(A) study on the two-dimensional analysis of thermal stress and distortion in welding process

Abstract

One of the major welding problems is residual stress and distortion. The uneven temperature distribution produced during welding gives rise to incompatible strains which in turn result in distortion and self equilibriating residual stress remaining in the structure after it has cooled down to the ambient temperature. The residual stress and distortion adversely affect the service behavior of welded structures including their brittle fracture, stress corrosion cracking, fatigue and buckling characteristics. In order to reduce, prevent, and/or relieve them, it is necessary to know the details of the thermal and mechanical response of weldments. Therefore the prediction of the residual stress and distortion is very important. In this study, the magnitude and mechanism of thermal stress and distortion were predicted and the effect of methods to reducing them was also quantitatively analyzed by using a thermo-elasto-plastic FEM analysis. Concepts of assumed boundary conditions and layered plate were introduced to improve the accuracy in prediction of welding distortion with 2-dimensional analysis. The result of the heat transfer analysis showed a highly nonuniform temperature distribution during the heating phase. With the thermal stress analysis, longitudinal tensile residual stresses were predicted to be a little higher than the yield stress at the room temperature, both in the weld metal and HAZ. The predicted residual stress agreed well with the experimental. The mechanical stress relieving treatment, which is accepted as a method of reducing the residual stress by a mechanical loading, was simulated by the same model used for the thermal stress analysis. The stress reduction by the MSR treatment can be explained by the fact that the plastic deformation in the high tensile stress region occurs during loading. The MSR results obtained from the numerical prediction agreed well with the experimental ones. Angular distortion and effect of prestraining on the distort...