Stefan Gach

Confirmation of tensile residual stress reduction in electron beam welding using low transformation temperature materials (LTT) as localized metallurgical injection – Part 1: Metallographic analysis

Abstract For the reduction of the distortion and the residual tensile stresses in welded seams on carbon manganese steels, low transformation temperature materials (LTT) were developed. These materials use the volume expansion effect during martensitic transformation. The volume expansion counteracts volume shrinkage during cooling. The positive effects of the low transformation temperature alloys on the residual tensile stresses were demonstrated in various investigations. The low transformation temperature materials were, so far, used as filler material in arc welding processes in large volumes. The use of modular thermal fields of the electron beam welding processes offers the potential of a temporally activated use of compressive stress induction by phase transformation of the low transformation temperature alloys. The aim is to exert influence in situ on the welding residual stress state and thus on the distortion of complex parts. The metallographic analysis of an electron beam welded seam in unalloyed steel with low transformation temperature filler material is demonstrated. The evaluation is made via electron backscatter diffraction (EBSD) and is shown in the first part. The second part will show the effect on near surface residual stresses examined by hole-drilling method in combination with an optical evaluation by electronic speckle pattern interferometry.

Residual stress reduction of laser beam welds by use of low-transformation-temperature (LTT) filler materials in carbon manganese steels—In situ diagnostic: Image correlation

Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacements or deformations, as they occur during the cooling down process, in the vicinity of a weld. Observation of the movement of surfaces using a stereo camera setup allows the recording of displacements in three dimensions. Technically, this is achieved by identifying recognizable points on the surface under observation and by tracking their movement via comparisons between several time-shifted images. Visualization of surface contractions, as well as the effects of solid-state phase transformations during the joining process, especially during cooling, is key to understanding and influencing the residual stress state in welds as well to reducing distortion. In this study, the used installation setup, and the methodology for sample preparation, shows the production of information relating to surface displacements in the direct vicinity of the laser weld during the cooling down stage. Different welding parameters were investigated, resulting in different dilution levels for the LTT-Filler material, thereby influencing the distances to the weld pool and temperatures at which transformations took place. In order to provide a point of reference, comparable welds, made with conventional filler wires, were also investigated. The displacements after welding are always lower when using an LTT filler wire when compared to a conventional wire, proving that LTT wires can be used to mitigate distortion during laser beam welding.Low-Transformation-Temperature (LTT) materials are used for residual stress reduction in weld seams and, subsequently, for the prevention of distortion. Typically, LTT materials are highly alloyed Fe-based materials with levels of chromium and nickel that ensure that austenite transforms to martensite at reduced temperatures. The dilatation associated with the transformation is prevented by the surrounding material. Consequently, compressive stresses develop within the transformed region, and these counteract the accumulation of stresses due to thermal contraction. The precise chemistry of the weld metal determines the transformation temperature as well as the magnitude of the volume expansion. The chemical composition of the weld metal can, in turn, be influenced by the energy input during welding. In recent years, LTT-filler materials have been applied successfully in arc welding processes for residual stress reduction. Digital image correlation provides one avenue for the detection of surface displacem...

Confirmation of tensile residual stress reduction in electron beam welding using low transformation temperature materials (LTT) as localized metallurgical injections – Part 2: Residual stress measurement

Abstract In welding, the change of the aggregate state solid-liquid-solid is, via a localized heat action at the welding point used for the material binding joining of parts. Due to the thermal expansion and also due to structural changes, heat residual stresses and transformation residual stresses develop which often exert a negative influence on the precision of the part. When the shrinkage-induced residual stresses exceed the temperature-dependent yield stress, weldment distortion is the consequence. For reduction of the distortion and the residual tensile stresses in welded seams on carbon manganese steels, low transformation temperature materials (LTT) were developed. These materials use the volume expansion effect during martensitic transformation. The volume expansion counteracts volume shrinkage during cooling. The metallographic analysis of an electron beam welded seam in unalloyed steel with LTT filler material has been demonstrated in Part 1. This second part shows the effect on near-surface residual stresses examined by hole drilling method in combination with an optical evaluation by electronic speckle pattern interferometry.