The grinding crack is obviously different from the general quenching crack, the grinding crack only occurs on the grinding surface, the depth is relatively shallow, and the depth is basically the same.The lighter grinding crack is perpendicular to or close to the parallel line of the grinding direction, and is a regular arranged strip crack, this is the first crack, the more serious crack is tortoise-shell shape (closed network shape), its depth is about 0.03 ~ 0.15 RAM, with acid corrosion crack is obvious, this is the second crack.

2. Causes of 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 microstructure of hardened steels is martensite and a certain amount of residual austenite, which is in an expanded state (untempered).The expansion and contraction of martensite increases with the increase of carbon content in the steel, which makes grinding cracks on the surface of bearing steel more important.Under the influence of grinding heat, the residual austenite in quenched steel decomposes and gradually transforms into martensite during grinding. The newly formed martensite concentrates on the surface of parts, causing local expansion on the surface of bearings and increasing the surface stress of parts, leading to the concentration of grinding stress. Continual grinding will accelerate the generation of surface grinding cracks.In addition, the new martensite 膪 is larger, and it is easy to accelerate the generation of grinding cracks during grinding.

On the other hand, when grinding parts on the grinder, both pressure and tension on the parts, promote the generation of grinding cracks.If the cooling is not sufficient during grinding, the heat generated during grinding is enough to make the grinding surface thin layer austenitized again, and then quenched again into quenched martensite, so that the surface layer produces additional organizational stress, coupled with the heat generated by grinding to make the bearing surface temperature rise very fast, cooling very fast,The superposition of microstructure stress and thermal stress may lead to grinding cracks on the grinding surface.

3, grinding crack prevention measures

It is known from the above analysis that the root cause of grinding crack is that the martensite is in a state of expansion during quenching and there is stress.To reduce and eliminate this stress, stress tempering should be quenched, in the tempering treatment, waste time must be more than 4h.With the increase of tempering time, the possibility of grinding cracks decreases.In addition, the bearing will crack in the rapid heating to about 100 degrees Celsius and rapid cooling.In order to prevent 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 shrink again and crack will occur. In order to prevent 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 to use.And after a tempering still produce grinding cracks, which can be secondary tempering or artificial aging treatment, this method is very effective.

The grinding crack is caused by the grinding heat, so reducing the grinding heat is the key to solve the grinding crack.Wet grinding is generally used, but no matter how the coolant is injected, the coolant cannot reach the grinding surface in time to reduce the grinding heat at the grinding point.The coolant can only make the grinding point of grinding wheel and parts be cooled instantly after grinding, and the coolant can quench the grinding point at the same time.Therefore, increasing the usage of coolant is one of the main measures to reduce the grinding heat in the grinding zone as far as possible.If the dry grinding method is adopted, the grinding feed is less and the grinding cracks can be reduced.But the effect of this method is not very significant, and dust flying, affect the working environment, should not be taken.

The grinding heat can be reduced by using the grinding wheel with soft hardness and coarse sand grain.However, the coarse particles will affect the surface roughness of the parts. For the parts with high surface roughness requirements, this method cannot be used, so it is limited to some extent.It is 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, fine grinding wheel is used for fine grinding, and the grinding feed is small.Rough grinding and fine grinding are carried out in two sets, which is an ideal method.

It is also an effective way to reduce grinding cracks to select grinding wheel abrasives with good self-sharpening performance, remove the waste material on the surface of grinding wheel in time, reduce grinding feed, increase grinding times and reduce the speed of worktable.

The rotation speed of the grinding wheel and parts is also one of the main influencing factors. The large rotation runout of the grinding wheel and the large channeling momentum of the parts are the inducements of grinding cracks.Improve the rotation precision of grinding wheel and parts in time so as to eliminate all kinds of factors that cause grinding cracks.

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

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

(1) Reduce grinding heat to solve grinding cracks.

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

③ Select the grinding wheel abrasive with good self-sharpening performance, remove the waste on the surface of the grinding wheel in time, reduce the grinding feed, increase the grinding times and reduce the speed of the worktable.

(4) improve the rotation accuracy of grinding wheel and parts in time so as to eliminate grinding cracks as far as possible.