The Effect of Heat and Surface Treatment on the Fatigue Behaviour of 56SiCr7 Spring Steel

Heat and surface treatments of spring steels can greatly affect fatigue life by increasing the number of cycles to failure. Heating, quenching and tempering of spring steel will increase its fatigue limit, as tempered martensite formation with an appropriate surface hardness is achieved from an initial ferritic/perlitic microstructure. This study deals with the parameters of the manufacturing process to transforming the initial microstructure of 56SiCr7 steel to tempered martensite, and increasing the surface hardness and fatigue resistance. Microscopy helps determine the decarburized surface layer thickness. Macro- and micro hardness measurements distinguish between core and surface microstructure hardness of the heat-treated steel. Shot-peening induces compressive residual stresses to the surface of the steel, therefore increasing its surface hardness and consequently the number of cycles to failure. This study shows significant hardness increase on the surface of the steel, and verifies how heat- treatment and shot-peening affect surface hardness. Experimental and analytically calculated (FKM guideline) S-N curves at 4- point cyclic bending quantify the individual effects resulting from the applied heat treatment and shot peening on fatigue life.