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Prevention method for grinding cracks of vacuum carburizing and quenching gears

There has been a long-term serious problem in the gear grinding process – cracks. Grinding cracks refer to the occurrence of the grinding surface, the depth is shallow, and the depth is basically the same, and the direction is perpendicular to the tooth direction, that is, perpendicular to the direction of the reciprocating motion of the grinding wheel. Arranged strip cracks can be observed with the naked eye. It is very necessary to study the causes and prevention measures of grinding cracks in carburized and quenched steel gears.

1.Causes of cracks and effective measures to prevent them

1.1 Causes of cracks

(1) The quality of gear heat treatment is an internal factor that causes wear and tear

The root cause of grinding cracks is grinding heat. During the carburizing process of gears, reticulated carbides or excessive free carbides are easily formed in the structure of the carburized layer. Due to the extremely high hardness of each material, during the grinding process, when the grinding wheel and the tooth surface are in contact, the temperature of the grinding zone is very high, and local overheating tendency and surface tempering may occur, which will change the metallographic structure. Carburizing and quenching gears, scrapping due to grinding cracks occurs in many factories, sometimes even serious.

According to the test of Russian scholars, when the grinding wheel speed is v=18mPs and the grinding depth is 0.05mm, the temperature of the grinding zone reaches 900~1100℃, so when the carburized and hardened tooth surface is ground, the surface of the tooth surface will be in a thin layer. The tempered martensite structure becomes a higher temperature (above 300C) tempered structure. Martensite precipitates carbides, and the retained austenite is further decomposed into tempered martensite or tempered troostite, and no structural change occurs in the subsequent cooling process. At this time, the specific volume decreases, the hardness decreases, and tensile stress is formed on the surface. In the tempered layer, due to the high grinding temperature (above 8000°C), it has been tempered, and the martensite structure is heated to above the critical temperature, and the quenched structure is generated in the subsequent cooling process. At this time, the specific volume increases. compressive stress is generated.

Below the tempered layer, it is less affected by the grinding heat (200~300℃), so only the transformation of retained austenite occurs, resulting in untempered martensite, and the specific volume of this transformation increases. During grinding, the retained austenite in the steel transforms into a secondary quenched structure at a temperature of 200-300 °C, and the volume expands to form a large internal stress.

In short, during grinding, the thermal stress and phase transition pressure generated inside the workpiece, plus the tearing stress caused by the grinding wheel grinding the workpiece, the combined maximum stress of the three is consistent with the direction of the tooth length due to the action of this force , The internal stress opposite to this combined stress is formed on the gear. If the strength of the carburized layer and the structural stress of the metal can resist the combined stress generated during grinding, cracks will not occur. On the contrary, the gear material will be damaged during the grinding process, resulting in cracks perpendicular to the direction of the resultant stress, and wear cracks appear.

(2) Thermal stress generated by grinding is an external factor that causes wear cracking

During the grinding process, thermal stress and mechanical stress will be generated between the grinding wheel and the workpiece, and gear grinding is no exception. The size of the generated stress is the external condition that constitutes the grinding crack or not. These all depend on the grinding conditions, such as the hardness of the grinding wheel, the particle size, the type of abrasive, the grinding speed, the coolant, the cutting depth, the amount of cutting, and the grinding wheel. The number of reciprocations per minute and the dressing of the grinding wheel, etc. Grinding cracking occurs when the grinding tensile stress exceeds the fracture of the metal. However, in actual production, the causes of grinding stress are very complicated. It is found through experiments that almost all grinding and cracked tooth surfaces are accompanied by tempering or re-quenching, which shows that the tensile stress peak is accompanied by heating and heating. Therefore, For carburized gears, the main reason for grinding tensile stress is that thermal stress and structural stress are instantaneously severe on the extremely thin surface of the gear, resulting in unbalanced stress within the surface organization. The comprehensive action of these conditions produces a certain stress, which constitutes the external cause of wear and tear.

2.Process measures to prevent grinding cracks

2.1 Analysis from the perspective of organizational structure

(1) Controlling the carbon concentration on the gear surface

The gears produced are carburized in a pit furnace, the carbon powder control is not very good, and the carbon content on the gear surface is often high, sometimes reaching 1.2%~1.3%. Reticular or angular carbides are formed in the corresponding structure with high carbon content, and there are more retained austenites. Excessive and poorly distributed carbides tend to brittle and form cracks during the grinding process. A large amount of retained austenite will occur due to the extrusion force during the grinding process. Martensitic transformation will occur and cause local volume. At the same time, it causes shrinkage under the action of grinding heat and finally forms tensile stress, which leads to cracks.

Based on the above analysis, the carbon powder in the carburizing process should be strictly controlled to reduce the surface carbon content to 0.8%~1.0%, and correspondingly, the carbide and retained austenite should be controlled below grade 3, which effectively improves the microstructure. .

(2) Controlling austenite grain size

The high temperature of carburizing and quenching will make the austenite grains coarse and form coarse needle martensite after quenching. The coarse acicular martensite has high brittleness and is prone to brittle cracking during grinding. Therefore, the original heat treatment process was adjusted: the carburizing temperature was reduced from 930°C to 900°C, and the quenching temperature was lowered from 850°C to 820°C. , the martensite obtained after improvement is relatively fine.

(3) Adjust the tempering process to reduce the quenching stress

The martensite formed by austenite quenching is in an expanded state, and the heat generated during the grinding process of the workpiece will cause the martensite to shrink during tempering, and this shrinkage is restricted by the rigid metal of the parent body, which will locally produce tensile stress on the surface. , when the tensile stress reaches a certain limit value, cracks will form. Based on this analysis, the original 170℃×2h tempering process was changed to 200℃×4h tempering process by appropriately increasing the tempering temperature and prolonging the holding time. Depending on the situation, especially in winter, measures to supplement secondary tempering are sometimes taken.

3.Conclusion

(1) The root cause of grinding cracks is that the high heat generated on the metal surface during the grinding process causes the martensite volume to shrink to form tensile stress, and the poor microstructure will further increase the sensitivity of grinding cracks.

(2) Strictly control the surface carbon content where the carbon potential is too low to avoid the formation of poor carbide morphology and distribution; appropriately reduce the carburizing temperature and quenching temperature, increase the tempering holding time, and make the martensite structure fine and retained austenite. The body content is not high, the quenching stress is reduced. Through the improvement of the above heat treatment process, the tendency of grinding cracks can be greatly reduced.

(3) Improve the grinding process. Reasonable selection of coarse and fine grinding wheel particle size and hardness and grinding parameters can effectively prevent the generation of local high heat, thereby preventing the formation of grinding cracks.

Of course, there are many reasons for grinding cracks, which should be comprehensively considered and controlled from the grinding process, heat treatment and parts material.

Selection of heat treatment equipment: The RVC series vacuum carburizing furnace produced by SIMUWU is a high-quality product for the vacuum carburizing process of tools and molds. Good temperature control accuracy and temperature control uniformity ensure the effective progress of the vacuum carburizing process.