Bearings in operation, due to various complicated reasons, such as unreasonable structure, poor material quality, low performance, defects on the working surface, impact, vibration, improper installation and poor lubrication, can cause early failure of the bearing. However, in some cases, some bearings are not put into use (running) at all, but the original intact appearance of the user during the installation process is damaged, and the use is lost, resulting in premature bearing failure. What is revealed in this paper is a practical example of this form of failure.
First, the failure background
When a power machine factory assembled the S195 diesel engine gearbox, there was a phenomenon that the inner ring of the 6205 deep groove ball bearing was intermittently or continuously cracked (block), occupying a certain proportion in quantity (four sets per gear box). According to the site, the total supply of the batch of bearings is 3,000 sets, and 1000 sets have been installed. The remaining 2,000 sets are not ready for installation due to the user’s “heart and soul”.
It has been verified that the dimensions of the gearbox shaft meet the requirements of the drawings, and the bearing installation method is correct, which is completed by a fixed skilled worker. The user has extracted 40 sets of the inner and outer rings of the bearing and measured the hardness of the inner and outer rings of the bearing. The hardness of the DZ-2000 is detected in the range of 61 to 65 HRC, which meets the JB1255 standard. No cracks are found by visual inspection. In view of this, in order to plead for the cause of cracking of the bearing ring, the damaged bearing on the site was returned, and conventional physical and chemical tests were carried out.
These ferrules are heated and quenched by a box type resistance furnace at 840 to 860 ° C, and tempered at 160 ° C × 2 ~ h low temperature oil.
Second, test methods and results
Macroscopic observation
Figure 1 is a photo of the bearing that was damaged during installation.
After careful observation of the real thing, we can know:
(1) No damage marks left by the installation of the machine on all the damaged bearings, and the steel balls, cages and other parts are intact.
(2) The cracking of the inner ring starts at the side of the center of the raceway and occupies about 1/2 or 1/4 of the diameter of the end face.
(3) There is a secondary longitudinal straight crack extending into the inner diameter surface at the fracture, and the crack is shaped like a knife and penetrates the wall surface.
(4) The shape of the entire crack is uneven, but the surface of the fracture is smooth and flat. There are no signs of plastic deformation and only a few tear marks.
(5) The fracture is gray and fine grained porcelain brittle fracture.
2. Acid etching test
The cracked inner ring and the complete inner ring were respectively subjected to hot pickling with a 1:1 aqueous hydrochloric acid solution at a temperature of 60 to 70 ° C for 10 minutes, and the results are shown in FIG. 2 .
(1) Strip-shaped grinding burns and grinding cracks exist in the raceway near the crack.
(2) The grinding crack is dendritic, distributed along one side of the raceway, and parallel to the grinding direction, and the cracking develops along the grinding crack.
(3) The secondary longitudinal straight crack and the cracking fracture and the grinding crack are substantially "T"-shaped perpendicular.
(4) The surface of the entire ferrule is distributed with loose black dark spots and pores, and its shape is circular or elliptical. Observed under a 40x magnification stereo microscope, the shape is irregular or round pinholes, which can be connected to each other in a grain-like shape, while the complete ferrule surface is quite smooth, and the above is not seen. defect.
3. Macro hardness and microstructure
In order to further understand the hardness distribution of the ferrule and the microstructure of the quenched and tempered microstructure and surface layer, select the partially cracked inner ring and its fragments, and set the macro hardness on the end face, and use the wire cutting method to crack. Longitudinal metallographic specimens were taken near the site and the results were:
(1) The macro hardness value is 64 to 65.2 HRC.
(2) The quenched and tempered structure conforms to the 3 to 4 grade of JB1255 standard, and the local area reaches 5 grades, which is fine crystalline martensite + cryptocrystalline martensite, ten fine needle-shaped martensite + a small amount of residual carbide + residual austenite body.
(3) Observed at 100 times, the martensite matrix showed an obvious black-and-white strip-like structure, which can be rated as 4 according to the YB9-59 standard, as shown in Fig. 3.
(4) It is observed that there is a layer of a semi-quenched white layer in the shape of a crescent and a high-temperature softening layer in the subsurface, that is, an "over-tempering layer". The white layer depth (the most surface to the crescent bottom) is about 0.033 ~ 0.056mm, and the depth of the over-tempered layer is about 0.293mm. It can be seen that the degree of burn burn is very serious, as shown in Figure 4.
