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 net-like cracks or more regular during the grinding process. The arranged small cracks are called grinding cracks, which not only affect the appearance of the bearing parts, but more importantly, directly affect the quality of the bearing parts. This article shares the characteristics and causes of bearing grinding cracks, as well as the corresponding preventive measures.
1. The characteristics of bearing grinding cracks
Grinding cracks are obviously different from general quenching cracks. Grinding cracks only occur on the grinding surface, the depth is relatively shallow, and the depth is basically the same. The lighter grinding cracks are perpendicular or close 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-shaped), and their depth Roughly 0.03 ~ 0.15ram, the cracks are obvious after acid corrosion, this is the second type of cracks.
2. Reasons for bearing grinding cracks
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 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 the carbon content in the steel, and it is particularly important to cause grinding cracks on the surface of the bearing steel. The retained austenite in the quenched steel is decomposed and gradually transformed into martensite during grinding under the influence of the grinding heat. This new martensite is concentrated on the surface of the part, causing local expansion of the bearing surface and increasing The surface stress of the part causes the concentration of grinding stress, and the continued 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 during grinding.
On the other hand, when grinding parts on a grinder, both pressure and tension are applied to the parts, which promotes the generation of grinding cracks. If the cooling is insufficient during grinding, the heat generated during grinding is sufficient to re-austenitize the thin layer of the ground surface, and then quench it into quenched martensite again, thus causing additional structural stress on the surface layer. Coupled with the heat generated by the grinding, the bearing surface temperature rises extremely fast and the cooling is extremely fast. The superposition of this structural stress and thermal stress may cause grinding cracks on the ground surface.
3. Preventive measures for grinding cracks
From the above analysis, it is known that the root cause of grinding cracks is that the martensite during quenching is in a state of expansion and stress exists. To reduce and eliminate this kind of stress, stress-relieving tempering, that is, quenching, should be carried out. When tempering is carried out, the squandering time must be more than 4h. 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 degrees Celsius and quickly 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 will shrink again and cracks will occur. To prevent cracks, the bearing should be tempered at about 300 degrees Celsius. It is worth noting that tempering of the bearing at about 300 degrees Celsius will reduce its hardness, so it is not suitable for use. Grinding cracks are still produced after one tempering, which can be used for 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 solving grinding cracks. Generally used wet grinding method, 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 instantaneously after the grinding passes, and at the same time, the coolant has a quenching effect on the grinding point. Therefore, increasing the amount of coolant used 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 rate is small, which can reduce grinding cracks. However, the effect of this method is not very significant, and the dust is flying, which affects the working environment, so it is not suitable to be adopted.
Choose a grinding wheel with softer hardness and coarser sand for grinding, which can reduce the grinding heat. However, the coarser particles will affect the surface roughness of the parts. For parts with high surface roughness requirements, this method cannot be used, so it is 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 is convenient for powerful grinding and improves efficiency. Then, a fine grinding wheel is used for fine grinding with a small grinding feed. Rough grinding and fine grinding are divided into two sets, which is an ideal method.
Selecting the grinding wheel abrasive with good self-sharpening performance, timely removal of the waste material on the grinding wheel surface, reducing the grinding feed rate, increasing the grinding frequency, and reducing the speed of the table, this is 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 large amount of rotation of the grinding wheel and the large amount of movement of the parts are all inducements of grinding cracks. Improve the rotation accuracy of the grinding wheel and parts in time so as to eliminate the various factors that cause grinding cracks as much as possible.
4. Some methods to prevent grinding cracks on the bearing steel surface
In the grinding process, the main methods to prevent grinding cracks on the bearing steel surface are:
① Reduce grinding heat to solve grinding cracks.
②It is divided into rough grinding and fine grinding. For rough grinding, a soft wheel with coarser particles is used for grinding.
③Select the grinding wheel abrasive with good self-sharpening performance, remove the waste material on the surface of the grinding wheel in time, reduce the grinding feed rate, increase the grinding frequency, and reduce the speed of the table.
④ Improve the rotation accuracy of the grinding wheel and parts in time to eliminate as much as possible the occurrence of grinding cracks.
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