting properties; The cutting process, especially the cutting speed and feed rate, will also have an important impact on the workpiece surface. The depth of turning lines on the workpiece surface is directly related to the formation of quenching cracks.

A batch of 6307 bearing outer rings were found to have cracks on the raceway surface through magnetic flaw detection during rough grinding. Metallographic examination shows that the quenched and tempered structure has no local overheating and no decarburization around the crack. After hot pickling, it is found that the cutting knife lines on the ferrule raceway are clearly visible, and the small cracks are just along the turning knife lines (see Fig. 9). No grinding burn is found in pickling. According to the above analysis, that is, there is no decarburization around the crack and the crack is fine and distributed along the direction of turning tool grain, it can be judged that it is a quenching crack caused by turning stripe. The main reason for its formation is that the feed is too large during turning, resulting in deep tool marks and large turning stress, and stress concentration during quenching, resulting in cracking.

Fig. 9 cracks on raceway of 6307 bearing outer ring

5. Cracks Caused by Poor Heat Treatment Process

Improper selection of heat treatment process specifications and incorrect heat treatment operation methods are easy to cause deformation and cracking of bearing and scrap them. There are many literatures discussing this issue, and the situation is also very complex. Here is just one example of a group of steel ball cracks: there are a batch of φ13mm 9Gr18 steel balls. During the inspection of the appearance and surface quality, 8 steel balls were found to have cracks on the surface of 1,500 finished steel balls (including 200 repaired products). The shape of the cracks after hot pickling is shown in Figure 10. The length and depth of cracks are different. For the crack of the same steel ball, the two ends of the crack are thin and shallow, the middle is thick and deep, and the deepest part is about 5mm. The shape of the crack is S-shaped and T-shaped. Under the microscope, there is no decarburization around the crack (see Fig. 11). Therefore, it can be determined that the steel ball crack belongs to typical quenching crack. This situation often leads to the scrapping of the whole batch of products.

Fig. 10 S-shaped crack and S-shaped + T-shaped crack

Fig. 11 13 mm steel ball crack (500) ×)

6. Cracks Caused by Grinding Burn

The research shows that the grinding instantaneous high temperature can make the local surface reach 1,000 ~ 1,500 ℃ in a very short time (10^-4 ~ 10^-6 s). Under the action of instantaneous high temperature, the workpiece is easy to cause different degrees of thermal damage (including surface burns and cracks) and form various grinding metamorphic layers. The generation of grinding cracks is only an extreme form of grinding thermal damage.

For example, the outer ring of bearing produced by GCr15 bearing steel was found to have micro cracks on the channel during magnetic flaw detection, which occurred when it fell underground. After hot pickling one of them, it is found that there are network turtle cracks on the channel surface (see Fig. 12). In addition, the microstructure of the cross section of the ferrule is observed under the microscope. It can be seen that there is a high-temperature tempering layer vulnerable to corrosion on the surface layer, and there is even a white bright layer in some parts. The hardness of the white bright area is the highest, while the hardness of the blackened high-temperature tempering layer is lower than that of the core structure (see Fig. 13).

Experiments show that the main reason for the grinding burn and crack is the unreasonable grinding process, which leads to the sharp rise of the instantaneous local temperature of the grinding surface. If the instantaneous local temperature of the surface of the grinding area exceeds the A_C1 point of the steel, it will be transformed into secondary quenched martensite under the action of coolant (see white bright area in Figure 13). Secondly, due to the large temperature drop gradient and short time, the surface layer can not reach A_C1 point, which can only form high-temperature tempering structure. This causes harmful tensile stress between different tissues of the surface and subsurface. The surface layer is a very thin layer of hard and brittle secondary quenched martensite, which can not bear such a large stress, resulting in grinding surface cracking.

The above mainly illustrates the characteristics of possible product surface cracks and the method of waste analysis from six process factors affecting the quality of bearing.

Fig. 12 hot pickling shows cracks on the surface of groove grinding

Fig. 13 metallographic structure of ferrule cross section (500) ×)

From the above crack characteristics, through acid pickling and metallographic structure analysis, supplemented by necessary process investigation, we can quickly determine the main process factors causing cracks and the scrapping range of workpieces, and can improve the process, guide production and improve product quality. Rapid waste analysis is very important in industrial production, but in production practice, the situation is very complex, and different situations must be analyzed one by one.

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