Grinding machining is a common method of metal cutting machining in the machinery manufacturing industry, in the bearing processing industry is also widely used, the bearing parts after heat treatment hardening, in the grinding process may appear a network of cracks or more regular arrangement of fine cracks, called grinding cracks, it not only affects the appearance of bearing parts, more importantly, also directly affects the quality of bearing parts. This paper shares the characteristics and causes of bearing grinding cracks, as well as the corresponding prevention measures.

1, bearing grinding crack characteristics:

The grinding crack is obviously different from the general quenching crack, the grinding crack only occurs on the grinding surface, the depth is shallow, and the depth is basically the same. The light grinding crack is perpendicular to or close to the parallel line perpendicular to the grinding direction, and is a regular arrangement of strip cracks, which is the first crack, the more serious crack is turtle-like (closed network), its depth is roughly 0.03 ~ 0.15ram, after acid corrosion cracks are obvious, which is the second crack.

2, bearing grinding crack causes:

Bearing grinding cracks are caused by grinding heat, and the bearing surface temperature can reach 800-1000 degrees Celsius or higher during grinding. The structure of the hardened steel is martensite and a certain amount of residual austenite, which is in an expanded state (not tempered). The expansion and contraction of martensite increases with the increase of carbon content in steel, so it is particularly important to produce grinding cracks on the surface of bearing steel. The residual austenite in the hardened steel is decomposed by grinding heat and gradually transformed into martensite. This new martensite is concentrated on the surface of the parts, causing local expansion of the bearing surface, increasing the surface stress of the parts, resulting in grinding stress concentration, and continuing grinding will accelerate the generation of surface grinding cracks. In addition, the new martensite is larger, and it is easy to accelerate the generation of grinding cracks.

On the other hand, when grinding parts on the grinder, both pressure and tension on the parts contribute to the generation of grinding cracks. If the cooling is not sufficient during grinding, the heat generated during grinding is enough to re-austenitize the thin layer of the grinding surface, and then re-quench into quenched martensite, so that the surface layer produces additional organizational stress, and the heat generated by grinding makes the bearing surface temperature rise very fast and cool very fast. The superposition of structural stress and thermal stress may lead to grinding cracks on the grinding surface.

3, grinding crack prevention measures:

From the above analysis, it is known that the root cause of grinding cracks is that the martensite is in a state of expansion during quenching, and there is stress. To reduce and eliminate this stress, stress tempering should be carried out, that is, quenching, in the tempering treatment, the waste time must be more than 4h. With the increase of tempering time, the possibility of grinding cracks decreases. In addition, cracks will occur when the bearing is rapidly heated to about 100 degrees Celsius and rapidly cooled. In order to prevent cold cracks, the parts should be tempered at about 150 ~ 200 degrees Celsius, if the bearing continues to heat up to 300 degrees Celsius, the surface shrinks again and cracks occur, in order to prevent cracks, the bearing should be tempered at about 300 degrees Celsius. It is worth noting that the temper of the bearing at about 300 degrees Celsius will reduce its hardness, so it is not suitable for use. After a tempering still produce grinding cracks, which can be secondary tempering or artificial aging treatment, this method is very effective.

Grinding cracks are caused by grinding heat, so reducing grinding heat is the key to solve grinding cracks. The wet grinding method is generally used, but no matter how the coolant is injected, the coolant can not reach the grinding surface in time during grinding, and therefore can not reduce the grinding heat at the grinding point. The coolant can only cool the grinding wheel and the grinding point of the part immediately after grinding, and the coolant can quench the grinding point. Therefore, increasing the use of coolant is one of the main measures to minimize the grinding heat in the grinding zone. If the dry grinding method is used, the grinding feed is less, and the grinding cracks can be reduced. However, the effect of this method is not very significant, and dusty, affecting the working environment, should not be taken.

The grinding wheel with softer hardness and coarser sand can reduce the grinding heat. However, the coarse particles will affect the surface roughness of the parts, and the parts with high surface roughness requirements can not use this method, so it is subject to certain limitations. It is divided into coarse and fine grinding, both coarse and soft grinding wheel with coarse particles is used for grinding, which is convenient for strong grinding and improve efficiency, and then fine grinding wheel with finer particle size is used for grinding, and the grinding feed is small. It is an ideal method to separate coarse grinding and fine grinding.

Selecting the grinding wheel abrasive with good self-sharpening performance, removing the surface waste of the grinding wheel in time, reducing the grinding feed, increasing the grinding times, and reducing the speed of the table 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 rotation of the grinding wheel is large, and the momentum of the parts is large, which is the incentive of grinding cracks. Improve the rotation accuracy of grinding wheel and parts in time to eliminate various factors causing grinding cracks as much as possible.

4, some methods to prevent grinding cracks on the surface of bearing steel:

In grinding processing, the main methods to prevent grinding cracks on the surface of bearing steel are:

① Reduce grinding heat to solve grinding cracks.

(2) Divided into coarse and fine grinding, both coarse grinding with soft grinding wheel with coarse particles.

③ Choose the grinding wheel abrasive with good self-sharpening performance, remove the surface waste of the grinding wheel in time, reduce the grinding feed, increase the grinding times, and reduce the speed of the table.

④ Timely improve the rotation accuracy of the grinding wheel and parts to eliminate as much as possible caused by grinding cracks.