Fatigue Property And Fatigue Cracking Risk Assessment Of A Surface Strengthened 50CrMnMoVNb Spring Steel

Nowadays,with the rapid development of transportation industry,lightweight has become a major trend in the development of commercial vehicles.Leaf spring is an important component whose main functions are vibration attenuation and load bearing.Fatigue is its main failure mode.For some commercial vehicles,lightweight commonly means to reduce weight,which may be potentially harmful to the fatigue reliability of the key components,e.g.,leaf spring.Surface strengthening,which can significantly suppress the crack initiation from the metal surface,is a good way to improve the fatigue reliability under the condition of lightweight.The microstructure evolutions and fatigue properties of the surface strengthened 50CrMnMoVNb spring steel treated by a new developed surface spinning strengthening-?(3S-?)and the traditional shot peening(SP)were investigated by means of microstructure characterization,microhardness measurement,residual stress test,surface roughness measurement,fractography analysis and so on.After 3S-? treatment,the microstructure in the surface layer of 50CrMnMoVNb spring steel has been greatly refined and,the grain size was reduced to the nano-scale,and correspondingly,the microhardness was obviously improved.After SP treatment,residual compressive stress was introduced into the surface layer of 50CrMnMoVNb spring steel.The surface roughness of the sample treated by 3S-? is obviously lower than that of the sample treated by SP,i.e.,the surface is much more smooth.At the low stress amplitude,the fatigue lives of the samples treated by 3S-? are obviously better than those of the samples treated by SP.At the high stress amplitude,the fatigue lives of the samples treated by SP exhibit quite different charateristics from those of the samples treated by 3S-?.Specifically,the fatigue lives of the samples treated by SP can be,in general,approximately described and evaluated by the Basquin equation(S-N curve).However,the fatigue lives of the samples treated by 3S-? do not follow the traditional rules.The fatigue lives of the samples treated by 3S-? become polarized,i.e.,the long life(>106 cycles)and the short life(<105 cycles).By analyzing the above experimental data,a fatigue risk factor R_f is proposed,to evaluate the fatigue risk of metallic materials or components.Moreover,the influences of mechanical property,applied load,residual stress and surface condition of component on the fatigue risk are comprehensively evaluated.Where,R_f is proportional to the applied tensile stress ?_e,the R_f max varies linearly with the change of ?max of the outer layer;R_f is inversely proportional to the yield strength ?y,and the position of R_f max can be pushed into the subsurface of the material by improving the mechanical properties of the surface layer;R_f is proportional to the residual stress ?r,and introducing high residual compressive stress on the surface layer can reduce R_f of the surface layer;the R_f of surface increases significantly because of the surface defects,and R_f max is proportional to the stress concentration factor Kt.The analysis shows that the locations of R_f max are consistent with the locations of fatigue sources of the samples treated by 3S-? and SP.In summary,surface strengthening,introducing residual compressive stress and reducing surface roughness and stress concentration are effective ways to reduce the fatigue risk factor R_f and make the position of R_f max move towards the direction far from the sample surface,which can effectively improve the fatigue lives of metallic components.