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Analyse lokaler Verformungsfelder mittels DIC-Technik während der Heißrissbildung beim Schweißen
(2013)
Für die Gewährleistung der Bauteilsicherheit ist während des Fertigungsprozesses das Vermeiden von Imperfektionen maßgeblich. Zu diesen Imperfektionen zählen auch beim Schweißen auftretende Heißrisse, die insbesondere die Bauteileigenschaft beeinflussen und demzufolge ausgeschlossen werden müssen. Aufgrund der hohen Komplexität, hervorgerufen durch das Zusammenwirken zahlreicher Einflussgrößen, existieren überwiegend vereinfachte Werkstoffmodelle, die das Heißrissphänomen nur unzureichend beschreiben. Da bislang eine Charakterisierung dieser Heißrissentstehung überwiegend auf spannungs- und dehnungsbasierten Modellen erfolgt, wurde in diesem Fall speziell geprüft, in wie weit vereinfachend eine Quantifizierung des Heißrissverhaltens anhand von Verschiebungen bzw. Verschiebungsraten möglich ist. Die Tatsache, dass Heißrisse in Werkstoffen auftreten, die aus metallurgischer Sicht als heißrissunkritisch eingestuft werden können, zeigt, dass weitere Faktoren, wie der konstruktive Einfluss, betrachtet werden müssen. In der vorliegenden Arbeit wurde dieser Aspekt unter Nutzung des Controlled-Tensile- Weldability(CTW)-Tests untersucht. Hierfür wurde die Digital-Image-Correlation( DIC)-Technik erfolgreich zur In-situ-Analyse der lokalen Querverschiebungen dy und Querverschiebungsraten dy/dt in direkter Schmelzbadnähe während des Schweißens eingesetzt.-----------------------------------------------------------------------------------------------------------------------------------------------------------------
In order to guarantee the component safety, it is essential to avoid imperfections during the production process. These imperfections also include hot cracks which arise during welding, exert an influence on the component property in particular and must consequently be excluded. Due to the high complexity caused by the interaction of a large number of influencing variables, there are mostly simplified material models which describe the hot cracking phenomenon only insufficiently. Since this hot crack initiation has, until now, been characterised mostly on models based on stresses and strains, it was especially checked in this case the extent to which the hot cracking behaviour can be quantified using a simplifying method taking account of displacements and displacement rates. The fact that hot cracks occur in materials that, from the metallurgical point of view, cannot be classified as hot-crack-critical shows that further factors such as the influence of design need to be considered. This aspect was investigated in the present study by using the Controlled Tensile Weldability (CTW) test. For this, the Digital Image Correlation (DIC) technique was successfully applied for the in situ analysis of the local transverse displacements dy and transverse displacement rates dy/dt in the direct weld pool vicinity during welding.
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metallic crystal lattice. For the investigations reported in this contribution, hydrogen thermal desorption analysis has been carried out to observe the influence of hydrogen desorption on the lattice of super martensitic stainless steel during continuous heating. The lattice expansion parameter and the phase transformations have been monitored during the thermal desorption process, and the influence of hydrogen on such characteristics has been evaluated. It was found that hydrogen has a significant influence on both the lattice parameter and on the thermal expansion. However, hydrogen has no influence on phase transformation during thermal desorption. The hydrogen's desorption behavior in this process was also observed and it turned out that hydrogen desorbs in two stages, i.e., firstly diffusible hydrogen and trapped hydrogen afterward.
A 20 mm thick heat resistant 13CrMoV9-10 steel was welded using a multi-pass submerged arc tandem setup on a 3-D testing facility designed to prevent shrinkage both during the welding and the subsequent cooling to room temperature. The effects of variations in pre-heating and interpass temperatures on the local stresses were monitored by in situ measurement of reaction forces and bending moments during welding and the subsequent dehydrogenation heat treatment. The austenite to bainite transformation and its effects on the overall loads were monitored through the use of a high sampling rate. Increasing the pre-heating and interpass temperatures led to a rise in reaction forces and bending moments. The bending moments originated from the eccentric force transmission over the backing, with a concomitant risk of fracture, especially in the critical root zone. Although the pre-heating and interpass temperatures had a strong effect on the reaction forces, their effect on the bending moments upon cooling to room temperature was negligible.
The purpose of the present study is to show the feasibility of examining hydrogen desorption in technical iron samples using neutron radiography at the ANTARES facility of the FRM II research reactor, Technische Universität München. It has been shown that this method is appropriate for in situ determination of hydrogen Desorption for concentrations as low as 20 ppmH. Experiments were carried out in the temperature range from room temperature up to 260 °C. Measurement was based on direct comparison between electrochemically hydrogen-loaded iron samples and hydrogen-free reference samples at the same temperature.
This enables the determination of hydrogen concentration as a function of time and temperature. Ex situ carrier gas hot extraction experiments using the same temperature–time profiles as the neutron radiography experiments have been used to calibrate the greyscale values of the radiographs to defined hydrogen concentrations. It can be stated that hydrogen desorption correlates with sample temperature.
Solid-state phase transformations and the evolution of thermal and elastic strains in novel low temperature transformation (LTT) weld filler materials in the near surface region are monitored in real time by means of an innovative experimental set-up at the PDIFF (powder diffraction) beamline at the synchrotron light source ANKA (Angströmquelle Karlsruhe) at the KIT (Karlsruhe Institute for Technology). The key components of the diffraction set-up are two fast microstrip line detectors, which enables the strain evolution to be followed as a function of time and temperature for a 0.5?s counting time. During controlled heating and cooling cycles, as well as during near welding cycles, the martensite–austenite–martensite phase transitions are analysed. The transformation kinetics are monitored during resistance heating of small chips of the pure LTT alloys and during gas tungsten arc welding of simplified LTT welds using a specially designed welding rig for in-situ studies on the diffraction instruments. Under the mechanically unconstrained condition allowing free thermal expansion and shrinkage, the LTT alloys are found to exhibit decreasing transformation temperatures Ac and MS and increasing phase fraction of retained austenite for increasing Ni content. The strain evolution during welding reveals increased compressive stresses upon welding, which is attributed to the martensite formation upon cooling, which counteracts the thermal contraction strains. Comparison of the transformation temperatures reveals higher values than in the pure LTT alloys, but no variation between the different alloys. On the one hand, this is attributed to preferred grain orientation affecting the diffraction measurements and the determination of the transformation temperatures. On the other hand, it is possible that with the different chemical compositions of the LTT alloys and the mechanical constraints during welding, the evolution of the residual strain and stress may vary and result in counteracting affects with respect to lowered martensite start temperatures.
In-situ analysis of solid state phase transformation in TRIP-aided steels by synchrotron diffraction
(2011)
Energy dispersive synchrotron diffraction (EDXRD) analysis and 3 dimensional digital image correlations were conducted to investigate the stress and strain effected transformation behavior during tensile loading of low alloyed TRansformation Induced Plasticity (TRIP) steel. This technique allowed for phase specific stress measurement during certain tensile load steps in the elastic and also plastic regime. Additionally the simultaneous determination of the load dependent phase content was realized. The results show that the martensite transformation starts only after exceeding the overall yield point and is finished before reaching the uniform elongation, whereas a large portion of the austenite remains unchanged in the structure. Furthermore, the martensite transformation related to the stress in the γ-phase and α-phase was analyzed and quantified.
Energy-dispersive x-ray diffraction offers the possibility for measurement and evaluation of diffraction spectra containing information of various diffraction lines of all contributing crystalline phases of a material. Combined strain imaging and diffraction analysis was conducted during the tensile test of a low alloyed transformation-induced plasticity (TRIP) steel in order to investigate the transformation induced plasticity, strain hardening, and load partitioning effects. Optical strain imaging allowed for determination of localized true strains from three-dimensional deformations measured in situ. High-energy synchrotron radiation has permitted diffraction analysis in transmission mode to gather information from the material interior. Phase-specific stress evolution during loading could be observed applying the sin2ψ technique during certain load steps. The strains of the individual lattice planes were determined in different locations under varying angles between loading and perpendicular direction. Using energy-dispersive methods it was also possible to determine the transformation behaviour during elastic and plastic regime taking into account a large number of diffraction lines. The results show that the approach practised here enables one to pull together macroscopic and phase-specific microscopic material behaviour in order to improve existing models for prediction of complex load situations.