Grinding is a common method of metal cutting in the mechanical manufacturing industry, and it is also widely used in the bearing processing industry. Bearing parts that have been heat-treated and quenched may have mesh like cracks or small cracks arranged in a regular pattern during the grinding process, known as grinding cracks. It not only affects the appearance of bearing parts, but more importantly, directly affects the quality of bearing parts. Below, we will share the characteristics and causes of bearing grinding cracks, as well as corresponding preventive measures.

1. Characteristics of bearing grinding cracks:

Grinding cracks are significantly different from general quenching cracks. Grinding cracks only occur on the grinding surface, with a shallow depth and basically the same depth. Lighter grinding cracks are parallel lines perpendicular to or close to the grinding direction, and are regularly arranged in the shape of a strip crack. This is a type of crack, while more severe cracks are in the shape of a turtle shell (closed network), with a depth of approximately 0.03-0.15ram. After acid corrosion, the cracks are obvious, which is the second type of crack.

2. Reasons for bearing grinding cracks:

The generation of bearing grinding cracks is caused by grinding heat, and the surface temperature of the bearing can reach 800-1000 degrees Celsius or higher during grinding. The structure of quenched steel is martensite and a certain amount of residual austenite, which are in a state of expansion. The expansion and contraction of martensite increase with the increase of carbon content in the steel, which is particularly important for generating grinding cracks on the surface of bearing steel. The residual austenite in quenched steel decomposes and gradually transforms into martensite under the influence of grinding heat during grinding. This newly formed martensite is concentrated on the surface of the part, causing local expansion of the bearing surface, increasing the surface stress of the part, leading to grinding stress concentration. Continuing grinding will accelerate the generation of surface grinding cracks; In addition, the newly formed martensite has a larger grain size, which can also accelerate the generation of grinding cracks during grinding.

On the other hand, when grinding parts on a grinder, both pressure and tension are exerted on the parts, which promotes the generation of grinding cracks. If the cooling is not sufficient during grinding, the heat generated during grinding is sufficient to re austenitize the thin layer on the grinding surface, which is then quenched again to form quenched martensite, resulting in additional structural stress on the surface layer. In addition, the heat generated during grinding causes the bearing surface temperature to rise extremely quickly and cool very quickly. The superposition of these structural and thermal stresses may lead to grinding cracks on the grinding surface.

3. Preventive measures for grinding cracks:

From the above analysis, it is known that the fundamental reason for the occurrence of grinding cracks is that the martensite during quenching is in an expanding state with the presence of stress. To reduce and eliminate this stress, stress relief tempering, also known as quenching, should be carried out. After tempering, the wasting time must be at least 4 hours. As the tempering time increases, the possibility of grinding cracks decreases. In addition, bearings will crack when rapidly heated to around 100 degrees Celsius and rapidly cooled. To prevent the occurrence of cold cracks, the parts should be tempered at around 150-200 degrees Celsius. If the bearing continues to heat up to 300 degrees Celsius, the surface will shrink again and cracks will appear. To prevent cracks from occurring, the bearing should be tempered at around 300 degrees Celsius. It is worth noting that tempering the bearing at around 300 degrees Celsius will cause a decrease in its hardness, and it is not suitable for use. Even after one tempering, grinding cracks still appear, which can be treated with secondary tempering or artificial aging. This method is very effective.

The generation of grinding cracks is caused by grinding heat, so reducing grinding heat is the key to solving grinding cracks. The commonly used wet grinding method, however, no matter how coolant is injected, the coolant cannot reach the grinding surface in time during grinding, and therefore cannot reduce the grinding heat at the grinding point. The coolant can only instantly cool the grinding points of the grinding wheel and parts after grinding, and at the same time, the coolant quenches the grinding points. Therefore, increasing the use of coolant is one of the main measures to minimize the grinding heat in the grinding area. If dry grinding method is used, the grinding feed rate is reduced, which can reduce grinding cracks. However, this method is not very effective and the dust is flying, affecting the work environment, so it is not suitable to adopt.

Choosing a grinding wheel with softer hardness and coarser sand particles can reduce grinding heat. However, coarser particles can affect the surface roughness of the parts. For parts with high surface roughness requirements, this method cannot be used and is therefore subject to certain limitations. It is divided into coarse and fine grinding. For coarse grinding, a soft grinding wheel with coarser particles is selected for grinding, which is convenient for strong grinding and improves efficiency. Then, a fine-grained grinding wheel is used for fine grinding, with a small grinding feed rate. Dividing into two machines for rough grinding and fine grinding is an ideal method.

Selecting grinding wheel abrasives with good self sharpening performance, timely removing surface waste of the grinding wheel, reducing grinding feed rate, increasing grinding frequency, and reducing worktable speed is also an effective way to reduce grinding cracks.

The rotation speed of grinding wheels and parts is also one of the main influencing factors. The large amount of grinding wheel rotation jumping and the large displacement of parts are the causes of grinding cracks. Timely improve the rotational accuracy of grinding wheels and parts 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 grinding, the main methods to prevent the occurrence of grinding cracks on the surface of bearing steel are:

① Reduce grinding heat to solve grinding cracks.

② It is divided into coarse and fine grinding. For coarse grinding, a soft grinding wheel with coarser particles is selected for grinding.

③ Select grinding wheel abrasives with good self sharpening performance, timely remove surface waste of the grinding wheel, reduce grinding feed rate, increase grinding frequency, and reduce workbench speed.

④ Timely improve the rotational accuracy of grinding wheels and parts to eliminate the occurrence of grinding cracks as much as possible.