Research And Emulation On Grind-hardening For 42CrMo Steel

Grind-hardening is a kind of green and environment-protective processing method, which can realize surface hardening in the process of grinding. The effect of grind-hardening is affected by many factors. The seeking of grind-hardening rules and ways of improving grind-hardening properties has very important theoretical research and practical application value for the engineering application of grind-hardening.42CrMo steel, which is widely used in industry, is used as the experimental material. The grind-hardening experiment is performed on the MM7132 surface grinding machine. The rules of grind-hardening are studied from grinding depth, grinding speed, original structure granularity of grinding wheel, grinding ways, etc. Microstructure of the specimens are observed by scanning electron microscopy (SEM) with the type of SSX-50, optical microscope (OM) with the type of XJG-04 and transmission eletron microscope with the type of F20; microhardness of the specimens are tested by microhardness meter with the type of HVS-10; wear resistance of the specimens are tested by abrasive wear tester with the type of ML-100 and adhesive wear tester with the type of M-200; surface roughness of the specimens are measured by tiny portable surface roughness tester with the type of TR110. Temperature field of the specimens after grind hardening and strain field of typical parts with different hardening methods are simulated with ANSYS software.(1)From grinding surface to the centre, specimens after grinding can be divided into three parts, i.e., fully hardened region, transitional region and the base. Microstructure in the fully hardened region is composed by lath martensite and acicular martensite; microstructure in the transitional region is composed by martensite and the base. The content of martensite becomes less when the site is closer to the centre.(2)From the grinding surface to the depth direction, when the grinding depth increases, martensite in the fully hardened region is finer, then coarser, and finer at last. Microhardness on the surface of specimens and the thickness of hardening layer are all increased. With the increase of grinding speed, microstructure becomes finer and the surface microhardness of specimens and the thickness of hardening layer are increased too. When the grinding depth is same, microstructure obtained by several reciprocating is finer than grinding with one time; the surface microhardness and the thickness of hardening layer are improved. Specimens with the state of tempering, annealing and normalizing are grinded, but lath martensite are all obtained. Microhardness at the hardened layer is almost same, while the thickness of hardening layer has a little difference. It decreased in the sequence of normalizing, tempering and annealing. The increase of granularity of grinding wheel can refine the grinding microstructure and improve the surface microhardness and the thickness of hardening layer. The ratio of height to diameter has certain effect on the grinding. When the ratio is below certain value, martensite obtained by dry grinding is finer than that by wet grinding; but when the ratio is increased to a certain extent, the grinding surface can not be fully hardened with dry grinding. The ratio has little effect on the grinding surface with wet grinding.(3)Grind hardening with liquid nitrogen can make the obtained martensite finer than that obtained by water. Thickness of the hardened layer, microhardness and the wear-resistance are improved. When deep cooling technology is performed on the specimens which have grind hardened with liquid nitrgon, microstructure is refined further and the wear-resistance of the specimens is improved further too.(4)The emulation results show that the highest temperature is different with the same depth and at the different surface location. When the grinding parameters are not changed, the region with the austenitizing temperature is increased and the hardened layer is also increased, which is consistent with the law of the grinding experiment. Compared with the strain before hardening, the strain of rack obtained by medium frequency hardening, high frequency hardening, and grind hardening is reduced by 27.3%,22.3%, and 45.3%, respectively; the strain of gear obtained by medium frequency hardening, high frequency hardening, and grind hardening is reduced by 31.3%,26.5%, and 46.6%, respectively; the strain of piston obtained by medium frequency hardening, high frequency hardening, and grind hardening is reduced by 27.3%,22.2%, and 43.4%, respectively. The deformation of the typical parts obtained by grind hardening is smallest than that obtained by medium frequency hardening, and high frequency hardening. Therefore, the manufacturing procedure can be simplified under a certain accuracy requirement.