Grinding is a common method of metal cutting in the mechanical manufacturing industry and is also widely applied in the bearing manufacturing sector. Bearing components that have undergone heat treatment and quenching may develop a network of cracks or finely arranged, regular fissures during the grinding process, termed grinding cracks. These not only affect the appearance of the bearing components but, more critically, directly impact their quality.

1. Characteristics of bearing grinding cracks:

Grinding cracks differ markedly from typical quenching cracks. They occur exclusively on ground surfaces, are relatively shallow, and exhibit consistent depth. Mild grinding cracks form parallel lines perpendicular or nearly perpendicular to the grinding direction, exhibiting a regular striped pattern. This constitutes the first type of crack. More severe cracks appear in a tortoise-shell pattern (closed network), with depths generally ranging from 0.03 to 0.15 mm. These cracks become clearly visible after acid etching, representing the second type.

2. Causes of Bearing Grinding Cracks:

Grinding cracks in bearings originate from grinding heat. During grinding, the bearing surface temperature can reach 800–1000°C or higher. The microstructure of quenched steel comprises martensite and a certain amount of retained austenite, both in an expanded state (untempered). The expansion and contraction of martensite increases significantly with rising carbon content in the steel, which is particularly crucial in causing grinding cracks on bearing steel surfaces. Residual austenite within quenched steel decomposes under grinding heat, gradually transforming into martensite. This newly formed martensite concentrates at the component surface, causing localised expansion of the bearing surface. This increases surface stress, leading to grinding stress concentration. Continued grinding accelerates the formation of surface grinding cracks. Furthermore, the large grain size of the newly formed martensite readily accelerates the generation of grinding cracks during the process. Additionally, when grinding components on a grinding machine, the simultaneous application of compressive and tensile stresses further promotes the formation of grinding cracks. Insufficient cooling during grinding allows the heat generated to re-austenitise the surface layer, which subsequently re-hardens into quenched martensite. This induces additional microstructural stresses in the surface layer. Coupled with the rapid temperature rise and subsequent rapid cooling of the bearing surface due to grinding heat, the combined effect of these microstructural and thermal stresses may induce grinding cracks in the surface.

3. Preventive Measures for Grinding Cracks:

Analysis indicates that grinding cracks fundamentally arise from residual stresses within the expanded martensite structure during quenching. To mitigate these stresses, stress-relief tempering must be performed: first quench, then temper for a minimum duration exceeding 4 hours. The likelihood of grinding cracks diminishes with extended tempering time. Additionally, bearings heated rapidly to approximately 100°C followed by swift cooling may develop cracks. To prevent cold cracks, components should be tempered at around 150–200°C. Should bearings be further heated to 300°C, surface contraction may cause cracking. To counter this, bearings should be tempered at approximately 300°C. It should be noted that tempering bearings at approximately 300°C reduces their hardness, making this approach unsuitable in certain instances. Should grinding cracks persist after initial tempering, secondary tempering or artificial ageing treatment may be applied, proving highly effective.

Grinding cracks originate from grinding heat, thus mitigating this heat is fundamental to their prevention. While wet grinding is generally employed, cooling fluid cannot reach the grinding surface promptly during the grinding process, regardless of the injection rate, thus failing to reduce heat at the grinding point. The fluid only provides instantaneous cooling to the grinding wheel and workpiece contact point after the grinding pass, simultaneously performing a quenching effect on the grinding point. Consequently, increasing coolant volume is a primary countermeasure to minimise grinding heat within the grinding zone. Dry grinding with reduced feed rates may reduce cracking, though this method proves less effective and generates significant dust, adversely affecting the working environment and rendering it impractical.

Selecting grinding wheels with softer hardness and coarser abrasive grains can reduce grinding heat. However, coarser particles adversely affect surface finish. This method is unsuitable for components requiring high surface precision, thus imposing limitations. A two-stage process—rough grinding followed by finishing—is preferable. Rough grinding employs a coarse-grained, soft wheel for high-efficiency, forceful grinding. This is followed by finishing with a finer-grained wheel using a reduced feed rate. Conducting rough and finish grinding on separate machines represents a more ideal approach.

Selecting grinding wheels with superior self-sharpening properties, promptly removing waste material from the wheel surface, reducing feed rates, increasing the number of grinding passes, and lowering table speeds also constitute effective methods for minimising grinding cracks.

The rotational speeds of both the grinding wheel and workpiece are significant influencing factors. Excessive rotational runout of the grinding wheel and high rotational speed of the workpiece are both contributing causes of grinding cracks. Promptly improving the rotational accuracy of both the grinding wheel and workpiece helps eliminate various factors that induce grinding cracks.

4. Methods to prevent grinding cracks on bearing steel surfaces:

During grinding operations, the primary methods to prevent grinding cracks on bearing steel surfaces are:

① Mitigate grinding heat to resolve cracking issues.

② Implement separate rough and finish grinding operations, employing coarse-grained, soft grinding wheels for rough grinding.

③ Select grinding wheel abrasives with superior self-sharpening properties, promptly remove waste material from the wheel surface, reduce feed rates, increase the number of grinding passes, and decrease table speed.

④ Promptly enhance the rotational precision of both the grinding wheel and workpiece to minimise factors contributing to grinding cracks.