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Organisationseinheit der BAM
The chemical composition of a weld metal determines the resulting solidification mode of stainless steel and the consequent weld metal quality. In this work tungsten inert gas (TIG) welding of EN grade 1.4435 austenitic stainless steel was monitored using laser-induced breakdown spectroscopy (LIBS) for the in situ measurement of chemical composition changes. This research aims to prototype a real-time chemical composition analysis system for welding applications and prove the feasibility of such quality control loop. LIBS was used to investigate in situ the monitoring of metal vaporization during TIG welding. We found Mn vapor formation above the weld pool and subsequent condensation of Mn on the weld metal surface using LIBS. Post-weld line scans were conducted by LIBS on various welds produced with different welding currents. Local changes of Ni and Mn were observed at higher welding currents. The results are in good agreement with the literature and proved that LIBS can be used in situ to inspect the TIG welding process.
A laser-induced breakdown spectroscopy (LIBS) system was combined with a bead-on-plate Tungsten Inert Gas (TIG) welding process for the in situ measurement of chemical compositions in austenitic stainless steels during welding.Monitoring the weld pool's chemical composition allows governing the weld pool solidification behavior, and thus enables the reduction of susceptibility to weld defects. Conventional inspection methods for weld seams (e.g. ultrasonic inspection) cannot be performed during the welding process. The analysis system also allows in situ study of the correlation between the occurrence ofweld defects and changes in the chemical composition in theweld pool or in the two-phase regionwhere solid and liquid phase coexist. First experiments showed that both the shielding Ar gas and the welding arc plasma have a significant effect on the selected Cr II, Ni II and MnII characteristicemissions, namely an artificial increase of intensity values via unspecific emission in the spectra.
In situ investigations showed that this artificial intensity increase reached a maximum in presence of weld plume. Moreover, an explicit decay has been observedwith the termination of thewelding plumedue to infrared radiation during sample cooling. Furthermore, LIBS can be used after welding to map element distribution. For austenitic stainless steels,Mnaccumulations on both sides of theweld could be detected between the heat affected zone (HAZ) and the base material.
One of the main advantages of metal additive manufacturing (MAM) techniques is their ability to produce components with site-specific microstructural features. Nevertheless, microstructural defects and lack of repeatability are still major concerns in MAM. In this study, a laser powder bed fusion (PBF-LB/M) IN718 material, produced using two different scan length vectors, is investigated using Bragg-edge neutron 2D imaging (BENI) combined with electron backscatter diffraction (EBSD) analysis. BENI is able to detect, on a macroscopic scale, process-induced changes in texture in a large field of view covering the entire sample (20×80 mm2). In addition, high-resolution BENI (HR-BENI), with a pixel size of 12.8 µm, provides a micro-scale examination of the local variations of texture and grain morphology, otherwise undistinguishable using the standard resolution. As such, HR-BENI offers a straightforward and detailed way of screening the integrity of MAM parts at cm-length scales.
The ISO 3690 standard “Determination of hydrogen content in arc weld metal” requires a thermal activation of the diffusible hydrogen in a piece of weld metal for the subsequent ex situ concentration measurement by carrier gas hot extraction CGHE or thermal desorption spectroscopy (TCD). Laser-induced breakdown spectroscopy (LIBS) offers a time and spatially resolved, almost non-destructive, in situ measurement of hydrogen at surfaces without sample preparation. We measured hydrogen in steels, which were charged either electrochemically or by high-pressure hydrogen gas, and compared the results. Further, the feasibility of quantitative hydrogen line scan measurements with LIBS was demonstrated by measuring hydrogen at water jet cut surfaces. The hydrogen concentrations measured with the help of LIBS were compared with CGHE measurements. It was observed that hydrogen can be reliably measured with LIBS for concentrations larger than 2 wt.-ppm. The maximum hydrogen concentration achieved using electrochemical charging was 85.1 ppm. The results show that LIBS is a promising technique for time- and spatially resolved measurements of hydrogen in steels.
Duplex stainless steels (DSS) are used in all industries where corrosion problems play a major role. Examples include the chemical industry, the food industry and shipping industries. DSS have a balanced phase ratio of ferrite (α) and austenite (γ). Unlike single-phase stainless steels, DSS combine the advantages of these and can therefore fit many industry requirements, such as weight saving or high mechanical strength. When these steels are welded, alloying elements can burn off and condense as thin layers on cold surface regions. This loss of chemical elements can lead to changes in the microstructure. With the help of Laser-Induced Breakdown Spectroscopy (LIBS), chemical element distributions were visualized. The results were compared with those of conventional measurement methods, such as energy dispersive X-ray analysis (EDS) and X-ray fluorescence analysis (XRF), and the results from LIBS could be validated. LIBS is suitable as a fast, straightforward measurement method for producing line scans along the weld seam and provides spatially resolved information on accumulation phenomena of burned off alloying elements. LIBS is very well suited for the detection of sub-surface elements due to the exclusively superficial ablation of the material. In addition, the measurement method has been calibrated so that quantitative statements about element concentrations can also be made.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron (and steel) and many other metals. The necessary contrast between hydrogen and these metals stems from the high difference in the total neutron cross section of both elements. This allows, e.g., the in situ measurement of hydrogen mass flow inside cm thick metal samples with a temporal resolution of at best 10 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations, e.g., at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-assisted damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20 μm - 30 μm. In this contribution, we give a short historical overview of neutron imaging and show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations, absorption processes, and diffusion. The examples are taken from the works of researchers dealing with titanium, palladium, zirconium, and iron or steel. More detailed descriptions of the experimental and analytic procedures are given for hydrogen detection using radiography and tomography on iron and steel samples.
While the problem of the identification of mechanisms of hydrogen assisted damage has and is being thoroughly studied, the quantitative analysis of such damage still lacks suitable tools. In fact, while, for instance, electron microscopy yields excellent characterization, the quantitative analysis of damage requires at the same time large field-of-views and high spatial resolution. Synchrotron X-ray refraction techniques do possess both features. In this work, we show how synchrotron X-ray refraction computed tomography (SXRCT) can quantify damage induced by hydrogen embrittlement in a lean duplex steel, yielding results that overperform even those achievable by synchrotron X-ray absorption computed tomography. As already reported in literature, but this time using a non-destructive technique, we show that the hydrogen charge does not penetrate to the center of tensile specimens. By the comparison between virgin and hydrogen-charged specimens, we deduce that cracks in the specimen bulk are due to the rolling process rather than hydrogen-assisted. We show that (micro)cracks propagate from the surface of tensile specimens to the interior with increasing applied strain, and we deduce that a significant crack propagation can only be observed short before rupture.
The Modified Varestraint Transvarestraint (MVT) test was used to investigate the solidification cracking susceptibility of an unstabilised, a Nb-stabilised and two (Ti + Nb) dual-stabilised ferritic stainless steels. Two different welding speeds of 6 and 3 mm/s using autogenous gas tungsten arc welding were employed. At the welding speed of 6 mm/s, the high-content (Ti + Nb) steel was resistant and the Nb-stabilised steel was marginally susceptible to solidification cracking. At the welding speed of 3 mm/s, the Nb and the high (Ti + Nb) steels were found to be marginally susceptible to solidification cracking while the unstabilised and low-content (Ti + Nb) grades were resistant to solidification cracking. The weld metal microstructures transverse to the welding direction revealed columnar grains in all the samples for both welding speeds. The ferritic stainless steels were generally resistant to solidification cracking, except the Nb-stabilised steel, which was marginally susceptible to solidification cracking.
When in situ techniques became available in recent years this led to a breakthrough in accurately determining diffusion coefficients for liquid alloys. Here we discuss how neutron radiography can be used to measure chemical diffusion in a ternary AlCuAg alloy. Neutron radiography hereby gives complementary information to x-ray radiography used for measuring chemical diffusion and to quasielastic neutron scattering used mainly for determining self-diffusion. A novel Al2O3 based furnace that enables one to study diffusion processes by means of neutron radiography is discussed. A chemical diffusion coefficient of Ag against Al around the eutectic composition Al68.6Cu13.8Ag17.6 at.% was obtained. It is demonstrated that the in situ technique of neutron radiography is a powerful means to study mass transport properties in situ in binary and ternary alloys that show poor x-ray contrast.
CONRAD-2 is an imaging instrument using low-energy (cold) neutrons. The instrument is installed at the end of a curved neutron guide which avoids the direct line of sight towards the reactor core. This ensures a very low background of high-energy neutrons and. photons at the sample position. The cold neutron beam provides a wavelength range which is suitable for phase-and diffraction-contrast imaging such as grating interferometry and Bragg edge mapping. The instrument is well suited for high resolution imaging due to the high efficiency of the very thin scintillators that can be used for the detection of cold neutrons. An instrument upgrade was performed recently as a part of an upgrade program for the cold neutron instrumentation at HZB. The parameters of the instrument as well as some research highlights will be presented.