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Paper des Monats
- ja (2)
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.
We discuss the stability of weld residual strain under static and quasi cyclic transverse tensile loading in the elastic and elastic–plastic region. The test welds were joined with low transformation temperature weld filler materials with 10 wt% Cr and varying Ni-content from 8 to 12 wt%. Using neutron diffraction the residual lattice strain in the martensitic α'- and austenitic γ-phase in the fusion zone as well as the ferritic α-phase in the heat affected zone and base metal as induced by welding, superimposed by stepwise tensile loading and after unloading was measured. The amount of retained austenite in the fusion zone increases with increasing Ni-content, but it decreases with increasing load level due to stress induced martensite formation. In the as-welded condition the transverse macroscopic residual lattice strain was found to be in low compression in the fusion zone in each weld, while the heat affected zone was in tension. Local plastic deformation of the γ-phase as a result of yielding during tensile loading in combination with the change in phase fraction resulted in increased macroscopic compression in the fusion zone. The reduced yield strength in the heat affected zone resulted in plastic deformation and a considerable shift into compression. Comparison with the cross weld distribution of the hardness and FWHM of the neutron diffraction interference lines supported the assumption of plastic deformation of the γ- and α-phase in the fusion and heat affected zone, respectively, while the α'-phase in the fusion zone was stressed within the elastic regime only. Microstructural observations as well as measurement of the local γ-phase fraction by means of laboratory X-ray diffraction in the fusion zone strengthen these observations.
Anwendung des neuen Controlled Tensile Weldability (CTW) Tests zur Untersuchung der Heißrissneigung
(2006)