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Premature bearing failure analysis (1)

Premature bearing failure analysis (1)

2020-04-29

Bearing in the work, due to a variety of complex reasons, such as unreasonable structure, poor material quality, low performance, work surface defects, impact, vibration, improper installation and poor lubrication, etc., can cause early bearing failure.However, in some cases, some bearings have not been put into use at all, but the original complete appearance of the bearings has been damaged in the process of installation by the user, resulting in the premature failure of the bearings.What this article will reveal is a practical example of this type of failure.

I. failure background

During the assembly of S195 diesel engine gearbox in a power plant, the inner ring of 6205 deep groove ball bearing discontinuous or continuous cracking (block) occurs, which occupies a certain proportion in quantity (each gearbox is packed with four sets).According to the site, the total supply quantity of this batch of bearings is 3000 sets, 1000 sets have been installed and the remaining 2000 sets dare not be installed because of the "residual fear" of users.

After verification, the size of gearbox shaft conforms to the requirements of the drawing, and the bearing mounting method is correct, which is completed by fixed skilled workers.The user once extracted 40 sets out of the box to measure the hardness of the inner and outer rings of the bearing, and tested by the dz-2000 large magnetic flaw detection machine, the hardness was in the range of 61-65hrc, in line with the JB1255 standard, and no cracks were found by visual inspection.In view of this, in order to find out the cause of bearing ring cracking, the damaged bearing was returned to the site for routine physical and chemical test.

These rings are heated and quenched at 840 ~ 860℃ in a box-type resistance furnace and tempered by oil at 160℃×2 ~ h at low temperature.

Ii. Test methods and results

1. Macro observation

FIG. 1 is a picture of the bearing damaged during installation.

Careful observation of the object shows that:

(1) no marks of damage were found on all damaged bearings, and the steel ball, cage and other parts were intact.

(2) the crack of the inner ring starts at one side of the raceway center, accounting for about 1/2 or 1/4 of the diameter of the end face.

(3) there are two vertical straight cracks extending into the inner diameter surface at the fracture, and the cracks look like knife cutting and penetrate the wall surface.

(4) the appearance of the whole fracture is uneven, but the fracture surface is smooth and flat.There is no sign of plastic deformation, only a few tears.

(5) the fracture is gray, fine-grain, porcelain-like, and brittle.

Bearing in the work, due to a variety of complex reasons, such as unreasonable structure, poor material quality, low performance, work surface defects, impact, vibration, improper installation and poor lubrication, etc., can cause early bearing failure.However, in some cases, some bearings have not been put into use at all, but the original complete appearance of the bearings has been damaged in the process of installation by the user, resulting in the premature failure of the bearings.What this article will reveal is a practical example of this type of failure.

I. failure background

During the assembly of S195 diesel engine gearbox in a power plant, the inner ring of 6205 deep groove ball bearing discontinuous or continuous cracking (block) occurs, which occupies a certain proportion in quantity (each gearbox is packed with four sets).According to the site, the total supply quantity of this batch of bearings is 3000 sets, 1000 sets have been installed and the remaining 2000 sets dare not be installed because of the "residual fear" of users.

After verification, the size of gearbox shaft conforms to the requirements of the drawing, and the bearing mounting method is correct, which is completed by fixed skilled workers.The user once extracted 40 sets out of the box to measure the hardness of the inner and outer rings of the bearing, and tested by the dz-2000 large magnetic flaw detection machine, the hardness was in the range of 61-65hrc, in line with the JB1255 standard, and no cracks were found by visual inspection.In view of this, in order to find out the cause of bearing ring cracking, the damaged bearing was returned to the site for routine physical and chemical test.

These rings are heated and quenched at 840 ~ 860℃ in a box-type resistance furnace and tempered by oil at 160℃×2 ~ h at low temperature.

Ii. Test methods and results

1. Macro observation

FIG. 1 is a picture of the bearing damaged during installation.

Careful observation of the object shows that:

(1) no marks of damage were found on all damaged bearings, and the steel ball, cage and other parts were intact.

(2) the crack of the inner ring starts at one side of the raceway center, accounting for about 1/2 or 1/4 of the diameter of the end face.

(3) there are two vertical straight cracks extending into the inner diameter surface at the fracture, and the cracks look like knife cutting and penetrate the wall surface.

(4) the appearance of the whole fracture is uneven, but the fracture surface is smooth and flat.There is no sign of plastic deformation, only a few tears.

(5) the fracture is gray, fine-grain, porcelain-like, and brittle.

 

 

 

2. Acid corrosion test

The split inner ring and the intact inner ring were hydrochloric acid washed with 1:1 hydrochloric acid solution, respectively, the temperature was 60 ~ 70℃, and the time was 10min. The results are shown in FIG. 2.

(1) strip grinding burn and grinding crack exist in the raceway near the crack.

(2) the grinding crack is dendritic, distributed along one side of the raceway, and is parallel to the grinding direction.

(3) the secondary longitudinal straight crack is roughly "T" perpendicular to the avulsion fracture and grinding crack.

(4) the whole ring surface is distributed with loose black spots and pores, and its shape is round or oval.Under the magnification of 40 times of the stereoscopic microscope, its appearance is irregular holes or round pinholes, they can be connected to each other into the shape of grain boundary, and the complete surface of the ring is quite bright and clean, not to see the above defects.

3. Macroscopic hardness and microstructure

In order to further understand the hardness distribution of the ring and the characteristics of quenched and tempered microstructure and surface microstructure, some cracked inner ring and its fragments were selected, the macroscopic hardness was determined on the side of the end face, and the longitudinal metallographic samples were intercepted near the crack by the linear cutting method. The results are as follows:

(1) the macroscopic hardness value is 64 ~ 65.2HRC.

(2) the quenched and tempered tissue conforms to grade 3-4 of JB1255, and the local area is up to grade 5. It is fine crystalline martensite + cryptocrystalline martensite 10 fine acicular martensite + a small amount of residual carbide + residual austenite.

(3) when observed at 100 times, the martensite matrix shows an obvious black and white banding structure, which can be rated as level 4 according to yb9-59 standard, as shown in FIG. 3.

(4) it was observed that there was a layer of crescent-shaped secondary hardening white layer and a high-temperature softening layer on the surface of the raceway, namely "over-tempering layer".The depth of the white layer (from the most surface to the bottom of the crescent moon) is about 0.033 ~ 0.056mm, and the depth of the over-tempering layer is about 0.293mm. It can be seen that the degree of grinding burn is very serious


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