MICROSTRUCTURAL INFLUENCE ON FATIGUE PROPERTIES OF A HIGH-STRENGTH SPRING STEEL

A study has been made to investigate the fatigue properties of a high- strength spring steel in relation to the microstructural variation via different heat treatments. Rotating-bending fatigue and fatigue crack growth (FCG) tests were conducted to evaluate the fatigue properties, and a transmission electron microscope (TEM) equipped with an energy dispersive X-ray (EDX) unit was used to characterize the tempered micr ostructure. The results indicate that the fatigue endurance sigma f in creases with increasing tempering temperature, reaching a maximum at 4 50 degrees C, then decreases. The increase of sigma(f) is mainly attri buted to the refined distribution of precipitation, together with the structural uniformity of tempered martensite. The softening of tempere d martensite due to excessive precipitation accounts for the decrease of sigma(f). By contrast, the FCC results show an insensitivity of the stage-II growth behavior to the microstructural changes for the whole range of tempering temperature tested. The insensitivity is interpret ed in terms of the counterbalancing microstructure-dependent contribut ions to the FCG behavior. (C) 1998 Elsevier Science S.A.