Fatigue Behavior of a Microalloyed Steel Under Uniaxial Loading

Fatigue data for a new microalloyed steel have been generated and compared with SAE 9259. The steel is designed to be used for suspension coil springs at high strength levels (~2100 MPa) in order to reduce weight. Both stress-controlled and strain-controlled tension-compression fatigue tests were conducted. The high strength microalloyed steel showed a 17 to 25% increase in endurance limit over SAE 9259 for polished specimens. Strain-controlled fatigue tests showed comparable strain resistance for these two steels. However, the microalloyed steel has higher stress carrying capability and less cyclic softening than SAE 9259 in the constant strain condition. Shot-peening is proved to improve its fatigue resistance under uniaxial tension-compression loading in stress amplitudes above 724 MPa (105 ksi). However, at stress amplitudes close to the endurance limit of the polished specimens (< 724 MPa), shot-peening was found to decrease the fatigue resistance, which is attributed to the residual tensile stress build-up at subsurface areas during shot-peening.