Cause Analysis on Microstructure Change for Rings of Failure Bearings

Abstract: During analyzing spalling failure of raceway for large size bearings made of steel M50，the microstructure change is found on raceway subsurface． The microstructure of life － limit element of bearings，inventory finished rings and machined forgings are compared． By combining with forging process for forgings，it is confirmed that the forging defect on rings is reasons for microstructure change, and the improvement measures are adopted. 

Key words: rolling bearing ; ring; microstructure variation; forging defect

1. Overview

The inner and outer ring diameters of a large-sized M50 steel bearing of a certain model are 133 35201. 725 mm, with inner and outer ring widths of 35 85, 39 4 mm, during use, the outer raceway peeling fault occurred multiple times (Figure 1), and the service life of this type of bearing is required to be 500 hours, while the service time of the faulty bearing is generally less than 100 hours. The peeling positions all occur on the working surface of the outer ring raceway. Except for differences in peeling area, the overall morphology of the peeling surface is basically similar. Upon microscopic observation, the peeling surface is characterized by rolling and extrusion.

Figure 1 Peeling morphology of the outer raceway of the bearing

2. Detection and analysis

A systematic dimensional accuracy test was conducted on the faulty bearing, and the cold and hot machining quality of the parts was investigated to confirm that the dimensional accuracy of the faulty bearing meets the requirements. The surface of the raceway is free of grinding burns and original crack defects, and the hardness and metallographic structure of the parts are qualified.

2.1 Microstructure of faulty bearings

Samples were taken from the peeling and non peeling areas of the faulty bearing, and their longitudinal sections were observed under a microscope. It was found that the sub surfaces of the peeling and non peeling areas of the raceway had patches of white tissue, similar to a butterfly shape (hereinafter referred to as "butterfly shaped" tissue). From the sample, it can be seen that the "butterfly" structure can reach a maximum depth of 0.8 mm and a maximum length of 0.2 mm from the surface of the raceway. Small microcracks can be seen at the edges of some tissues, and the crack tails bifurcate. In contrast, the "butterfly" structure is more distributed on the sub surface of the peeling area (Figure 2). Under the scanning electron microscope, the "butterfly shaped" tissue appears slightly watermarked (Figure 3), which is basically the same color as the matrix, indicating that the composition of the "butterfly shaped" tissue is similar to that of the matrix, and there are crack defects at its edges.

Figure 2 Morphology of white "butterfly shaped" tissue under the microscope

Figure 3 Morphology of "butterfly shaped" tissue under scanning electron microscope

Observing under a microscope, the core structure of the sample consists of tempered martensite and small uniformly distributed carbides, and its heat treatment quality meets the requirements of JB/T 2850-2007 "Technical Conditions for Heat Treatment of Cr4Mo4V High Temperature Bearing Steel Parts for Rolling Bearings".

2.2 Composition and microhardness of faulty bearing micro areas

Perform energy spectrum analysis on the "butterfly" structure and matrix in the faulty bearing, with scanning positions shown in Figure 4.

Figure 4 Scanning positions of energy spectrum points

Select the butterfly shaped and matrix tissues of three peeled samples for microhardness testing

2.3 Microstructure of Shou Bearing

Three sets of bearings with good service condition and no faults were taken, and under the microscope, no obvious defects such as white tissue, inclusions, and cracks were observed on the sub surface of the raceway (Figure 5).

Figure 5 Morphology of microstructure at the edge of the raceway of Shou bearing

3.Cause analysis

3.1 The emergence of "butterfly shaped" organizations

The butterfly shaped structure is the white etched part formed by microstructural variation. It is generally believed that its formation is due to the heavy load borne by the bearing, which causes microstructural variation in rolling contact parts; On the other hand, bearing parts have internal defects, and the microstructure near the defect undergoes variation under the action of alternating stress. During the working process of this type of bearing, it needs to withstand an axial load of 30000 N (short-term 70000 N), a radial load of 1100 N (short-term 5500 N), and a speed of 13 725 r/min. Overall, the bearing's load-bearing and speed requirements are relatively high. However, when multiple sets of bearings were taken for observation, no "butterfly" structure and crack defects were found on the surface of the sample raceway, indicating that the performance of this type of bearing can meet the working condition requirements, and the influence of excessive load on the structural transformation can be ruled out. Therefore, the occurrence of the "butterfly" structure of the faulty bearing may be related to internal defects.

Using the water immersion ultrasonic non-destructive testing method, internal defect inspections were conducted on more than 160 outer rings in four different states: faulty bearings, expired bearings, unused finished rings in factory inventory, and forged rings. It was found that the outer race sub surface of the faulty bearings had defects, and some faulty bearings also had defects on the outer diameter sub surface; In addition, there are 25 finished rings and forgings in stock that also have defects on the outer diameter surface or the sub surface of the raceway, and all bearings are in good condition.

By coupling the defect locations of the faulty rings, inventory finished rings, and forged rings mentioned above, it was found that the defects were mainly distributed about 2.4-6.4 mm below the outer diameter surface and on the sub surface of the raceway. Some of the ring defects were also located near the load-bearing side of the raceway (Figure 6).

Figure 6 Location of defect concentration in water immersion ultrasonic testing

From the defective forging rings, forgings with larger signal amplitudes were selected, and metallographic specimens were cut from the signal display area. After grinding and polishing, no butterfly shaped structure was observed under optical and scanning electron microscopes. However, cracks were found at the defect display area, as shown in Figure 7. Energy spectrum analysis was conducted on the defects, and no other elements were found except for the matrix. Each defect has a depth, indicating that these defects belong to the matrix. Based on the comprehensive analysis of the location, morphology, and metallographic structure of the defects, it is believed that the defects are forging defects and belong to forging internal cracks.

Figure 7 Morphology of crack defects

The defects of multiple forged rings are similar to those of faulty bearings, and no "butterfly" structure or crack defects were found in the bearings. This indicates that the formation of the "butterfly" structure is related to the presence of internal defects, which is caused by the variation of the microstructure near the subsurface defects of the raceway under the action of alternating stress. This is also the reason why there is no significant difference in the composition between the "butterfly" structure and the matrix structure.

3.2 Causes of crack defects

Through the investigation of the raw materials of the bearings and the production and processing process of the rings, it was found that the internal crack defects of the bearings were formed during the forging process.

The outer ring of the bearing is processed by forging a section of material with a diameter of 80 mm x 103.7 mm at a forging temperature through processes such as upsetting, composite extrusion punching, punching and skin removal, and rolling, ultimately forming a circular forging with an outer diameter of 206 mm and a width of 44.5 mm. The forging process flow diagram is shown in Figures 8-10.

Figure 8 Schematic diagram of forging process flow

During the forging process, the forming die and rolling wheel used in the composite extrusion punching and de punching and skin hole processes, as well as the rolling wheel used in the rolling process, were not preheated (at a room temperature of around 20 ℃), while the temperature of the material segment was around 1000 ℃. The first processed material segment rapidly cooled down due to the contact between its outer surface and the mold or rolling wheel at room temperature, causing a large temperature difference between its surface and the core material, resulting in inconsistent metal flow and deformation speed, forming significant internal stress, and internal defects on the subsurface; In addition, during the rolling process of forgings, when the temperature difference between the center and the surface reaches a certain degree, under the combined effect of external factors such as rolling force and rolling speed, when the shear stress on the surface and subsurface is greater than the tensile strength of the material, internal cracks and defects can also occur.

4. Mechanism analysis

In summary, forging defects in the ring are the direct cause of the variation of the secondary surface microstructure, forming a "butterfly" structure.

Due to the extremely high operating speed and working load of the bearing, during the working process, the material near the subsurface defect in the raceway undergoes elastic deformation under the action of high stress cyclic rolling contact. After the rolling element leaves this area, the elastic deformation recovers. The frequent changes from deformation to recovery in the defect area below the raceway generate a large amount of deformation heat at the edge of the defect, which spreads to the surrounding area, forming a "butterfly shaped" heat affected zone, causing the microstructure of the material in this area to undergo variation, forming a "butterfly shaped" structure. Due to its slightly higher hardness than the matrix structure, under the action of alternating loads, defects propagate along the edges of the "butterfly" structure, and the two promote each other's development. As the working time increases, fatigue peeling eventually occurs on the surface of the raceway. Due to the difficulty of etching the mutated butterfly shaped tissue, it appears white under an optical microscope; But because their composition is the same as the matrix element, the colors of the two are similar under scanning electron microscopy.

5. Improvement measures

To avoid the recurrence of similar faults and ensure the performance and reliability of bearings, the following measures are taken for the forging processing and defect control of bearings:

1) Improve the processing quality of ring forgings, optimize the forging process, effectively preheat the mold and rolling wheel, reasonably determine the position, size, and removal method of punching and connecting the skin, and strictly control the forging temperature to avoid the occurrence of forging defects.

2) Conduct water immersion ultrasonic non-destructive testing on ring forgings to effectively remove forgings with internal defects. Through verification and field use, it has been confirmed that the improvement measures are effective and avoid forging defects. At the same time, this measure has been promoted and applied to the process control of multiple key models of large-sized M50 steel bearings, effectively improving the machining quality of bearing forgings and ensuring that the performance of bearings meets reliability requirements.

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