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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 the 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.
The development of high-strength structural steels with yield strengths up to 1000 MPa results in the requirement of suitable filler materials for welding. Recently designed low transformation temperature (LTT) alloys offer appropriate strength. The martensitic phase transformation during welding induces compressive residual stress in the weld zone. Therefore, the mechanical properties of welded joints can be improved. The present paper illustrates numerical simulation of the residual stresses in LTT-welds taking into account the effect of varying Ms/Mf-temperatures, and therefore different retained austenite contents, on the residual stresses. Residual stress distributions measured by synchrotron diffraction are taken as evaluation basis. A numerical model for the simulation of transformation affected welds is established and can be used for identification of appropriate Ms-temperatures considering the content of retained austenite.
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.
For the safety and cost efficiency of welded high-strength steel structures, precise knowledge of the level and distribution of welding- and cooling-specific stresses and residual stresses is essential, since they exert a decisive influence on strength, crack resistance, and finally on the bearable service load. This paper presents innovative filler materials, of which the phase transformation temperature was deliberately adjusted via the chemical composition. The transformation behaviour of these martensitic Low Transformation Temperature (LTT-) filler materials shows direct effects on the local residual stresses in the weld and the HAZ. These effects can purposefully be exploited to counteract the thermally induced shrinkage of the material and to produce significant compressive residual stresses in the weld. Comparative welding experiments were carried out on 690 MPa high-strength base materials using various LTT-filler materials. High energy synchrotron radiation was used for residual stress measurement. Particularly the use of high energy synchrotron radiation makes it possible to detect the residual stress condition fast without destruction of material. Thereby, residual stress depth gradients can be determined simultaneously without removing material. In steel, gradients of up to 150 µm can be resolved in such a way. Furthermore, the application of high energy radiation permits determination of residual stresses of any available residual austenite contents. Results show significant dependence of transformation temperatures on the resulting residual stress level and distribution.
Cold cracking resistance is a relevant evaluation criterion for welded joints and affected by residual stresses which result from the welding procedure. Compressive residual stresses can thereby have a positive influence on preventing cracking. A unique possibility of generating compressive residual stresses already during the welding procedure is offered by the so-called Low Transformation Temperature (LTT) filler wires. Compared to conventional wires, these materials show decreased phase transformation temperatures which can work against the cooling-specific contraction. 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. Investigations were carried out to determine the phase transformation behaviour of different LTT-filler materials. Transformation temperatures were identified using Single Sensor Differential Thermal Analysis (SS-DTA). Additionally Synchrotron radiation was used to measure the transformation kinetics of all involved crystalline phases during heating and cooling of a simulated weld thermal cycle.
Direct measurement and quantification of phase transformation in a low-alloyed transformation induced plasticity steels depending on the tensile load as well as determination of the real true stress and true strain values were carried out in-situ using high energy synchrotron radiation. Digital image correlation technique was used to quantify more precisely the true strain values. The aim of the work was to obtain a better understanding of the phase transformation of commercial low-alloyed transformation induced plasticity steel depending on the true strain and true stress values.
Realization and safe operation of modern welded structures are progressively requiring for base and filler materials to cope with continually increasing loads, Analyses with a view to crack prevention therefore need to accommodate particularly the structural design (restraint intensity) and the thermomechanical effects in terms of stresses introduced during welding, Against this background, multi-run welding experiments were carried out in an IRC - (Instrumented Restraint Cracking) Test under defined restraint intensity in order to examine the influence of heat control (preheating and interpass temperatures) on the weldinq-specific forces and stresses, The experiments revealed significant rises in the reaction force and stress after welding with increasing preheating and interpass temperature, Furthermore, various concepts for calculating the preheating temperature are discussed, It is demonstrated that the hitherto existing concepts for high-strength finegrained structural steels can lead to divergent results.
Modifying the level of mostly detrimental welding residual stresses already during the welding process would be highly attractive as time- and cost-consuming post processing may be prevented. The nature of stress buildup during welding-associated cooling is highly affected by phase transformations. Up to now, it is not clear in which way this is applicable to real component welding exhibiting high shrinkage restraint and complex heat input. In this study, two different low transformation temperature (LTT) alloys have been investigated concerning the stress development in restrained multi-run butt welding in order to evaluate the potential of stress reduction. Pulsed gas metal arc welding (P-GMAW) welding was executed on a testing facility designed to simulate real lifelike restraint conditions of component weldments. The effect of reducedMS-temperatures and the heat control on the globally acting stresses was monitored by in-situ measurement of the reaction forces during welding fabrication. Additional local residual stress measurements allowed analyzing global as well as local loading of the welded construction. Although phase transformation has a significant influence on unloading the joint during each weld pass, the reaction stress upon cooling to room temperature seems to be determined mainly by the heat input. On the surface, low longitudinal residual stresses were observed in case of LTT whereas transverse residual stresses are less affected.
Low transformation temperature (LTT) alloys allow to control residual stresses already during the welding process. Especially high-strength structural steel applications may benefit from the LTT effect as they are sensitive to residual stresses due to a limited ductility. Within this study, two modified LTT alloys were tested concerning their weldability under varying conditions. Beside the transformation behavior, basic material properties were determined from all weld metal. Hot cracking as well as cold cracking susceptibility was evaluated using specific tests. The materials' capability for residual stress control was characterized by online measurements of the occurring loads during double-sided multipass fillet welding in a special test facility. Varying heat control parameters were found to affect the stress buildup significantly. In the specific case, the results revealed that higher working temperatures may favor lower stress buildup despite the higher overall heat input. Local residual stress measurements using X-ray diffraction support this finding.