Experimental and numerical study of strength mismatch in cross-weld tensile testing

The mechanical properties of welded boiler tubes used in power plants can be significantly altered as a result of the fabrication history, such as pre-straining and heat treatment. The primary aim of the study was to determine the effect of fabrication history on local tensile properties across the welds. This was achieved by testing cross-weld specimens machined from welded thin-walled tubes (with unstrained or pre-strained base metal) before and after heat treatment. Digital image correlation, which is a full-field strain measurement technique, was implemented in order to obtain the local stress–strain curves and to extract the corresponding local tensile properties such as offset proof stress. Evidence of strain hardening due to the constraint and thermo-mechanical cycles during the welding process was found in the heat-affected zone and evidence of softening was observed in the pre-strained base metal. It was found that the heat treatment process removed the effect of pre-straining and welding on the proof stress and the strength along the specimen was nearly homogenized. However, mapping the local stress–strain curves in the as-welded cross-weld specimens with pre-strained base metal has revealed abnormal strain relaxation with increase in load in the weld-affected region. For a better understanding of this behaviour, a tensile test of a cross-weld specimen with a large strength mismatch between the weld metal and the base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen causes the development of biaxial stresses in the heat-affected zone once local yielding begins, and the use of global axial stress to construct the local stress–strain curve results in an apparent ‘reduced-strain’ anomaly. Nevertheless, for the strength mismatch ratios studied, this anomalous behaviour did not seem to significantly affect the determination of the local proof stress in the specimens.