Fundamental Studies Of Laser-induction Hybrid Cladding Technique On Full-scale Rail Surface

With the rapid development of high-speed and heavy-haul trains,surface damages of rails are becoming more and more severe,which has become the bottleneck affecting the efficiency of railway transportation,how to promote the surface strength of rails and prolong their service life with high efficiency are becoming extremely important.Laser cladding?LC?,with small heat affected zone?HAZ?and low dilution,has attracted a lot of notices in repairing and hard-facing rails.However,there are two great barriers for the traditional LC to apply on full-scale rails:one is how to prevent the coating from cracking under the rapid heating and cooling cycle;the other is how to eliminate the martensite structure in HAZ.In this dissertation,laser-induction hybrid cladding?LIHC?technology,which was innovatively subdivided into the LIHC with induction pre-heating?pre-LIHC?and the LIHC with induction post-heating?post-LIHC?,was proposed to deposit Ni-based coatings on a full-scale rail.The evolutions among the parameters,microstructures,thermal cycles and mechanical properties of the coatings and HAZs by LC,pre-LIHC and post-LIHC were established.Then,the bending properties,fracture mechanisms,rolling contact wear and fatigue damage behaviors of rails by different technologies were studied systemically.Further,the cladded rails were verified by the on-site testing,and the failure behaviors after offline were also analyzed.The main results are summarized as follows:?1?Among the LC,pre-LIHC and post-LIHC technologies,the cracking and martensite transformation occur in the HAZ of rails can only be prevented by the optimized post-LIHC technology.Compared with the traditional LC technology,the effect of the pre-LIHC technology on the rails mainly reflects in the cladded coatings.The increase of the induction preheating power in pre-LIHC will improve the molten pools'temperature and decrease the molten pools'cooling rate,increase the dilution ratio of the coating,enlarge the second dendrite arm spacing?SDAS?and reduce the strengthening phases'content of the coating,leading to the decrease of the hardness and strength.Comparetively,the post-LIHC technology has no obvious effect on the cladded coatings,while the influence mainly reflects in the HAZs.Under the induction post-heating window of post-LIHC technology,the martensite transformation in HAZs can be avoided absolutely,while fine pearlite structure with hardness about HV350-380 forms instead.It makes the microhardness decreased gradually along the depth direction of rails,avoiding the abrupt change of microstructure and mechanical properties in the HAZs by LC and pre-LIHC.?2?In the processes of LC and pre-LIHC,the HAZs undergo a continuous cooling process?CCT?,and t_8/5s under all parameters are much less than the critical cooling time required to achieve complete martensitic transformation.Differently,the post-LIHC makes the cooling process of the HAZ changed to an approximate isothermal transformation process?TTT?,and the cooling rates under the optimized parameters meet the critical conditions for getting pure pearlite microstructure.Compared with the rail substrate,the pearlite structure in the HAZs by optimized post-LIHC technology has lower interlamellar spacing,higher ferrite grain misorientation,and smaller grain size and pearlite block size,endowing both the strength and toughness of rails be promoted significantly.?3?Under the bending stress,both of the maximum stress and damage of the specimen by LC and pre-LIHC generate in the HAZ,while that by opitimized post-LIHC parameters initiates from the surface.Since the cracks initiating and expanding rate of the specimen by opitimized post-LIHC technology are both lower than that by LC and pre-LIHC,thus the bending strength,bending fracture strain and facture work are all improved significantly.?4?The wear and fatigue damage behaviors of rails by LC and post-LIHC under different contact stresses were investigated comparatively by the wheel/rail rolling contact tests.When?_max=800MPa,the wear and fatigue characteristics of specimens by LC and post-LIHC are almost the same,both of which show the lateral creep and fatigue damage on the contact surface,while the LC specimen have both the lower volume wear rate and local dimensional wear rate.When the?_max increases to 1230MPa and 1500MPa,the damage behaviors of specimens by post-LIHC have no obvious changes,while the coating and HAZ at the contact edge of the specimens by LC spall and fracture severely,leading to the sharp increase of the volume wear rate,as well as the significant reduction of the rail's surface roundness and smoothness.On the other hand,the volume wear rates of rails by both LC and post-LIHC are higher than the rail substrate under the same contact stress.However,the poorer plastic deformation resistance of the rail substrate makes it has much larger lateral creep and local dimensional wear rate,as well as the poorer surface roundness and smoothness than the specimen by post-LIHC.Therefore,compared with the rail treated by LC and the rail substrate,the rail by optimized post-LIHC can better maintain the profile and smoothness of the contact surface.?5?The on-site testing results show that the rail by post-LIHC has much higher contact fatigue resistance and much lower wear rate and edge width than the rail by LC,which also shows the service life of about 1.87 times compared with the untreated U71Mn rail substrate.In summary,this study establishes the foundation for the application of LIHC technology in railways,and has made the preliminarily verification of the post-LIHC technology,which has great potential to be used in rail hardfacing and repair in practical engineering.