Characteristics and causes of bearing grinding cracks and preventive measures

• Shanghai Zhio Technology Co.,Ltd 
• 2023-08-29

Grinding is a common method of metal cutting in the machinery manufacturing industry. It is also widely used in the bearing processing industry. The bearing parts that have been heat-treated and quenched may appear mesh-shaped cracks or more regular during the grinding process. The small cracks arranged, called grinding cracks, not only affect the appearance of bearing parts, but also directly affect the quality of bearing parts. Let's share the characteristics and causes of bearing grinding cracks, as well as corresponding preventive measures.
1. Characteristics of bearing grinding cracks:
Grinding cracks are obviously different from general quenching cracks. Grinding cracks only occur on the grinding surface, the depth is shallow, and the depth is basically the same. The lighter grinding cracks are perpendicular or close to the parallel lines perpendicular to the grinding direction, and are regularly arranged strip cracks, which are a kind of cracks. It is 0.03 ~ 0.15ram, and the crack is obvious after etching with acid, which is the second kind of crack.
2. Reasons for bearing grinding cracks:
The occurrence of bearing grinding cracks is caused by grinding heat, and the bearing surface temperature can reach 800-1000 degrees Celsius or higher during grinding. The structure of quenched steel is martensite and a certain amount of retained austenite, which are in an expanded state. The expansion and contraction of martensite increases with the increase of carbon content in the steel, so it is particularly important to generate grinding cracks on the surface of the bearing steel. The retained austenite in the quenched steel is decomposed under the influence of the grinding heat during grinding and gradually transformed into martensite. This new martensite is concentrated on the surface of the part, causing local expansion of the bearing surface, increasing the size of the bearing surface. The surface stress of the parts leads to the concentration of grinding stress, and continued grinding will accelerate the generation of surface grinding cracks; in addition, the new martensitic mash is large, and it is easy to accelerate the generation of grinding cracks during grinding.
On the other hand, when grinding parts on a grinding machine, both pressure and tension are exerted on the parts, which promotes the generation of grinding cracks. If the cooling during grinding is not sufficient, the heat generated during grinding is enough to re-austenize the thin layer on the ground surface, and then quench it again to quenched martensite, thus causing additional structural stress on the surface layer. Coupled with the heat generated by grinding, the temperature of the bearing surface rises very quickly and cools very quickly. The superposition of this structural stress and thermal stress may lead to grinding cracks on the grinding surface.
3. Preventive measures for grinding cracks:
From the above analysis, we know that the root cause of grinding cracks is that the martensite during quenching is in an expanded state and there is stress. To reduce and eliminate this stress, stress relief tempering, that is, quenching, should be carried out. In tempering treatment, the waste time must be more than 4h. As the tempering time increases, the likelihood of grinding cracks decreases. In addition, the bearing will crack when it is rapidly heated to about 100 degrees Celsius and cooled rapidly. In order to prevent cold cracks, the parts should be tempered at about 150 to 200 degrees Celsius. If the bearing continues to heat up to 300 degrees Celsius, the surface shrinks again and cracks occur. To prevent cracks, the bearing should be tempered at about 300 degrees Celsius. It is worth noting that tempering the bearing at about 300 degrees Celsius will reduce its hardness, so it is not suitable for use. After one tempering, grinding cracks still occur, which can be treated by secondary tempering or artificial aging, which is very effective.
Grinding cracks are caused by grinding heat, so reducing the grinding heat is the key to solving grinding cracks. The wet grinding method is generally used, but no matter how the coolant is injected, the coolant cannot reach the grinding surface in time during grinding, so the grinding heat at the grinding point cannot be reduced. The coolant can only cool the grinding point of the grinding wheel and the parts after grinding, and the coolant can quench the grinding point at the same time. Therefore, increasing the use of coolant is one of the main measures to reduce the grinding heat in the grinding area as much as possible. If the dry grinding method is used, the grinding feed is small and grinding cracks can be reduced. However, the effect of this method is not very significant, and it is dusty and affects the working environment, so it should not be used.
Use a grinding wheel with softer hardness and coarser sand for grinding, which can reduce the grinding heat. However, the coarser particles will affect the surface roughness of the parts. For parts with high surface roughness requirements, this method cannot be used, so it is subject to certain restrictions. It is divided into coarse grinding and fine grinding. For coarse grinding, soft grinding wheel with coarse particles is used for grinding, which is convenient for strong grinding and improves efficiency. Then, fine grinding wheel is used for fine grinding, and the grinding feed is small. It is an ideal method to carry out rough grinding and fine grinding in two sets.
Selecting the grinding wheel abrasive with good self-sharpening performance, timely removing the waste on the surface of the grinding wheel, reducing the grinding feed, increasing the grinding times, and reducing the table speed are also an effective way to reduce grinding cracks.
The rotation speed of the grinding wheel and the parts is also one of the main influencing factors. The large amount of grinding wheel rotation and the large amount of movement of the parts are the causes of grinding cracks. Improve the rotation accuracy of grinding wheel and parts in time to eliminate various factors that cause grinding cracks as much as possible.
4. Some methods to prevent grinding cracks on the surface of bearing steel:
In the grinding process, to prevent the occurrence of grinding cracks on the bearing steel surface, the main methods are:
①Reduce grinding heat to solve grinding cracks.
②It is divided into coarse grinding and fine grinding. For rough grinding, use a soft grinding wheel with coarse particles for grinding.
③Choose the grinding wheel abrasive with good self-sharpening performance, remove the waste on the surface of the grinding wheel in time, reduce the grinding feed, increase the grinding times, and reduce the speed of the worktable.
④ Improve the rotation accuracy of the grinding wheel and parts in time to eliminate grinding cracks as much as possible.