Investigations On Laser Shock Peening The Fe Based Coatings Prepared Via Laser Cladding On U71Mn Steel

U71Mn steel is widely uses as railway tracks.The requirement for U71Mn steel increases as expansion of railway,the increasing of transportation capacity of the railway,and the increasing of the train speed.However,the railway tracks are easy to fail due to the serious friction and rolling during service.This,in turn,will decrease the service safty and shortern the lifetime of the railway tracks.In order to change the worn tracks,the railway course should be closed.Meanwhile,the operation time is strictly limited to guarantee the railway unblocked,it is needed to spend a lot of manpower and material resources.This is regarded as a sevious waste in money and materials.For the purpose of improving the wear resistance and prolonging the service life of the railway tracks,laser cladding technology and laser shock peening(LSP)technology were applied to modify the surface properties of U71Mn steel.Two types of Fe based coatings were fabricated via laser cladding technique,followed by laser shock peening treatment to eliminate the tensil residual stress in the coatings.The influeces of LSP on the phase constituents,the microstructure,wear and corrosion resistance of the coatings were invesitgated.It can be concluded as following.The main phases in 43X coating can be characterized as α’-Fe、γ-Fe、CrFe4 and Fe19Ni,while that in SP 465 coating include α’-Fe、γ-Fe 和 CrFe4.The phase constituents were not changed after LSP treatment,no shifts of the peaks was observed.However,the peaks corresponding to higher diffraction angle appeared with increasing of laser power density and impulse times,indicating that the preferential orientation was caused during LSP treatment.The microstructure in the coatings before LSP can be characterized as coarse columnar crystals.The crystals in the coatings were significantly refined after LSP.The strengthening depth increased with increasing of laser power density and impulse times.The surface roughness of the coatings was significantly decreased by LSP treatment.The surface roughness decreased with increasing of laser power density and impulse times.The tensil residual stress was observed in the coatings before LSP processing.The stress decreased after LSP treatment.After being LSP twice,the tensil residual stress transform to compressive stress.The crack propagation in the coatings is expected limited by LSP.The microhardness of the two coatings is much higher than that of U71Mn steel.Moreover,the microhardness and wear resistance of the coatings were significantly improved after LSP treatment.It can be concluded that a higher power density and more impulse times are beneficial for improving wear resistance of the coatings.The wear mechanisms of the U71Mn steel and Fe based coatings can be characterized as abrasive and adhesive wear,and just abrasive wear,respectively.The corrosion resistance of the two coatings against 3.5%NaCl solution was improved by LSP treatment.But the two coatings present different results with increasing the impulse times.The corrosion resistance for 43X coating decreased when the impulse time was 3.The corrosion resistance for SP465 coating always increased with increasing of power density and impulse time.