Grinding is a common method of metal cutting in the machinery manufacturing industry. It is also widely used in the bearing processing industry. Bearing parts that have been heat treated and quenched may have network-like cracks or more regular cracks during the grinding process. The arranged small cracks are called grinding cracks, which not only affect the appearance of bearing parts, but more importantly, also directly affect the quality of bearing parts. Below, China Bearing Network (abbreviated as: China Bearing Network) 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 obviously different from general quenching cracks. Grinding cracks only occur on the grinding surface, with a shallow depth and basically the same depth. The lighter grinding cracks are perpendicular to or nearly perpendicular to the parallel lines perpendicular to the grinding direction, and are regularly arranged strip cracks. This is the first type of crack. The more serious cracks are tortoise-shell-shaped (closed network-like), and their depth is It is roughly 0.03~0.15ram. The cracks are obvious after acid corrosion. This is the second type of crack.
2. Causes of bearing grinding cracks:
The generation of bearing grinding cracks is caused by grinding heat. The bearing surface temperature can reach 800-1000 degrees Celsius or higher during grinding. The structure of quenched steel is martensite and a certain amount of retained austenite, which are in an expanded state (without tempering treatment). The expansion and contraction of martensite increases with the increase of carbon content in steel, which is particularly important to produce grinding cracks on the surface of bearing steel. The residual 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 part leads to the concentration of grinding stress, and continued grinding will accelerate the generation of surface grinding cracks; in addition, the newly formed martensite is larger, and it is easy to accelerate the generation of grinding cracks during grinding.
On the other hand, when grinding parts on a grinder, there is both pressure and tension on the parts, which promotes the occurrence of grinding cracks. If cooling is insufficient during grinding, the heat generated during grinding is enough to re-austenitize the thin layer on the grinding surface and then quench it again into quenched martensite, thus causing additional structural stress on the surface layer. In addition, the heat generated by grinding causes the bearing surface temperature to rise and cool down extremely quickly. This superposition of structural stress and thermal stress may cause grinding cracks on the grinding surface.
3. Preventive measures for grinding cracks:
From the above analysis, we know that the fundamental reason for grinding cracks is that the martensite during quenching is in an expanded state and stress exists. To reduce and eliminate this stress, stress relief tempering, that is, quenching, should be performed. During the tempering treatment, the waste time must be more than 4 hours. As tempering time increases, the possibility of grinding cracks decreases. In addition, cracks will occur in bearings that are rapidly heated to about 100 degrees Celsius and then cooled rapidly. To prevent cold cracks from occurring, parts should be tempered at about 150 to 200 degrees Celsius. If the bearing continues to heat up to 300 degrees Celsius, the surface will shrink again and cracks will occur. To prevent cracks from occurring, the bearings should be tempered at about 300 degrees Celsius. It is worth noting that tempering the bearing at around 300 degrees Celsius will reduce its hardness and should not be used in emergency situations. If grinding cracks still occur after one tempering, secondary tempering or artificial aging treatment can be performed. This method is very effective.
Grinding cracks are caused by grinding heat, so reducing grinding heat is the key to solving grinding cracks. The wet grinding method is generally used, but no matter how 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 cool the grinding points of the grinding wheel and parts instantly 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 reduce the grinding heat in the grinding area as much as possible. If the dry grinding method is used, the grinding feed amount is small, which can reduce grinding cracks. However, the effect of this method is not very obvious, and it is dusty and affects the working environment, so it should not be adopted.
Using a grinding wheel with softer hardness and coarser sand grains for grinding can reduce grinding heat. However, coarser particles will 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 restrictions. It is divided into rough grinding and fine grinding. For rough grinding, a soft grinding wheel with coarser particles is used for grinding, which facilitates strong grinding and improves efficiency. Then a grinding wheel with finer particle size is used for fine grinding, and the grinding feed amount is small. It is an ideal method to carry out rough grinding and fine grinding in two machines.
Selecting grinding wheel abrasives with good self-sharpening performance, promptly removing waste material on the surface of the grinding wheel, reducing the grinding feed, increasing the number of grinding times, and reducing the workbench speed is also an effective way to reduce grinding cracks.
The rotation speed of the grinding wheel and parts is also one of the main influencing factors. The large rotational runout of the grinding wheel and the large movement of parts are all causes of grinding cracks. Improve the rotation accuracy of grinding wheels and parts in a timely manner 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, the main methods to prevent the occurrence of grinding cracks on the surface of bearing steel are:
① Reduce grinding heat and solve grinding cracks.
② It is divided into rough grinding and fine grinding. For rough grinding, a soft grinding wheel with coarser particles is used for grinding.
③Select grinding wheel abrasives with good self-sharpening performance, remove 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 workbench.
④ Improve the rotation accuracy of the grinding wheel and parts in a timely manner to eliminate the occurrence of grinding cracks as much as possible.
2023-09-25