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Influence of mechanically induced hardening on the generation of welding residual stresses

Subject Area Mechanical Properties of Metallic Materials and their Microstructural Origins
Term from 2015 to 2022
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 273371116
 
Final Report Year 2022

Final Report Abstract

In this research project, it should be clarified to what extent the thermal cycle of rapid, localized heating and subsequent cooling associated with fusion welding leads to plastic deformations of the weld seam environment resulting in a strengthening effect. Since deformation-induced hardening limits the local yield point, on which the maximum level of residual stresses depends, residual stresses in the area around the weld above the base metal limit are generally possible. This may explain why the results of residual stress calculations often deviate significantly from the corresponding measurement results. If the strengthening behaviour is not taken into account adequately in the calculation, in particular, it had to be clarified to what extent deformation-induced hardening is retained during the thermal cycle and it can be represented in a suitable manner by an isotropic or kinematic hardening model. In the first project phase, all investigations were focused on an austenitic and a ferritic chromium steel. In the second project phase it was tried to apply the knowledge from the first phase in order to improve the reliability of numerical calculations with regard to weldable structural steels of different strengths. The most important results can be summarized as follows. • None ofthe hardening models enables a full agreement between the calculation and the experimental results. A qualitative comparison, however, provides that isotropic hardening seems to play a major role in all cases, since it always provides findings closer to the experimental results enabling also the representation of localized residual stress peaks. This also means that in the case of high-alloy steels, the Bauschinger effect seems to play no or only a minor role for the stress-strain relationships induced by the thermal cycle. • Kinematic or mixed calculations should only be used if the hardening exponents have temperature-independent values. It should also be noted that these models provide non-conservative values and do not always reproduce the residual stress distributions in a qualitatively correct manner. For this reason, an isotropic model is recommended for carrying out such calculations, because here the best agreement could be achieved in all cases. • The methods used to analyze the hardening state enable a differentiated assessment of the local deformation processes as a result of the plasticization caused by the thermal cycles and their consequences for the hardening state. In particular, the characteristic values obtained from the interference line profile analysis enable a qualitatively targeted selection of the hardening model to be used for the calculation. • The locations of the highest residual stresses always match with those where the highest hardening was detected. • In the case of structural steels with pahes transformations, no influence of the plastificationinduced hardening on the position and level of the residual welding stresses can be demonstrated or it is completely covered by the strength changes primarily effective in the weld and in the HAZ as a result of the phase transformations. The secondary maxima of the longitudinal residual stresses that occur in the base material close to the weld cannot therefore be associated with plastification, but can also be attributed to the transformation processes in the weld as an equilibrium reaction. • A connection between the hardening behaviour and the suitability of one of the calculation models used cannot therefore be clearly derived. Nevertheless, all calculation results indicate that with an isotropic material model, the best correspondence between calculated and measured residual stresses can consistently be achieved and that local residual stress peaks and sinks can also be mapped qualitatively well. If the boundary conditions during welding are known exactly, the thermal cycles are precisely represented in the calculation and the temperaturedependent mechanical properties of the materials are available, accuracies of >100 MPa can be achieved between calculation and measurement. In any case, this requires realistic consideration of the phase transitions. Neglecting the transformations is not permitted. • In-situ measurements represent, as far as feasible, an important instrument for the time- and temperature-dependent monitoring of the microstructural processes and the transient voltage development during welding, which provide important additional knowledge for understanding the overall context.

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