Characteristics And Causes Of Bearing Grinding Cracks And Preventive Measures

Grinding processing is a common method of metal cutting processing in the machinery manufacturing industry, which is also widely used in the bearing processing industry. The bearing parts quenched by heat treatment may appear in the grinding process as a network of cracks or regular arrangement of small cracks, called grinding cracks, which not only affect the appearance of bearing parts, but also directly affect the quality of bearing parts.

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 arranged strip crack, which is the first kind of crack, the more serious crack is tortoise shell (closed network), its depth is about 0.03 ~ 0.15 ram, the crack is obvious after acid corrosion, which is the second kind of crack.

2，Causes of bearing grinding cracks:

Bearing grinding cracks are 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 is in a state of expansion (without tempering). 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 hardened steel is decomposed under the influence of grinding heat during grinding, 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, leading to the concentration of grinding stress, and continuing grinding will accelerate the generation of surface grinding cracks. In addition, the new martensite Chuai is bigger, also easy to accelerate grinding grinding cracks.

On the other hand, when grinding parts on a 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 make the thin layer of grinding surface austenitization, and then quenched again into quenched martensite, so that the surface layer generates additional tissue stress, coupled with the heat generated by grinding to make the bearing surface temperature rise very fast, cooling very fast, This 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 fundamental 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 quenching should be carried out, in the tempering treatment, the waste time must be more than 4h. With the increase of tempering time, the possibility of producing grinding cracks decreases. In addition, bearings in the rapid heating to about 100 degrees Celsius and rapid cooling will produce cracks. In order to prevent the occurrence of 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 will shrink again and produce cracks. In order to prevent the occurrence of cracks, the bearing should be tempered at about 300 degrees Celsius. It is worth noting that the bearing temper at about 300 degrees Celsius will make its hardness decrease, something should not be used. Again after a tempering still produce grinding cracks, which can be secondary tempering or manual 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 cannot reach the grinding surface in time during grinding, so the grinding heat at the grinding point cannot be reduced. The coolant can only make the grinding point of the grinding wheel and parts cooled immediately after grinding, and the coolant can quenched the grinding point. Therefore, increasing the amount of coolant is one of the main measures to reduce the grinding heat in the grinding zone as much as possible. If the dry grinding method is used, the grinding feed is less, and the grinding crack can be reduced. But the effect of this method is not very significant, and dust, affect the working environment, not suitable to take.

The grinding heat can be reduced by choosing the grinding wheel with softer hardness and coarser sand. However, coarse particles will affect the surface roughness of parts. For parts with high surface roughness requirements, this method cannot be used, so it is limited to a certain extent. It is divided into coarse and fine grinding. The soft grinding wheel with coarse particles is used for grinding in coarse grinding, which is convenient for strong grinding and improves efficiency. Then, the grinding wheel with fine particle size is used for fine grinding, and the grinding feed is small. Two sets of rough grinding and fine grinding, this is a more ideal method.

It is also an effective way to reduce grinding cracks by selecting grinding wheel abrasives with good self-sharpening performance, timely removing grinding wheel surface waste, reducing grinding feed, increasing grinding times and reducing table speed.

The rotation speed of grinding wheel and parts is also one of the main influencing factors. The rotation runout of grinding wheel is large, and the momentum of parts is large, which is the cause of grinding cracks. Improve the rotation precision of grinding wheel and parts in time so as to eliminate all kinds of factors that cause grinding cracks.

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

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

① Reduce the grinding heat to solve the grinding crack.

(2) It is divided into coarse and fine grinding, and the soft grinding wheel with coarse particles is used for grinding.

(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 Improve the rotation accuracy of grinding wheel and parts in time so as to eliminate the occurrence of grinding cracks as much as possible.