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Common Cracking Morphology and Causes of Bearing Rings

Time:2021.06.24  Source:Roller Bearings Suppliers

Common Cracking Morphology and Causes of Bearing Rings


Bearing ring is one of the most important parts of bearing components. In the process of bearing use, bearing ring cracking and fracture are common forms of damage. Some bearings crack on the ring in the early use process, and some bearings fracture due to fatigue in the use process.


1.Macro morphology diagram and causes of cracking of outer ring of deep groove ball bearing


Macro morphology diagram of bearing fracture shows that the outer ring of the bearing is fractured, and the section is basically perpendicular to the surface, as indicated by the arrow in Figure 1. The section starts from the lower surface of the outer inner diameter on the right side of the raceway in the figure, and rapidly expands to the outer surface and the left to the fracture.The bearing outer ring fracture has no obvious plastic deformation and shows brittle fracture characteristics.


After analysis, the protective atmosphere in the heat treatment furnace of the bearing ring is a mixture of various gases, including oxidizing gas, neutral gas, reducing gas and carburizing gas.It's a very complex chemical reaction when you heat it at high temperatures, whether it's decarbonization, or.Is a carburization reaction, in addition to the participation of free oxygen atoms, can reach equilibrium under certain conditions, and even reversible reaction.The level of carbon potential in furnace has great influence on the thickness of carburizing layer, carbon content and carbon concentration gradient.


Generally speaking, the higher the carbon potential in the furnace, the steeper the carbon concentration gradient, and the greater the tendency to form coarse and networked carbides.In order to avoid the carbon potential is too high, resulting in the surface of bearing ring carburization phenomenon, heat treatment can not be heated sharply, it is necessary to use appropriate heating temperature, so that the steel grain does not grow up as well.Correct selection and design of heating medium, heating speed, heating temperature and holding time and other heating parameters;Strictly control the uniformity of furnace temperature, can not fluctuate too much.By controlling the carbon potential in the furnace, the carbon concentration and concentration gradient of the bearing ring are strictly controlled, so as to ensure the heat treatment quality and service life of the ring.Need WSBC 22218 E,Click here to learn more.

WSBC 22218 E.jpg

2. Macro morphology diagram and causes of cracked bearing rings

The bearing cracks occur in the process of use. The cracks on the bearing ring fracture extend from the bottom of the pit to the surface of the bearing ring in a river pattern, so it can be seen that the crack source is at the bottom or near the bottom of the pit on the surface of the bearing ring.When the fracture is artificially opened, the macro morphology of the bearing ring fracture after opening is obtained.


Once the bearing ring spalling with contact fatigue occurs, it will lead to its 

instability. In addition to the high hardness and brittleness of the material, one-time brittle fracture begins to occur in the local spalling area, that is, relatively straight macro crack, and relatively straight micro fracture, showing cleavage characteristics and rapid expansion, and the rapid expansion area accounts for most of the fracture section.The factors that cause contact fatigue include material structure, surface strengthening process, workpiece surface roughness, lubricant and stress.


Through analysis, it can be seen that the structure of the workpiece is uniform and normal, the surface has not been strengthened, the working surface of the workpiece has not seen rough features, and there is no obvious organizational change at the fracture source, so it can be ruled out that the cause of surface organizational change leads to contact fatigue.


That is to say, the contact fatigue on the surface of the bearing ring may be caused by the joint action of the contact stress and the micro-slip caused by the cyclic stress field.


3. Macro morphology diagram and causes of surface cracking of bearing outer ring groove

After acid-dipping treatment of the outer ring of a fractured bearing, severe black burn marks on the surface of the groove were observed by naked eye and parallel transverse cracks were distributed basically perpendicular to the grinding direction. 


The crescene-shaped burn layer was presented on the broken surface, and the fracture was fine porcelain shaped. The crack at the groove was rapidly pushed inward to complete fracture.Click here to learn more about WSBC 22222-E1-K.

WSBC 22222-E1-K.jpg

The parallel crack on the surface of bearing outer ring raceway is a typical grinding crack. The grinding crack is mainly caused by excessive grinding amount and bad grinding conditions.Secondly, insufficient tempering of bearing outer ring also increases its sensitivity to grinding cracking.It is suggested that the tempering process of bearing outer ring should be improved by adopting 170C× 4H tempering, which can reduce the grinding tensile stress and make the metal volume more stable.In addition, the grinding amount should be strictly controlled, the grinding wheel should be fully cooled and the grinding wheel should be dressed in time to effectively prevent grinding cracking.

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