Grinding is a common method of metal cutting in the mechanical engineering industry and is also widely used in the bearing manufacturing sector. Bearing components that have undergone heat treatment and quenching may develop a network of cracks or fine, regularly arranged cracks during the grinding process; these are known as grinding cracks. Not only do they affect the appearance of the bearing components, but more importantly, they directly impact their quality. Below, we shall discuss the characteristics and causes of grinding cracks in bearings, as well as the corresponding preventive measures.

1. Characteristics of grinding cracks in bearings:

Grinding cracks differ markedly from typical quenching cracks. They occur exclusively on the ground surface, are relatively shallow, and are generally of uniform depth. Mild grinding cracks form as parallel lines perpendicular or nearly perpendicular to the grinding direction, arranged in a regular pattern; this constitutes one type of crack. More severe cracks exhibit a tortoise-shell pattern (a closed network), with a depth of approximately 0.03–0.15 mm; these cracks become clearly visible after acid etching, representing the second type.

2. Causes of Bearing Grinding Cracks:

The formation of bearing grinding cracks is caused by grinding heat; during grinding, the surface temperature of the bearing can reach 800–1000 °C or higher. The microstructure of quenched steel consists of martensite and a certain amount of retained austenite, which are in a state of expansion. The expansion and contraction of martensite increase with the carbon content of the steel, a factor of particular significance in the formation of grinding cracks on the surface of bearing steel. The residual austenite in quenched steel decomposes under the influence of grinding heat during the grinding process, gradually transforming into martensite. This newly formed martensite concentrates on the surface of the component, causing localised expansion of the bearing surface, increasing surface stress, and leading to stress concentration. Continued grinding will accelerate the formation of surface grinding cracks; Furthermore, the newly formed martensite has a larger grain size, which also facilitates the formation of grinding cracks during the grinding process.

On the other hand, when parts are ground on a grinding machine, they are subjected to both compressive and tensile forces, which further promotes the formation of grinding cracks. If cooling during grinding is inadequate, the heat generated by the grinding process is sufficient to cause a thin layer of the ground surface to re-austenitise, followed by re-hardening into quenched martensite. This results in additional microstructural stresses in the surface layer. Combined with the rapid rise and fall in temperature of the bearing surface caused by the heat generated during grinding, the superposition of these microstructural and thermal stresses may lead to the formation of grinding cracks on the ground surface.

3. Measures to Prevent Grinding Cracks:

From the above analysis, it is clear that the fundamental cause of grinding cracks lies in the fact that the martensite formed during quenching is in an expanded state and contains residual stresses. To reduce and eliminate these stresses, stress-relief tempering must be carried out, i.e., quenching followed by tempering, with the tempering time set to at least 4 hours. As the tempering time increases, the likelihood of grinding cracks occurring decreases. Furthermore, cracks may form if the bearing is rapidly heated to approximately 100°C and then cooled rapidly. To prevent cold cracks, the component should be tempered at around 150–200°C. If the bearing is heated further to 300°C, the surface will contract again and cracks may form; to prevent this, the bearing should be tempered at around 300°C. It is worth noting that tempering bearings at around 300°C reduces their hardness, so this method should not be used in certain cases. If grinding cracks still occur after a single tempering, a second tempering or artificial ageing treatment can be carried out; this method is highly effective.

Grinding cracks are caused by grinding heat, so reducing this heat is key to resolving the issue. The wet grinding method is generally employed; however, regardless of the amount of coolant injected, it cannot reach the grinding surface in time during the grinding process, and thus fails to reduce the grinding heat at the grinding point. The coolant can only provide momentary cooling to the grinding point on the grinding wheel and the workpiece after the grinding action has passed, whilst simultaneously acting as a quenching agent on the grinding point. Therefore, increasing the volume of coolant used is one of the primary measures to minimise grinding heat in the grinding zone. If dry grinding is employed, a reduced grinding feed rate can help minimise grinding cracks. However, this method is not particularly effective and generates significant dust, which adversely affects the working environment; consequently, it is not recommended.

Selecting a grinding wheel with a softer hardness and coarser grit can help reduce grinding heat. However, coarser grains affect the surface roughness of the workpiece; this method cannot be used for parts requiring high surface finish, and is therefore subject to certain limitations. Separating the process into rough and finish grinding—using a soft grinding wheel with coarser grains for rough grinding to facilitate aggressive grinding and improve efficiency, followed by a finer-grained wheel for finish grinding with a smaller feed rate—is a relatively ideal approach. Using two separate machines for rough and finish grinding is the most ideal approach.

Selecting grinding wheel abrasives with good self-sharpening properties, promptly removing debris from the wheel surface, reducing the grinding feed rate, increasing the number of grinding passes, and lowering the worktable speed are also effective ways to minimise grinding cracks.

Grinding wheels and parts