Papers by Keyword: Low Transformation Temperature

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.

Abstract: Residual stresses in welded components are consequence of stress and/or thermal gradients and influenced by factors such as joint geometry, variation in strength of the material, preheat temperature, heat input, post-weld heat treatment and phase transformation strains. During the 70’s, it was observed that the level of residual stress accumulated in a constrained sample during cooling from austenite could be reduced after transformation to martensite or bainite. Some works have evaluated effect of welding using a low transformation temperature martensitic filler metal on the level of residual stress in single pass joints. According to these studies, martensite start temperature in the range 200–250°C can be extremely effective for mitigation of tensile residual stresses. The outcome of most of these works was on one hand increase of fatigue life due to the mitigation of tensile residual stresses via transformation strains, on the other a significant reduction of the fracture toughness.In the present study, sections of API 5L class B steel tubes were multipass welded using a 12Cr-5Ni low transformation temperature filler metal in addition to a conventional filler metal. Residual stresses in the inner and outer surfaces were measured by X-ray diffraction. Aspects related to the improvement of toughness in the weld metal due to the tempering of one pass by the subsequent were also discussed.

Abstract: Residual stress distributions in fillet welds in 8 mm 900 MPa steel have been mapped perpendicular and parallel to the weld line and also through the thickness in the vicinity of weld toe position. Measurements were carried out on four welds when two of them were performed with conventional and two with the so called LTT (low transformation temperature) filler materials. Both neutron and X-ray diffractions were used for determination of the residual stress distribution. Fatigue properties have also been evaluated for all test welds. Neutron diffraction measurements showed that the stress profiles perpendicular to the weld toe qualitatively did not depend on filler material type although the absolute stress levels differed. Trends were similar for positions 2, 4 and 6 millimetres below the surface for all three stress components; σ_x (direction perpendicular to the weld), σ_y (parallel to the weld) and σ_z (through the thickness). X-ray diffraction showed difference in residual stress level at the weld toe. Lower residual stress levels have been identified for LTT filler material when compared to the conventional consumable compositions. The effect of residual stress is discussed in relation to fatigue properties of all four welds. Remarkable higher fatigue strength has been measured for welds prepared by the LTT filler materials.

Abstract: Low Transformation Temperature (LTT) alloys were developed in order to control the residual stress development by the martensitic phase transformation already during cooling of the weld metal. The positive effect of such LTT alloys on the mitigation of detrimental tensile residual stresses during welding has already been confirmed on the basis of individual laboratory tests. Within the current project it was experimentally investigated whether the phase transformation mechanisms are effective under increased restraint due to multi-pass welding of thicker specimens. The local residual stress depth distribution was analyzed non-destructively for V-type welds processed by arc welding using energy dispersive synchrotron X-ray diffraction (EDXRD). The use of high energy (20 keV to 150 keV) EDXRD allowed for the evaluation of diffraction spectra containing information of all contributing phases. As the investigated LTT alloy contains retained austenite after welding, this phase was also considered for stress analysis. The results show in particular how the constraining effect of increased thickness of the welded plates and additional deposited weld metal influences the level of the residual stresses in near weld surface areas. While the longitudinal residual stresses were reduced in general, in the transition zone from the weld to the heat-affected zone (HAZ) compressive residual stresses were found.