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Independent from the interpass temperature residual stresses in longitudinal direction were lower than in transverse direction. This applies for the surface as well as for the bulk of the welds.
Residual stresses in the bulk were in general lower compared to stresses found on the surface.
Compressive residual stresses as a result of the martensite formation were formed in the bulk weld metal, only.
Independent from the LTT filler used high interpass temperatures were beneficial to reduce the stresses mainly in longitudinal direction.
Lower interpass temperatures tend to narrow the tensile zone in the weld metal but they also prevent the formation of compressive residual stresses.
Independent from the interpasstemperature residual stresses in longitudinal direction were lower than in transverse direction. This applies for the surface as well as for the bulk of the welds.
-Residual stresses in the bulk were in general lower compared to stresses found on the surface.
•Compressive residual stresses as a result of the martensite formation were formed in the bulk weld metal, only.
•Independent from the LTT filler used high interpasstemperatures were beneficial to reduce the stresses mainly in longitudinal direction.
•Lower interpasstemperatures tend to narrow the tensile zone in the weld metal but they also prevent the formation of compressive residual stresses.
Low Transformation Temperature (LTT) alloys are high alloyed filler materials, which exhibit a martensitic phase transformation at comparatively low temperatures in order to prevent high tensile residual stresses. A number of publications have already shown that even compressive residual stresses can be observed when using LTT filler materials. Up to know it is not clear in which way this it applicable to multi-run welding exhibiting high shrinkage restraint and complex heat input. In this study the potential for stress reduction during welding of LTT alloys was studied by numerical simulation. This allows for evaluation of the stress development in every single weid run. Additionally, the impact of the structural stiffness was incorporated by modelling a special sample geometry exhibiting a high intensity of restraint. The results show that the stress formation in weid longitudinal direction is determined by the phase transformation as high compressive residual stresses were found here independent from the weid run. On the other hand the transformation induced stresses in weid transverse direction are superimposed by tensile stresses originated from shrinkage restraint. With increasing number of runs the tensile residual stress level is raised. The results were confirmed by residual stress measurements using diffraction methods.
In situ EDXRD study of MAG-welding using LTT weld filler materials under structural restraint
(2017)
Welding using low transformation temperature (LTT) filler materials is an innovative approach to mitigate detrimental welding residual stresses without cost-intensive post weldtreatments. Due to the local Generation of compressive residual stresses in the weld line by means of a delayed martensite transformation a significant enhancement of the cold cracking resistance of highly stressed welded components can be expected. For the effective usage of These materials a deeper understanding of the microstructural evolution inside the weld material is necessary to determine the complex processes that cause the residual stress formation during welding. Solid-state phase transformation kinetics and the evolution of strain in LTT weld filler materials are monitored in-situ at the instrument ID15A at the ESRF in Grenoble, France. The transferability to real components is implemented by using a realistic MAG welding process under consideration of structural restraint. During welding of multilayer joints, the phase Transformation and phase specific strain evolution of each individual layer is investigated in transmission geometry by means of energy-dispersive X-ray diffraction EDXRD using high energy synchrotron Radiation with a counting rate of 2.5 Hz. The measurement results of a 10% Cr / 10% Ni LTT weld filler are compared to data monitored for the conventional weld filler material G89. The in-situ data clearly indicate a strong effect on the local strain evolution and the formation of compressive strain. This results from the restraint volume expansion during the postponed austenite to martensite transformation of the LTT weld filler, which counteracts the thermal shrinkage. In contrast, for the conventional weld filler material the thermal contraction strains lead to tensile residual strain during welding. Furthermore, the results of in-situ observation during welding Show that the transformation kinetic is dependent on the welding sequence.
Investigations on weldability often deal with hot cracking as one of the most prevalent failure mechanisms during weld fabrication. The modified varestraint transvarestraint hot cracking test (MVT) is well known to assess the hot cracking susceptibility of materials. The shortcoming of this approach is that the information is only from the very near surface region which inhibits access to the characteristic of the hot crack network in the bulk. Here, we report about an alternative approach to monitor the entire 3D hot crack network after welding by means of microfocus X-ray computer tomography (µCT). However, to provide sufficient high spatial resolution small samples must be sectioned from the MVT-welded joint. The sampling is accompanied by local relaxation of the residual stress distributions that are induced by welding, which can have an impact on the crack volumes prior to the sampling. The studies were carried out to investigate the hot cracking susceptibility of low transformation temperature filler materials (LTT). As high compression residual stresses up to -600 MPa in the area of the crack networks were determined by means of the contour method, stress relaxation caused by sectioning for µCT sample extraction can affect the detectability of the cracks later on. X-ray diffraction studies revealed surface residual stress relaxations up to about 400 MPa due to cutting. To investigate this effect, the specimens with hot cracks were subjected to a load test with known stress states. The results clearly show that local stress relaxations will have a strong impact on the volume images reconstructed from tomography analysis. This effect must be considered during hot crack assessment on basis of µCT data.