Study On Step-tempering Mechanism Of Martensite Structures In Laser Cladding Of High Carbon Steel

Regardless of economic factors or safety considerations,the application of laser cladding technology to preparation of high-performance rails is an inevitable requirement for the continuous growth of railway traffic.Although the fatigue and wear of curved rails and turnouts in service are the most serious.laser cladding technology is difficult being applied for the preparation of the rail in the turnout.Because of the special structure and uneven surface of curved rails in the turnout,it is difficult for the traditional laser cladding process to perform an effective and accurate cladding.In addition,the latest hybrid laser cladding with the aid of induction is also difficult being applied for the preparation of turnouts,resulting in the inability to eliminate the martensite structure generated on the surface of the substrate during cladding and produce harmful phases.In addition,the latest hybrid laser cladding with the aid of induction and other processes are difficult to apply for the preparation of turnouts,which leads to the inability to eliminate the harmful martensite phase generated on the surface of the substrate during cladding.A laser cladding process for rails based on step tempering was first proposed in this paper.This process uses an scanning galvanometer,which can adjust the cladding width of a single track.The workpiece is heated more uniformly during the cladding process,and the cladding layer is thin and uniform,more importantly,this method can eliminate the martensite structure on the surface of the substrate.During laser cladding,when the thickness of a single cladding track is relatively thin,the martensite structure on the surface of the substrate will be subjected to step tempering during multi-layer cladding,thereby transforming the martensite structure into tempered sorbite,and the microhardness of the martensite zone decreased from 66 HRC to 44 HRC.This tempering method can also be used in in multiple cladding processes such as cladding with coaxial powder feeding and cladding with wide-spot.A thermocouple was used to detect the temperature cycle curve of laser cladding,and the temperature field and tempering process of laser cladding were simulated by the software such as ANSYS and Jmat Pro.The phase analysis of the tempered martensite structure in the cladding layer by the X-ray diffraction,reveals the evolution mechanism of the martensite structure in the step tempering process.The research results show that the effective tempering temperature range for the martensite structure is 600? to 700?,and the increase of the number of tempering makes the tempering effect better.The results of the three-point bending test show that the better the tempering effect,the better the bending performance of the sample.Compared with the sample with a untempered martensite layer,the ultimate bending strength of the sample with a fully tempered martensite layer increased to 2.0 times,the fracture strain increased to 5.5 times,and the fracture work increased to 15 times.