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.
Insights into early damage mechanisms during high-temperature gas corrosion provide important aspects for the prediction of the long-term stability of hightemperature materials exposed to a hot and corrosive environment. The current work presents a real time study concerning the combined oxidation and sulfidation of ferritic model alloys in a hot SO2 containing atmosphere using energy dispersive X-ray diffraction. The applied high temperature reactor allows the transmittance of high energetic X-rays in order to collect X-ray diffraction pattern of the sample surface in situ during the reaction with the reactive environment. The results revealed that the first phases crystallizing on iron with 2 wt% chromium and with 9 wt% chromium are oxides. The aging experiments at T = 650° C with 0.5% SO2 and 99.5% Ar were followed in situ and caused an external mixed oxide-sulfide layer on top of iron with 2 wt% chromium. On the higher alloyed material the external scale consists of iron oxides and the inner scale of mixed (Fe,Cr)-oxides and chromium-sulfides. The oxide content continuously increases parallel to an increase of the sulfide amount. Thus, the initially formed (Fe,Cr)-oxides do not have a protective character and support the transport of sulfur through the growing oxide scale.
A range of heat treatments have been developed for wrought Inconel 718 to obtain desired properties. For additively manufactured Inconel 718, the recently developed standard ASTM F3301 provides guidance for heat treatment of powder bed fusion specimens. Although this standard is based on standards developed for wrought Inconel 718, it does not include direct aging. Since direct aging reduces the number of processing steps, it can result in a post processing cost reduction if the desired properties are obtained. In this study, we characterized the microstructure and tensile behavior of Inconel 718 specimens produced by a laser powder bed fusion process. The specimens were heat treated according to two different routines after stress relieving: a full heat Treatment versus a one-step direct aging process. Differences in the resulting texture and grain morphology were observed.
The ex-situ stress-strain behavior was broadly similar. However, a slight increase in yield strength was observed for the direct aged specimen. In order to understand this behavior, investigations with in-situ synchrotron Energy dispersive X-ray diffraction tensile testing revealed differences in the load partitioning among different Crystal directions. Importantly, the elastic anisotropy expressed by the magnitude of the diffraction elastic constants showed a dependency on the microstructures.