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 quenched by heat treatment may have reticular cracks or regular cracks during the grinding process. Arranged small cracks are called grinding cracks, which not only affect the appearance of bearing parts, but more importantly, directly affect the quality of bearing parts. This article shares 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, and the depth is shallow and basically the same. Lighter grinding cracks are perpendicular to or close to the parallel lines perpendicular to the grinding direction, and are regularly arranged strip cracks. This is the first type of crack. It is roughly 0.03-0.15ram, and the crack is obvious after acid corrosion, which is the second type of crack.
2. Causes of bearing grinding cracks
The generation of bearing grinding cracks is caused by grinding heat, and the bearing surface temperature can reach 800-1000°C or higher during grinding. The structure of quenched steel is martensite and a certain amount of retained austenite, which are 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 retained austenite in the quenched steel is decomposed under the influence of grinding heat during grinding, and gradually transforms into martensite. This new martensite is concentrated on the surface of the part, causing local expansion of the bearing surface, increasing the the surface stress of the parts leads to the concentration of grinding stress, and continuous grinding will accelerate the generation of surface grinding cracks; in addition, the new martensite is relatively 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, it is both pressure and tension on the parts, which promotes the generation of grinding cracks. If the cooling is insufficient during grinding, the heat generated during grinding is enough to re-austenize the thin layer on the grinding surface, and then quench it into quenched martensite again, thus causing additional structural stress on the surface layer. Coupled with the heat generated by grinding, the temperature of the bearing surface rises and cools extremely fast, and the superposition of this structural stress and thermal stress may cause grinding cracks on the grinding surface.
3. Measures to prevent grinding cracks
From the above analysis, we know that the root cause of grinding cracks is that the martensite during quenching is in a state of expansion, and there is stress. To reduce and eliminate this kind of stress, stress relief tempering, that is, quenching, should be carried out. After tempering, the squandering time must be more than 4 hours. As the tempering time increases, the possibility of grinding cracks decreases. In addition, the bearing will crack when it is rapidly heated to about 100°C and then cooled rapidly. In order to prevent cold cracks, the parts should be tempered at about 150-200°C. If the bearing continues to heat up to 300°C, the surface will shrink again and cracks will occur. To prevent cracks, the bearing should be tempered at about 300°C. It is worth noting that tempering the bearing at around 300°C will reduce its hardness, so it is not suitable for use. Again, grinding cracks still occur after one tempering, which can be carried out for secondary tempering or artificial aging treatment, which is very effective.
Grinding cracks are caused by grinding heat, so reducing grinding heat is the key to solving grinding cracks. Generally, the wet grinding method is 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 the parts be cooled instantly after grinding, and the coolant can quench the grinding point at the same time. Therefore, increasing the amount of coolant used 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 rate is small, which can reduce grinding cracks. However, the effect of this method is not very significant, and it is dusty and affects the working environment, so it is not suitable to adopt.
Use a grinding wheel with softer hardness and coarser sand grains for grinding, which can reduce the grinding heat. However, coarser particles will affect the surface roughness of 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 and fine grinding. For coarse grinding, a soft grinding wheel with coarser particles is used for grinding, which is convenient for strong grinding and improves efficiency. Then, a finer grinding wheel is used for fine grinding, and the grinding feed rate is small. It is an ideal method to use two sets for rough grinding and fine grinding.
It is also an effective way to reduce grinding cracks by selecting grinding wheel abrasives with good self-sharpening performance, removing the waste on the surface of the grinding wheel in time, reducing the grinding feed rate, increasing the number of grinding times, and reducing the speed of the table.
The rotation speed of the grinding wheel and parts is also one of the main influencing factors. The large amount of rotation runout of the grinding wheel and the large amount of movement of the parts are the causes of grinding cracks. Improve the rotation accuracy of the 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 surface of bearing steel, the main methods are:
① Reduce grinding heat to solve grinding cracks. the
② It is divided into coarse grinding and fine grinding. For rough grinding, a soft grinding wheel with coarser particles is used for grinding. the
③ Select grinding wheel abrasives with good self-sharpening performance, remove the waste on the surface of the grinding wheel in time, reduce the grinding feed, increase the number of grinding times, and reduce the speed of the table. the
④ Improve the rotation accuracy of the grinding wheel and parts in time so as to eliminate the occurrence of grinding cracks as much as possible.
