Rail grinding is a process of material removal by rotating grinding wheels. The grinding mileage is quite long, using cutting fluid will not only increase the maintenance cost, but also cause widespread pollution. Without cooling and lubrication, the heat generated in the grinding process can’t be released in time, thus the rail burns are often observed after rail grinding processes due to the dry conditions, high rotary speed of grinding wheels (~3600 rpm) and grinding load (~2000 N) [1-4], as given in Fig.1. To further improve the grinding efficiency and obtain good surface integrity, designing and manufacturing pores in grinding wheels is an economical and effective way [5].
Fig.1. The grinding induced burns and white etching layers on railhead.
Chinese scholars have prepared porous grinding wheels and characterized their grinding performance on a self-designed rig [5]. It can be noticed that once the pores were generated in the grinding wheels, the maximum compressive strength reduced by 35% from 83.74 MPa to 54.53 MPa. The results of grinding experiments presented that with the increase of porosity of grinding wheels, the grinding volume was slightly improved, the grinding temperature decreased and the wheel load was reduced. The results indicate that the grinding wheel with higher porosity owns a better self-dressing ability, which benefits to prevent wheel loading.
Fig. 2. Surface morphology of grinding wheels before and after test with different porosity: 8.12%(a) & (e), 15.81%(b) & (f), 18.60%(c) & (g) and 21.18%(d) &(h).
The hard and brittle white etching layer were observed on all ground railhead due to the grinding heat, and the thickest WEL were given by the lowest porosity of grinding wheels, as given in Fig.3 and Fig.4. Bellow the WEL is a deformed pearlite layer formed by deformation under shear stress of abrasive grits. The hardness of WEL is 5.77 GPa, about 2~3 times harder than the matrix of pearlite. Many scholars have concluded that the WEL has a close relationship with rail fracture [6-8]. Caused by mixed tensile and shear stresses of wheels during the service of rails, cracks may appear on the surface. The formed crack would rapidly propagate through the WEL layer because of its brittle nature, extend at the WEL and perlite interface or even propagate down into the pearlite matrix forming severer rail defects[9]. Hence, the hard and brittle would cause the premature failure of ground rail and can be effectively controlled by the porosity of grinding wheels.
Fig. 3. Hardness of WEL and deformed layer.
Fig. 4. OM of cross sections of the rail ground by different porosity of grinding wheels: 8.12%(a), 15.81%(b), 18.60%(c) and 21.18%(d).
The grinding mechanism of grinding wheel with pore structures can be illustrated in Fig. 5. Due to the high negative rake angle and a relatively high active grit density, the grinding chips first melt under such a high temperature and then get stuck on the wheel surface deteriorating the grinding ability of the grinding wheel and increasing the grinding heat. In contract, the porous grinding wheel owns a better self-dressing ability and contributes to a milder damage on rail surface[8]. On the one hand, the pore structures increase the space between abrasive grits which provide sufficient space for storing chips and releasing the heat. The chips can be curled in the pore and eliminated by the subsequent interaction of abrasives, and can also transfer a portion of heat from the contact zone. On the other hand, the stress and protrusion height for each active grit are larger than ordinary grinding wheel, which increase the uncut chip thickness and reduce rubbing effect between abrasive grit and rail surface to reduce the pre-fatigue caused by rail grinding as discussed. Therefore, depending on the outstanding grinding performance and respectively lower damage effect on the rail surface, the grinding wheel with pore structure has great potential to be applied in rail grinding technology under its high speed and dry grinding condition.
Fig. 5. Grinding mechanism of grinding wheel with pore structures.
References
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