S164 Load Partitioning in a Duplex Stainless Steel with Surface Strength Gradient and Residual Stresses

Abstract

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.

Neutronographic Analysis of Load Partitioning and Micro Residual Stress Development in Duplex Stainless Steels

“…For the X2CrNiMoN25-7-4 duplex stainless steel, it was shown that both phases transferred almost the same phase-specific stress. The load carried by the ferrite phase was only minimally higher [8]. For the X2CrNiMoN22-5-3 duplex stainless steel, which was also investigated in the present work, a slightly higher phase-specific stress was transferred by the austenite phase [10].…”

“…Regarding diffraction analyses on the load-partitioning behaviour of duplex stainless steels, numerous works have already been published (e.g., [8][9][10][11][12][13]). In particular, investigations of the X2CrNiN23-4 duplex steel with the same nominal phase content of 50:50, which was one of the duplex stainless steels studied within this work, have shown that the austenite phase exhibited significantly higher phase-specific stresses than the ferrite phase and that the load transfer changed depending on the texture orientation [9].…”

SCC of 2304 Duplex Stainless Steel—Microstructure, Residual Stress and Surface Grinding Effects

“…Above a certain loading, local yielding occurs in the surface or subsurface, in one or both phases, depending on both the local strength and residual stresses. The relaxation of local residual stresses also results in redistribution of both the residual stresses and applied stresses and thus can be expected to affect the surface phase stresses/macrostress evolution [ 28 ]. With increasing loading, the extent of plastic deformation and stress redistribution will increase so the calculated stresses from four-point bending will not be attained.…”

“…Different trends of in-situ surface stress measurements were observed for the ground specimens. The surface layer was highly deformed so the actual measured surface stresses depend on the interaction between the applied and the residual stresses as well as the strength gradient under the surface [ 28 ]. Grinding operations generated predominantly tensile stresses parallel to the grinding direction and perpendicular compressive stresses, thus the actual measured surface stresses appreciably depending on whether the load was applied parallel or perpendicular to the grinding directions.…”

Analysis of Residual Stress in Stress Harps of Grey Iron by Experiment and Simulation

“…Secondly, the macrostresses related to differential cooling rate between the side and centre bars can be treated as "external load". The influence of the microstresses can then be analyzed by making analogy with analysis of load partitioning in a steel plate with surface compressive residual stress and subsurface tensile residual stress [8] in which the surface started to carry the applied load only after its compressive residual stress was canceled by the applied tensile stress. In the side bar of the stress harp, the ferrite is subjected to the compressive "load" only after its residual microstress is canceled.…”