Grinding is a kind of common machinery manufacturing, metal cutting processing method, the bearing has been widely used in the processing industry, the bearings components after the heat treatment quenching, may occur in the process of grinding are arranged in a cracked or the rules of the reticular tiny cracks, called grinding cracks, it not only affect the appearance of the bearing parts, more important is also directly affect the quality of the 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 cracks are obviously different from the general quenching cracks. The grinding cracks only occur on the grinding surface, the depth is shallower and the depth is basically the same. The light grinding cracks are perpendicular to or close to the parallel lines perpendicular to the grinding direction, and are regular arranged strip cracks, this is the first kind of crack, the more serious cracks are tortoise-shaped (closed network), its depth is about 0.03 -- 0.15 RAM, after acid corrosion cracks are obvious, this 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 a quenched steel is martensite and a certain amount of residual austenite in an expanded state (without tempering). The expansion and contraction of martensite increases with the increase of carbon content in the steel, which makes the grinding crack on the surface of bearing steel particularly important. The residual austenite in the quenched steel is decomposed and gradually transformed into martensite under the influence of grinding heat during grinding. This new martensite is concentrated on the surface of the parts, causing the local expansion of the bearing surface, increasing the surface stress of the parts, resulting in the grinding stress concentration. The continuous grinding will accelerate the production of surface grinding cracks. In addition, the new martensite 膪 is large, which is easy to accelerate the grinding crack.



On the other hand, when grinding parts on the grinder, the parts are both pressure and tension, which promotes the generation of grinding cracks. If the cooling is not sufficient during grinding, the heat generated during grinding is enough to reaustenitize the thin layer of the grinding surface, and then quenching into quenching martensite again, so that the surface layer generates additional microstructure stress, and the heat generated by grinding makes the bearing surface temperature rise very quickly and cool very quickly. The superposition of the microstructure stress and thermal stress may lead to the grinding crack on the grinding surface.



3, grinding crack prevention measures



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



Grinding crack is caused by grinding heat, so reducing grinding heat is the key to solve grinding crack. In general, the wet grinding method is used, but no matter how to inject the coolant, the coolant can not reach the grinding surface in time during grinding, so it can not reduce the grinding heat of the grinding point. The coolant can only cool the grinding wheel and the grinding point of the parts immediately after grinding, and at the same time, the coolant can do the quenching effect on the grinding point. Therefore, increasing the use of coolant is one of the main measures to reduce the grinding heat in the grinding zone. If the dry grinding method is used, the grinding feed is less, and grinding cracks can be reduced. But the effect of this method is not very significant, and dusty, affect the working environment, should not be taken.



The grinding wheel with soft hardness and coarse sand grains can reduce the grinding heat. However, the coarse particles will affect the roughness of the surface of the parts. For the parts requiring high surface roughness, this method cannot be used, so it is limited to some extent. It can be divided into coarse grinding and fine grinding. For coarse grinding, soft grinding wheel with coarse particles is selected for grinding, which is convenient for strong grinding and improves efficiency. Then, grinding wheel with fine particle size is used for fine grinding with small grinding feed. This is an ideal method for coarse and fine grinding.



It is also an effective way to reduce grinding cracks by selecting the grinding wheel abrasive with good self-sharpening performance, removing the grinding wheel surface waste in time, reducing the grinding feed, increasing the grinding times and reducing the speed of the working table.

The rotation speed of grinding wheel and parts is also one of the main influencing factors. The large rotation runout of grinding wheel and large channeling momentum of parts are the inducement of grinding cracks. To improve the rotation accuracy of grinding wheel and parts in time so as to eliminate the various factors causing grinding cracks as much as possible.



4. Some methods to prevent surface grinding cracks of bearing steel



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



① Reduce grinding heat to solve grinding cracks.



(2) Coarse and fine grinding. The soft grinding wheel with coarse particles is selected for coarse grinding.



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



④ Improve the rotation accuracy of grinding wheel and parts in time to eliminate grinding cracks as far as possible.