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- Beitrag zu einem Tagungsband (3) (entfernen)
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- Welding (2)
- Component Testing (1)
- Influence (1)
- Interpass temperature (1)
- LTT (1)
- Low transformation (1)
- Phase transformation (1)
- Residual Stress (1)
- Residual stresses (1)
- Sectioning Method (1)
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.
The current paper presents residual stress analyses of large scale LTT (Low Transformation Temperature) welds. LTT filler materials are specially designed for residual stress engineering by means of an adjusted martensite phase transformation. Controlling the level of mostly detrimental residual stresses already during the welding process would be highly attractive as time and cost consuming post processing may be prevented. In large scale welds the residual stress state is influenced by the heat control (e.g. interpass temperature) during welding. Therefore, welding residual stresses are studied here putting the focus on the influence of welding process parameters while joining heavy steel sections with a thickness of 25 mm. The residual stress state was determined at the top surface using X-ray diffraction as well as in the bulk by neutron diffraction. The results show that control of the interpass temperature is vital for the residual stresses present in the joints. This accounts for the top surface but is most pronounced for the bulk of the welds. While high interpass temperatures are appropriate to induce compressive residual stresses in the weld metal, low interpass temperatures favor unwanted tensile residual stresses instead.
Residual stresses and distortions in welded I-girders for steel construction are relevant when evaluating the stability of steel beams and column members. The application of high strength steels allows smaller wall thicknesses compared to conventional steels. Therefore, the risk of buckling has to be considered carefully. Due to the lack of knowledge concerning the residual stresses present after welding in high strength steel components conservative assumptions of their level and distribution is typically applied. In this study I-girders made of steels showing strengths of 355 MPa and 690 MPa were welded with varying heat input. Due to the dimension of the I-girders and the complex geometry the accessibility for residual stress measurement using X-ray diffraction was limited. Therefore, saw cutting accompanied by strain gauge measurement has been used to produce smaller sections appropriate to apply X-ray diffraction. The stress relaxation measured by strain gauges has been added to residual stresses determined by X-ray diffraction to obtain the original stress level and distribution before sectioning. The combination of both techniques can produce robust residual stress values. From practical point of view afford for strain gauge application can be limited to a number of measuring positions solely to record the global amount of stress relaxation. X-ray diffraction can be applied after sectioning to determine the residual stresses with sufficient spatial resolution.