Study On Experiment And Mechanism Of External Surface Grind-Hardening Technology

Grind-hardening is a new technology which utilizes grinding heat to induce martensitic phase transformation and strengthen workpiece surface in gringding process by raising surface temperature above Ac3 instantaneously and cooling quickly. The application of this new technology can reduce production cycle, improve working efficiency, and decrease manufacturing cost by integrating the two operations of grinding and surface heat treatment into one, which has the great social and economical benefits.Grind-hardening experiments for external grinding mode are carried out. The results indicate that the hardened workpiece surface consists of three parts of hardened layer, transition layer, and body. The metallurgical structures of the parts are martensite, mixture of martensite and ferrite, and mixture of ferrite and pearlite respectively. Under the experiment condition, the top value of hardness and hardened depth is HV824.1 and 1.1mm respectively, which achieves the effect of high-frequency hardening.The orthogonal experiments with different cut depths, workpiece speeds and wheel characteristic are performed, contribution and pattern of the factors'influence to hardening effect are analyzed. The results show that hardened layer thickness rises with increasing cut depth, and rises and falls with increasing workpiece speed. White alundum wheel increased more hardened layer thickness than pink alundum, while small grain granularity products more hardened layer thickness than big one. The order of factors is cut depth, workpiece speed, and wheel characteristic according to the contribution. The results also indicate that surface hardness is only affected by cut depth, and the hardness value rises with increasing cut depth. Under the present experiment condition, the adoption of 0.4mm cut depth, 0.5m/s workpiece speed and wheel WA46L8V may achieve good comprehensive grind-hardening results of both hardened layer thickness and surface hardness.40Cr steel and 45 steel are adopted simultaneously in the experiments to research hardening effects with different materials. The results show good hardened effectiveness of both materials as well as the similar pattern of metallurgical structure and hardness distribution. It has been found that 40Cr steel has more hardened layer thickness while 45 steel has higher surface hardness. The difference derives from the material property.The obsevation of the metallurgical structure and test of hardness distribution for the overlap hardened zone demonstrate the rehardening is occurred and the metallurgical structure or hardness distribution changes little. Due to the inertance of conductivity, the residual heat in the zone tempered the material at its end part. A little tempered sorbite appears while the hardness value drops accordingly. However, because the tempered area is small in size (1.0mm) and even the bottom value (HV479.6) can satisfy the hardness requirement, it has little influence to the total hardened effectiveness.Fine grinding stage is taken for the surface roughness and quality after the hardened layer is obtained in the coarse grinding stage, and the fine grinded sample is analyzed. The result indicates the same metallurgical structures and surface hardness to the sample of coarse stage. Therefore, with rational choiced grinding parameters and condition, requirement of surface hardness and machining precision may be met simultaneously.Grinding force and temperature are measured in the experiments by adopting elastic core clampers and infrared thermometer, which provides data for theory study. Moreover, the variation regularity and influence factors are analyzed. The results indicate that grindng force is proportional to workpiece speed and cut depth in external grind-hardening process, and the ratio of normal force to tangential force is more than that of common grinding. Grinding temperature is influenced by cut depth, wheel characteristics and workpiece speed in the contribution order, and has the same change regularity of hardened layer thickness.Calculation equations of grinding force for external grind-hardening are derived through the study of grain cutting mechanism and the utilization of grain energy cost and distribution. Plough stage and large cut depth of grain are considered for accurate results. It has been shown that the calculation results have consistency with measurment results.Process of material remove is considered to consist of various stages. The compound heat and heat distribution ratio are modelled for the whole process. Moreover, the influence pattern of grinding condition to distribution of heat is evaluated. The results demonstrate that grain granularity and wheel velocity are the major factors. Both increasing grain granularity number and decreasing wheel velocity may reduce the heat conducted into workpiece surface.A computational finite element method software, ANSYS, is introduced to simulate the process of external grind-hardening. The dynamic states of temperature field and temperature history of points in various model placements are investigated by simulation. It has been demonstrated that the simulation results are in accordance with measurment results due to the reliable simulation method and the accurate heat model. Therefore the simulation may be studied instead of experiment.