Influence of mechanically induced hardening on the generation of welding residual stresses
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.
Publications
- Effects of heat source geometric parameters and arc efficiency on welding temperature field, residual stress, and distortion in thin-plate fullpenetration welds. In: International Journal of Advanced Manufacturing Technology, 99 (2018). London: Springer, 2018: S. 497–515
Sun, J.; Klassen, J.; Nitschke-Pagel, T.; Dilger, K.
(See online at https://doi.org/10.1007/s00170-018-2516-6) - Experimental and Computational Analysis of Residual Stress and Mechanical Hardening in Welded High-Alloy Steels. In: Seefeldt, M. [Hrsg.]: Materials Research Proceedings, Volume 6 (2018); Residual Stresses 2018 (ECRS-10); 10th European Conference on Residual Stresses, 11.–14.09.2018, Leuven, Belgien
Hempel, N.; Nitschke-Pagel, T.; Rebelo-Kornmeier, J.; Dilger, K.
(See online at https://doi.org/10.21741/9781945291890-36) - Influence of heat input model parameters on the simulated properties in ferritic steel weldments. Dissertation, Technische Universität Braunschweig. Forschungsberichte des Instituts für Füge- und Schweißtechnik, Band 53, Shaker Verlag Düren, 2019
Jiamin Sun
- Solid-state phase transformation and strain hardening on the residual stresses in S355 steel weldments. Journal of Materials Processing Technology (2019) Volume 265; 173-184
Sun, J.; Hensel J.; Klassen, J.; Nitschke-Pagel, T.; Dilger, K.
(See online at https://doi.org/10.1016/j.jmatprotec.2018.10.018) - Influence of temperature- and phase-dependent yield trength on residual stresses in unltra-high strength steel S960 weldments. Journal of Materials Research and Technology 15 (2021) 1854-72
Sun, J.; Nitschke-Pagel, Th.; Dilger, K.
(See online at https://doi.org/10.1016/j.jmrt.2021.09.050) - Effect of phase- and temperature-dependent strainhardening slopes on the calculated welding residual stresses in S235 steel. 13th International Seminar Numerical Analysis of Weldability, 4 - 7 September 2022 Graz - Seggau – Austria
Sun, J., Nitschke-Pagel, Th., Dilger, K.
- Influence of strain hardening models on the predicted welding residual stresses. 11th International Conference on Residual Stresses, 7.-10.3.2022, Nancy, France
Nitschke-Pagel, Th.; Sun,J.
- Influence of strain-hardening models and slopes on the predicted residual stresses in structural steel S235 weldments. Journal of Materials Research and Technology 19 (2022) 4044-4062
Sun, J., Nitschke-Pagel, Th., Dilger, K.
(See online at https://doi.org/10.1016/j.jmrt.2022.06.134) - On the influence of cyclic plasticity on the residual stress state in welded high-alloy steels. 13th International Seminar Numerical Analysis of Weldability, 4 - 7 September 2022 Graz - Seggau – Austria
Hempel, N.; Nitschke-Pagel, Th.; Rebelo-Kornmeier, J.; Dilger, K.
- Zum Einfluß zyklischer Plastizität auf die Eigenspannungsentstehung beim Schweißen hochlegierter Stähle. Dissertation, Technische Universität Braunschweig. Forschungsberichte des Instituts für Füge- und Schweißtechnik, Band 61, Shaker Verlag Düren, 2022
Nico Hempel
