Uniaxial ratcheting behaviour of 304L stainless steel and ER308L weld joints

In this study, a series of uniaxial strain cycling and ratcheting tests were conducted on 304L stainless steel and weld metal made with ER308L stainless steel welding wire. The ratcheting strain and fatigue life of the materials were measured at different loading levels and their cyclic behaviours are discussed. Microstructural evolution during cyclic loading was investigated by electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM). The experimental results show that the cyclic behaviours of the base metal and the weld metal are different. The cyclic behaviour of the base metal includes primary hardening, slight softening, and significant secondary hardening, while the weld metal exhibits a short hardening stage for several early cycles and later exhibits cyclic softening until failure. The evolution of dislocations during strain cycling and ratcheting deformation is discussed and the cyclic and ratcheting behaviours of the two materials are explained qualitatively. The final ratcheting strain of the materials depends greatly on the mean stress and stress amplitude. However, the fatigue life variation in the two materials is different. The base metal exhibited minimum fatigue life when a mean stress of 30 MPa was loaded; at constant mean stress, the fatigue life decreases when the stress amplitude is increased. In the case of the weld metal, the fatigue life decreases with increasing mean stress and stress amplitude.