<i>In Situ</i> EDXRD Study of MAG-Welding Using LTT Weld Filler Materials under Structural Restraint

Abstract

Welding using low transformation temperature (LTT) filler materials is an innovative approach to mitigate detrimental welding residual stresses without cost-intensive post weld-treatments [1, 2]. 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@ESRF in Grenoble. 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.

Hot crack assessment of LTT welds using μCT

“…The volume expansion during martensite formation is more pronounced at lower temperatures due to the higher coefficient of thermal expansion (CTE) for austenite compared to martensite. In-situ analysis using high energy synchrotron X-ray diffraction during realistic MAG welding showed a significantly higher decrease of residual strain due to the hindered volume expansion for an LTT weld filler material compared to a conventional weld filler material [3,4]. The subsequent residual stress analysis using the contour method, which gives an entire two-dimensional residual stress map normal to the cut face through the component, revealed a higher compressive residual stress level for the investigated LTT filler materials compared to a conventional weld filler material [5].…”

Solidification Cracking Assessment of LTT Filler Materials by Means of Varestraint Testing and µCT

“…The volume expansion during martensite formation is more pronounced at lower temperatures due to the higher coefficient of thermal expansion (CTE) for austenite compared to martensite. In-situ analysis using high-energy synchrotron X-ray diffraction during a realistic Metal Active Gas (MAG) welding process showed a significantly higher decrease of residual strain due to the hindered volume expansion for a LTT weld filler material compared to a conventional weld filler material [20,21]. The subsequent residual stress analysis using the contour method, which gives an entire two-dimensional residual stress map, revealed a higher compressive residual stress level for the investigated LTT filler materials compared to a conventional weld filler material [22].…”

In situ analysis of the strain evolution during welding using low transformation temperature filler materials

“…In situ diffraction studies dealing with the phase transformation kinetics and the strain evolution of LTT welds based on the main alloying elements chromium (Cr) and nickel (Ni) have already been reported by the authors [19][20][21][22]. After first investigations including simple heating and cooling of LTT alloys, re-melting of the welds was realised by Gas Tungsten Arc Welding (GTAW) using a special sample geometry at different beamlines featuring varying experimental conditions.…”

“…Especially the restraint intensity, which represents a measure for the hindered shrinkage of the weld due to the stiffness of the joint surrounding, affects the residual stress formation considerably [23][24][25]. Therefore the latest publication by the authors dealt with the influence of the global restraint on the strain evolution during Gas Metal Arc Welding (GMAW) in multi-pass welds using energy-dispersive X-ray diffraction (EDXRD) [22]. Despite a very high intensity of restraint, it was possible to induce compressive strains in case of the LTT CrNi welds, while the conventional weld remains in tension.…”