Worm is an important part of machine tool rotating parts. Worm spiral surface and worm gear tooth surface slip relatively when working, which is easy to wear. Therefore, the worm will usually carburizing quenching treatment to obtain higher hardness to prevent worm spiral surface wear. Worm parts require high precision, so the processing technology is complex and long. If there is any problem during processing, it will cause big loss. A kind of worm parts in our company appeared grinding cracks during grinding, resulting in parts scrapped, seriously affecting the production schedule.
1. Worm material and main process flow
The worm material is 20CrMnTiH and the heat treatment requires carburizing and quenching with a depth of 1.1-1.5mm and a surface hardness of 58HRC after quenching.
Blanking→Forging→Normalizing→Rough machining→Stress relieve→Fine machining→carburizing→Carbon remove on the thread→Quenching→Rough grinding→Test→Aging→Fine grinding
Heat treatment process: Forging normalizing→ Carburizing →Quenching →Low temperature tempering →Straightening→Stress relief→Low temperature aging.
After carburizing and quenching, the carburized depth layer is 1.32mm and the surface hardness is 59 to 60HRC.
2. Crack phenomenon description
After rough grinding, the crack appears on the tooth surface of the worm. The cracks are very fine, linear and shallow in depth. The shape of the cracks is usually called “hairline”. The cracks are not parallel and scattering lines.
3. Analysis of grinding cracks
It is obviously different from common quenching cracks. The quenching cracks are thick and deep, and the number of cracks is small. It only occurs on the grinding surface, and the number is large, the depth is shallow, but the depth is basically the same. The lighter ones are stripe cracks perpendicular to or nearly perpendicular to the grinding direction. The worm tooth surface is a spiral surface.
In the grinding process of heat-treated parts, the instantaneous high temperature in the grinding zone can reach 400-1500 â because the grinding wheel is blunt, the grinding depth is too deep and the cooling is insufficient. As a result, the grinding burn occur, resulting in the changes of surface microstructure, microhardness, residual stress and mechanical properties of the parts.
The formation of cracks is related to the hardness of the workpiece. Although the workpiece with hardness less than 55HRC may be burned, the occurrence of grinding cracks is rare. The workpiece with hardness above 60HRC will greatly increase the possibility of crack. The measured hardness of the part is 59 ~ 60HRC, which greatly increases the tendency of grinding cracks. On the one hand, when grinding conditions are improper or grinding is not standard, grinding surface will firstly form a strong burn center, resulting in uneven thermal stress. In alternate high temperature and rapid cooling, the stress value gradually increased. When the stress exceeds the material strength limit, the grinded surface will produce cracks. On the other hand, the retained austenite in the surface carburized and quenched structure will be transformed into new martensite by the influence of intense grinding heat and the cooling of coolant during grinding, which will cause the local volume expansion on the surface of the part, increasing the tensile stress on the surface of the part and lead to stress concentration. Continuous grinding will easily accelerate the grinding crack generation.
From the above analysis, we know that the root cause of grinding cracks lies in the existence of stress. To prevent this phenomenon, it is necessary to reduce and eliminate various stresses on the surface of parts.