Load Partitioning in a Duplex Stainless Steel with Surface Strength Gradient and Residual Stresses

The load partitioning between surface and bulk was studied by means of in-situ synchrotron energy dispersive diffraction in an SAF 2507 superduplex stainless steel having a strain hardened surface layer with compressive residual stresses. Two loading cycles with a peak tensile strain of 0.5% and 0.87%, respectively, were successively applied and the evolution of stresses in the respective constituent phases (austenite and ferrite) were measured in a 0.026 mm thick surface layer and at 0.75 mm depth, from which the macrostress development was derived, respectively, for the two locations. It was found that the surface and the bulk followed different loading paths and that such inhomogeneous load sharing was related to variation in strength and residual stresses over the load-carrying cross-section. During macroscopic elastic loading, the load partitioning, characterized by increasing tensile stress in the bulk and decreasing compressive stress in the surface, depended mainly on the initial residual stress distribution. Upon yielding of the bulk, a rapid load transfer from the bulk to the hard surface layer started and continued until the surface also reached its elastic limit. At the 0.87% peak strain, the plastic incompatibility over the cross-section resulted in a much higher stress in the surface (1070 MPa) than in the bulk (680 MPa), which also led to a large tensile residual stress in the surface after unloading. In the paper, the observed inhomogeneous microscopic load partitioning was also presented.