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Anwendung des neuen Controlled Tensile Weldability (CTW) Tests zur Untersuchung der Heißrissneigung
(2006)
CT-01 Formation of welding residual stresses in low transformation temperature (LTT) materials
(2008)
Crack and fatigue resistance are relevant evaluation criteria for welded joints and are decreased by tensile residual
stresses resulting from the welding and cooling process, while compressive residual stresses can have a positive
influence on the characteristics mentioned. In order to generate compressive residual stresses, a set of post weld
treatment procedures is available, like shot peening, hammering, etc. These procedures have the disadvantage that
they are time and cost extensive and have to be applied after welding. As another point, such technologies can only
produce compressive stresses at the top surface, i.e. can only contribute to the reduction of the risk of cracks initiated
at the surface, like fatigue cracks. A chance to generate compressive stresses over the complete weld joint during the
welding procedure is offered by the so-called Low Transformation Temperature (LTT -) filler wires. Compared to
conventional wires, these materials show lower phase transformation temperatures, which can work against coolingrelated
tensile stresses, resulting from respective shrinkage restraint. In consequence, distinct compressive residual
stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially
applicable to high-strength steel welding. Welds produced with different LTT filler wires have shown different levels
and distributions of the resulting residual stresses depending on the specific transformation temperature. The
transformation temperatures are determined by temperature measurement. Classical X-ray diffraction as well as
diffraction methods using high energy synchrotron radiation have been used for residual stress analysis. By means of
high energy synchrotron diffraction in reflection mode residual stress depth gradients can be determined nondestructively.
The phase selective nature of the diffraction measurements enables the simultaneous determination of
the phase specific residual stresses of all contributing crystalline phases within one experiment. The application of
white beam diffraction implies recording of a multitude of diffraction lines within the energy range of the provided
energy spectrum of the white beam. By this means phase specific residual stress depth distributions up to distances of
150 ìm below the surface can be analysed for steel using the energy dispersive set-up of the HMI-beamline EDDI at
the Bessy site, Berlin, providing an energy range between 20-150 keV. As a side effect quantitative phase analysis can
be carried out using white energy dispersive diffraction e.g. the determination of the content of retained austenite in the
weld.
Novel martensitic filler materials with specially adjusted martensite start temperatures (Ms) can counteract the cooling specific shrinkage due to expansion effects of the weld metal associated with phase transformations. That can be exploited to create compressive residual stresses in the weld and adjacent areas, i.e. beneficial for increasing fatigue strength. The Ms temperature is shifted via the chemical composition, mainly by the alloying elements nickel and chromium, resulting as well in different retained austenite contents. Investigations were made using different Low Transformation Temperature (LTT) alloys with varying nickel content. The resulting phase transformation temperatures were - for the first time - detected using high energy synchrotron diffraction and Single Sensor Differential Thermal Analysis (SS-DTA). Compared to angle dispersive diffraction, energy dispersive diffraction offers the possibility to measure residual stresses of the martensite and austenite phase parallel fast in one experiment up to depths of 100 µm. The residual stresses show significant distributions dependent on Ms temperature. The effect on the cold cracking behaviour of these alloys was investigated using the Tekken test. Results show that cold cracking can be avoided when appropriate contents of retained austenite are existent.