Microstructure evolution and mechanical properties changes in the weld zone of a structural steel during low-cycle fatigue studied using instrumented indentation testing

Abstract

Room-temperature low-cycle fatigue (LCF), instrumented indentation, tensile tests, finite element (FE) analysis, and optical microscopy examinations were carried out to investigate the mechanical properties and microstructures in the weld zone of a structural steel, SS400, in cyclic loading conditions. Five indentation arrases were performed across the weld zone of five specimens, including one virgin specimen and four specimens subjected to LCF loading at different strain levels. The mechanical properties, such as elastic modulus E, yield strength σy, and strain hardening exponent n, were then estimated from indentation testing and FE analysis results. The obtained results and optical microscopy examinations show that the mechanical properties and microstructures in the individual region of the weld zone are not only decided by the metallurgical conditions resulting from the welding but also were highly influenced by the strain level applied during cyclic loading. In the as-welded condition, the values of elastic modulus, yield strength, and strain hardening exponent in the weld metal (WM) are higher than those in the base metal (BM), while materials in heat affected zone (HAZ) reveal a gradual decrease of elastic modulus, yield strength, and strain hardening exponent values from the WM to BM region. It is also shown that with increasing strain amplitudes during cyclic loading, the elastic modulus E displays decrease in all regions of the weld zone, both yield strength σy and strain hardening exponent n exhibit decreasing characteristic in WM and increasing characteristic in BM region, while the yield strength in HAZ region increases, the strain hardening exponent in this region keeps almost unchanged. The dependence of the mechanical properties on the applied strain level is also discussed in terms of microstructural changes in fatigue failure specimens.