Residual Stress Calculations And Measurements — Review And Assessment Of The Iiw Round Robin Results

Abstract

A Round Robin Programme was established by a working group of Commission X in 1997 in order to evaluate the possibilities of residual stress and distortion prediction (RSDP) in welded structures and to validate and benchmark prediction codes based on finite element simulation of the welding process. Calculations on residual tresses have been carried out on an austenitic steel plate with well-defined material and geometrical data, welding conditions and with the proposal to use the kinematic hardening model. In addition, a second programme for measurements of residual stresses in this plate has been started in 2003. Measurements have been taken on three austenitic steel plates using the X-ray measuring method as well as neutron diffraction methods and different kinds and variations of the hole drilling method. Although a rough agreement between the measured results and the calculated ones was found in principle, distinct differences have been observed in detail — especially for the longitudinal residual stresses. The calculations indicated nearly constant tensile stresses of the order of the initial yield strength in the weld seam and in the adjacent heat affected zone (HAZ). The measurements however revealed stress maxima in the HAZ with values clearly higher than the initial yield strength. Therefore additional calculations have been carried out with different hardening models in order to investigate their influence on the residual stress results. In fact, calculations performed with the isotropic hardening model could approve these stress maxima quite exactly. In further detailed investigations, it could be shown that the reason for the stress maxima is the strain hardening in the HAZ due to thermal stresses during the welding process. Finally basic considerations could explain that the kinematic hardening model, including the Bauschinger effect, cannot account for this hardening effect. Literature indicates that the Bauschinger effect is at least not fully effective at high working temperatures of austenitic steels. Altogether, the calculation programme has shown that a rough estimation of the residual stress state after welding is possible without any special distinction of the hardening model to be used. For a more precise and detailed information, the chosen hardening model is of significant relevance.