Characteristics and causes of bearing grinding cracks and measures to prevent them

The grinding process is a common method of metal cutting in the machinery manufacturing industry, and is widely used in the bearing processing industry. The heat-treated and quenched bearing parts may appear as a net-like crack or a more regular arrangement of small cracks during the grinding process, called grinding cracks, which not only affect the appearance of the bearing parts, but more importantly, also directly affect the quality of the bearing parts. This article shares the characteristics of bearing grinding cracks and the reasons for them, as well as the corresponding preventive measures.

1, bearing grinding crack characteristics

Grinding cracks are obviously different from general quenching cracks, grinding cracks only occur on the grinding surface, the depth is shallow, and the depth is basically the same. The lighter grinding crack is perpendicular or close to the parallel line perpendicular to the grinding direction, and is a regular arrangement of stripe cracks, which is a kind of crack, the more serious cracks are tortoise-like (closed network-like), its depth is roughly 0.03 ~ 0.15ram, the crack is obvious after corrosion with acid, which is the second crack.

2, bearing grinding cracks generated by the reasons

Bearing grinding cracks are caused by grinding heat, and the temperature of the bearing surface can be 800-1000 degrees Celsius or higher when grinding. The organization of quenched steel is martensite and a certain amount of residual austenite, which are in an expanded state (without tempering treatment). The expansion and contraction of martensite increases with the increase in carbon content in the steel, making it particularly important to produce grinding cracks on the surface of the bearing steel. The residual austenite in the quenched steel is decomposed by the grinding heat during grinding and gradually transformed into martensite, which is concentrated on the surface of the parts, causing local expansion of the bearing surface, increasing the stress on the surface of the parts, leading to a concentration of grinding stress, and continuing grinding will accelerate the production of surface grinding cracks; in addition, the new martensite is larger, and the grinding is also likely to accelerate the production of grinding cracks.

On the other hand, when grinding parts on the grinding machine, it is both pressure and tension on the parts, which contributes to the generation of grinding cracks. If the cooling is not sufficient during grinding, the heat generated during grinding is sufficient to re-austenitize the thin layer of the grinding surface and subsequently quench it again into quenched martensite, thus causing additional tissue stress on the surface layer, plus the heat generated by grinding makes the bearing surface temperature rise extremely fast and cool extremely fast, and the iteration of this tissue stress and thermal stress may lead to grinding cracks on the grinding surface.

3, grinding crack prevention measures

From the above analysis, we know that the root cause of grinding cracks lies in the quenching of the martensite in a state of expansion and the presence of stress. To reduce and eliminate this stress, stress tempering should be carried out, that is, quenching, in the tempering process, the volatile time must be more than 4h. As the tempering time increases, the possibility of producing grinding cracks decreases. In addition, bearings that are heated rapidly to around 100 degrees Celsius and cooled rapidly can produce cracks. To prevent cold cracking, the parts should be tempered at around 150 to 200 degrees Celsius. If the bearing continues to heat up to 300 degrees Celsius, the surface will shrink again and cracks will be produced. It is worth noting that tempering the bearing at around 300 degrees Celsius will make its hardness drop, so it is not suitable. Again, if grinding cracks still occur after one tempering, this is possible with a second tempering or artificial ageing treatment, which 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 much coolant is injected, the coolant cannot reach the grinding surface in time for grinding and therefore cannot reduce the grinding heat at the grinding point. The coolant can only cool the grinding wheel and the grinding point of the part instantly after the grinding process, while the coolant quenches the grinding point. Therefore, increasing the amount of coolant used is one of the main measures to minimise 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. However, the effect of this method is not very significant, and it is not suitable to be adopted as it is dusty and affects the working environment.

Choosing a softer, coarser grit grinding wheel for grinding can reduce grinding heat. However, the coarser particles will affect the roughness of the part surface and this method cannot be used for parts with high surface roughness requirements and is therefore subject to certain restrictions. Separate coarse and fine grinding, both coarse grinding with a coarse particle size soft grinding wheel is chosen for grinding to facilitate strong grinding and improve efficiency, and then fine grinding with a finer particle size wheel for fine grinding with a small grinding feed. This is the ideal way to carry out rough and fine grinding in two stages.

Choose grinding wheel abrasives with good self-sharpening properties, remove the grinding wheel surface waste in time, reduce the grinding feed, increase the number of grinding operations and reduce the table speed, which is also an effective way to reduce grinding cracks.

The rotational speed of the grinding wheel and the part is also one of the main influencing factors. The large rotational runout of the grinding wheel and the large tampering of the part are the causes of grinding cracks. Improve the rotational accuracy of the grinding wheel and parts in time to eliminate the factors that cause grinding cracks as far as possible.

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

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

① Reduce grinding heat to solve grinding cracks.

②Separate coarse and fine grinding, both coarse grinding choose coarse particles of soft grinding wheel for grinding.

③ Select grinding wheel abrasives with good self-sharpening properties, remove the grinding wheel surface waste in time, reduce the grinding feed, increase the grinding times and reduce the table speed.

④Improve the rotational accuracy of the grinding wheel and the part in time so as to eliminate as far as possible the cracks caused by grinding.