Filtern
Erscheinungsjahr
Dokumenttyp
- Vortrag (25)
- Zeitschriftenartikel (7)
- Beitrag zu einem Tagungsband (5)
- Posterpräsentation (3)
- Beitrag zu einem Sammelband (1)
- Sonstiges (1)
- Forschungsbericht (1)
Referierte Publikation
- nein (43) (entfernen)
Schlagworte
- Neutron imaging (7)
- Neutron radiography (7)
- Hydrogen (6)
- Hydrogen embrittlement (4)
- In situ measurement (4)
- Neutron tomography (4)
- Diffusion (3)
- LIBS (3)
- Steel (3)
- TIG welding (3)
Organisationseinheit der BAM
Eingeladener Vortrag
- nein (25)
Chemical compositions of a weld can be varying locally as a result of the welding process. These local variations can be due to the vaporization of individual alloying elements. In this work, tungsten inert gas (TIG) bead-on-plate stainless steel welds of EN grade 1.4404 and 1.4435 were investigated using laser-induced breakdown spectroscopy (LIBS) on the completed welds. This study aims to reveal the welding parameters’ influence on the resulting local chemical compositions of the stainless steel welds. We demonstrated Mn vaporize before Cr due to its lower latent enthalpy of vaporization. Hence, Mn accumulates on the heat-affected zone (HAZ) both sides across the weld bead by being swept away through the circulation flow of the welding plasma. Additionally, increasing the heat input tends to enhance the accumulated Mn content on the HAZ as well as increasing the shielding gas flow rate. The results are in good agreement with the literature and proved that LIBS is an effective method to inspect completed welds.
Duplexstähle besitzen ein zweiphasiges Gefüge und werden aufgrund ihrer verbesserten Schweißeignung gegenüber den Vollausteniten häufig im Anlagen- bzw. Apparatebau eingesetzt. Aufgrund der hohen Abkühlgeschwindigkeit und dem Abbrand von Legierungselementen kommt es zu einer Ferritisierung des Schweißguts und damit zu einer Degradation der mechanischen Eigenschaften. Zur Vorhersage des Phasenverhältnisses im Schweißgut wird das WRC1992-Diagramm genutzt. Dieses Diagramm zeigt einige Ungenauigkeiten und benötigt zur genaueren Vorhersage der Phasenverhältnisse eine Überarbeitung. Um den Einfluss einzelner Elemente auf das Schweißnahtmikrogefüge besser zu verstehen, wurden drahtförmige Schweißzusatzwerkstoffe mit dem Ferrit-Bildner Nb und dem Austenit-Bildner Cu beschichtet und für Schweißungen verwendet. Die Messmethode der Laser-induzierten Plasmaspektroskopie (LIBS) bietet hier eine gute Möglichkeit der in situ Überwachung der chemischen Konzentrationen, während des WIG-Schweißens von Duplexstählen. Die LIBS-Messergebnisse, konnten mit der Ferritnummer und der Schweißnahtmikrostruktur korreliert werden.
Many applications in industry require a material-to-material joining process of Duplex Stainless Steels (DSS). Therefore, it is essential to investigate the material’s properties during a welding process to control the weld quality. With the help of Laser-Induced Breakdown Spectroscopy (LIBS), the chemical composition during the Tungsten Inert Gas (TIG) welding process of DSS could be monitored in situ. The chemical composition could be quantitatively measured using pre-established calibration curves. Although the surface temperature and the welding plasma have a high influence on the spectral intensities, reliable composition measurements were possible. The concentration of alloying elements could be mapped during the TIG welding process.
Duplex stainless steels (DSS) are frequently used, especially in applications requiring high strength combined with high corrosion resistance in aggressive media. Examples include power plant components and maritime structures. During welding of these steels, local variations in chemical composition can occur. This results in ferritization of the material and negatively affects the mechanical properties of the components. In this work, tungsten inert gas (TIG) welding experiments were performed with DSS. Chemical composition analysis was realized in situ by using Laser Induced Breakdown Spectroscopy (LIBS). The aim of the work is to quantitatively measure the chemical composition in the weld seam of various DSS and to identify possible influences of welding parameters on the microstructure of the material. The chemical concentrations of the main alloying elements Cr, Ni, Mn on the surface of the sample during the welding process and the cooling process were measured. Mn and Ni are austenite stabilizers and their content increases during welding by using certain high alloyed filler material. Spectra were recorded every 1.3 s at a spacing of approximately 2 mm. During the cooling process the location of the measurement was not changed. The LIBS method is proofed to be suitable for the quantitative representation of the chemical compositions during the welding process.
Hydrogen-charged supermartensitic steel samples were used to systematically investigate imaging artifacts in neutron radiography. Cadmium stencils were placed around the samples to shield the scintillator from excessive neutron radiation and to investigate the influence of the backlight effect. The contribution of scattered neutrons to the total detected intensity was investigated by additionally varying the sample-detector distance and applying a functional correlation between distance and intensity. Furthermore, the influence of the surface roughness on the edge effect due to refraction was investigated.
3D Crack analysis in hydrogen charged lean duplex stainless steel with synchrotron refraction CT
(2016)
Hydrogen in metals can cause a degradation of the mechanical properties, the so-called hydrogen embrittlement. In combination with internal stresses, hydrogen assisted cracking (HAC) can occur. This phenomenon is not completely understood yet. To better characterise the cracking behaviour, it is important to gain information about the evolution of the 3D crack network. For this purpose samples of lean duplex stainless steel were loaded with hydrogen by means of electrochemical charging and investigated by means of synchrotron refraction CT and SEM fractography after uniaxial tensile loading. Synchrotron refraction CT is an analyser-based imaging (ABI) technique. It uses a Si (111) single crystal as analyser, which is placed into the beam path between sample and detector. According to Bragg’s law only incident x-rays within a narrow range around the Bragg-angle are diffracted from the analyser into the detector. Hence, the analyser acts as an angular filter for the transmitted beam. This filtering allows to turn the refraction and scattering of x-rays into image contrast. Refraction occurs at all interfaces, where the density of the material changes and is more sensitive to density changes than the attenuation. Therefore, it is possible to detect smaller cracks than with classical x-ray imaging techniques, like CT, with comparable spacial resolution. It also visualises the 3D structure of the cracks and gains quantitative information about their morphology and distribution. Since cracks introduced by HAC are usually very small and have a small opening displacement, synchrotron refraction CT is expected to be well suited for imaging this cracking mechanism and can be a valuable tool to characterise the formation and the evolution of a 3D crack network.
To better understand the mechanism of hydrogen assisted cracking (HAC), it is important to investigate the 3D structure of the cracks non-destructively. Since, cracks introduced by HAC are usually very small, conventional x-ray imaging methods often lack the required spatial resolution.
However, the detection of those cracks can be enhanced by taking advantage of refraction at interfaces within the sample.
To image this refractive deflection we employ analyser based imaging (ABI). In this work we aim at proving the enhanced crack detection of ABI by investigating an alluminum alloy weld.
Hydrogen in metals can cause a degradation of the mechanical properties with possible subsequent hydrogen assisted cracking (HAC). Though, the mechanism of HAC is not completely understood yet and thus suitable methods for in situ investigations to characterise the crack formation are needed. X-ray computed tomography (CT) is a well-known tool for analysing these properties. However, the effective resolution of the detector system limits the detection of small defects by CT. Analyser based imaging (ABI) takes advantage of x-ray refraction at interfaces between volumes of different density, i.e. of cracks, pores, inclusions, etc., within the sample to detect defects smaller than the resolution of the detector system. In this study, measurements on an aluminium alloy weld showed that ABI allows us to resolve the 3D structure of cracks undetected by absorption based CT. Prospective investigations will analyse HAC in steels.
This study was carried out to investigate the neutron transmission signal as a function of sample temperature during a welding process. A theoretical description that includes the Debye-Waller factor was used to describe the temperature influence on the neutron crosssections. Neutron imaging using a monochromatic beam helps to observe transmission variations related to the material temperature. In-situ neutron imaging of welding experiments show the distribution of the temperature in bulk steel samples. The performed finite element modelling of expected temperature distributions shows good agreement with the obtained experimental data.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel.
The necessary contrast between hydrogen and iron 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 steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 μm.
In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed
at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in
Garching.
Polychromatic and wavelength-selective neutron transmission radiography were applied during bead-on-plate welding on 5 mm thick sheets on the face side of martensitic low transformation temperature (LTT) steel plates using gas tungsten arc welding (GTAW). The in situ visualization of austenitization upon welding and subsequent α’-martensite formation during cooling could be achieved with a temporal resolution of 2 s for monochromatic imaging using a single neutron wavelength and of 0.5 s for polychromatic imaging using the full spectrum of the beam (white beam).
The spatial resolution achieved in the experiments was approximately 200 µm. The transmitted monochromatic neutron beam intensity at a wavelength of λ = 0.395 nm was significantly reduced during cooling below the martensitic start temperature M s since the emerging martensitic phase has a ~10% higher attenuation coefficient than the austenitic phase. Neutron imaging was significantly influenced by coherent neutron scattering caused by the thermal motion of the crystal lattice (Debye–Waller factor), resulting in a reduction in the neutron transmission by approx. 15% for monochromatic and by approx. 4% for polychromatic imaging.
Neutron cameras allow visualizing hydrogen distributions with radiographic or tomographic imaging methods in iron and steel. The necessary contrast between hydrogen and iron 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 steel samples with a temporal resolution of 20 s using neutron radiography as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples with neutron tomography. We could detect directly gaseous hydrogen in the crack cavities and we measured the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of hydrogen-induced damage mechanisms. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 µm. In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography methods in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed at the research reactor BER II of the HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in Garching.
Neutron imaging has become a valuable tool for measuring hydrogen distributions qualitatively and quantitatively in metals. Hydrogen mass flow can be measured inside cm thick Steel samples with 10 s temporal resolution. Hydrogen accumulations around craclcs in embrittled iron samples can be visualized three-dimensionally. The gas pressure of hydrogen in crack cavities has been measured to be in the ränge of 5 MPa to 15 MPa. This quality of information allows new insights for the analysis of damage mechanisms on a micrometer scale, e.g., of hydrogen blistering. Further, this method is nondestructive and provides local information in situ and in three dimensions with a spatial resolution of 20 µm - 30 µm.
In Neutron-Bragg-Edge Imaging (NBEI) experiments, we studied the phase transition during butt-welding of supermartensitic steel plates. Gas tungsten arc welding (GTAW) was used with a motorized torch allowing for automated weldments. The austenitization in the heat affected zone (HAZ) underneath the welding head could be clearly visualized at λ = 0.39 nm, a wavelength smaller than the Bragg edge wavelengths of both austenite and martensite. Also, the re-transformation into the martensitic phase upon cooling was detected. However, we observed an unexpected additional change in transmission at λ = 0.44 nm that is a wavelength larger than the wavelength of the Bragg edges of both the martensitic and austenitic phases. We attribute this change to the Deybe-Waller-Factor that describes the temperature dependence of coherent scattering at a crystal lattice. The observed two-dimensional attenuation map corresponds well with a temperature distribution modelling by software macros in ANSYS. Here, the absolute temperature values could be achieved by calibrating the modelled attenuation with help of a thermocouple placed at the steel plate. This allows in return for a direct two-dimensional temperature reading based on the Debye-Waller-relation between neutron attenuation and sample temperature.
Revealing hydrogen embrittlement mechanisms in steels is of great interest to scientists and engineers. Neutron radiography makes it possible to measure in-situ hydrogen diffusion with high spatial and temporal resolution at concentrations as low as 20 ppm. We compare hydrogen-charged specimens with hydrogen-free reference specimens and use calibration standards to normalize the hydrogen concentrations. This allows quantitative tracking of the hydrogen concentration evolution as a function of time, space and temperature. Furthermore, a view into the material with 'neutron eyes' facilitates the detection of cavities that contain molecular hydrogen.
We report about investigations of typical material problems in steels. First, residual stresses in and around weld seams with low martensite transition temperature filler material were analyzed w/ and w/o additional transverse tensile load. The investigation was accompanied by x-ray diffraction measurements to track stress-induced phase transitions. Martensite formation during welding under external load yield strain gradients near the surface that improve the integrity of the weld. Further, the residual stress distribution close to laser-treated steel surfaces was analyzed. The method could be improved to reduce analysis artifacts. Finally, hydrogen diffusion in different steels was measured using neutron radiography and subsequent image analysis. Diffusion coefficients were derived by analyzing the time-dependent mass flux. In an outlook we describe the optimal ESS instrumentation from a metallurgists point of view.
Hydrogen assisted cracking of metals limits the durability of welds and constructions. Still
the failure mechanisms are not fully understood, for instance the role of hydrogen mass
transport during crack initiation. Measuring the hydrogen concentration as a function of
time (and space) would allow to determine diffusion coefficients helping to enlighten this
lack of understanding. Therefore, neutron radiography is a valuable tool for measuring in
situ hydrogen transport in iron and steels [1].
The influence of grain boundaries on the hydrogen diffusion behaviour can be determined
by comparing the diffusion behaviour of samples with e.g. different microstructure. In a
first study we investigated in situ the simplest case, the hydrogen effusion out of pure iron
(ARMCO) using neutron radiography at ANTARES [2]. Samples were charged electrochemically
ex situ with a well defined amount of hydrogen, stored in liquid nitrogen and were then
heat treated during a neutron radiography measurement whilst the change of neutron transmission
was recorded. For analysis the gray values in the pictures were transformed into
concentrations with help of an ex post calibration. Therefore, the experimental temperaturetime
profile of the heat treatment has been repeated with identical samples in the home lab
and the amount of effusing hydrogen was measured by carrier gas hot extraction. In a first
approximation the concentration of hydrogen in the sample can be calculated by subtracting
the hydrogen concentration in the atmosphere from the initial one in the sample. We
were able to measure hydrogen concentrations as low as 65 wt.ppm.
Simultaneously monitoring of hydrogenous
sample and hydrogen-free reference allowed
for in situ measurement of hydrogen effusion.
Sample heating was done by infrared radiation.
Diffusion coefficients for hydrogen effusion
from iron have been calculated. They coincide
fairly with literature data.
We could show in GTAW experiments with steel samples that the attenuation of neutron transmission underneath the welding torch due to the Debye-Waller-effect correlates well with sample temperatures obtained by FE numerical simulations. This would allow principally to determine sample temperatures by measuring the neutron beam attenuation.
Die Ergebnisse von in-situ Experimenten am Berliner Elektronenspeicherring (BESSY II) werden vorgestellt. Es wurden Zugversuche an wasserstoffbeladenen und wasserstofffreien supermartensitischen Proben durchgeführt und Diffraktionsspektren und radiographische Bilder aufgenommen. Die energiedispersive Röntgendiffraktion ermöglichte dabei in-situ die dehnungsinduzierte Phasenumwandlung von Restaustenit zu Martensit zu beobachten. Die Radiographiebilder der zerreisenden Probe gaben Einblicke in das Bruchverhalten in Abhängigkeit vom Wasserstoffgehalt.
In Neutron-Bragg-Edge Imaging (NBEI) in situ experiments, we studied the phase transitions in martensitic steel sheets during butt-welding. Gas tungsten arc welding was used with a motorized torch allowing automated weldments. The austenitization in the heat affected zone underneath the welding head could be clearly visualized. Also, the retransformation into the martensitic phase upon cooling. However, we observed an unexpected additional change in transmission at λ = 0.44 nm that is at a wavelength larger than the wavelength of the Bragg edges of both the martensitic and austenitic phases. We attribute this change to the Deybe-Waller-Factor that describes the temperature dependence of coherent scattering at a crystal lattice. With help of temperature field simulations that were calibrated by the reading of anattached thermo couple during welding, we could show that the Debye-Waller factor can produce an additional image contrast.
We report about diffusion measurements and molecular dynamic simulations
in Al-Ni and Al-Cu melts. Capillary methods were used to measure Ni and
Cu self diffusion and to measure interdiffusion. The combination of the
capillary set-up with X-ray radiography allowed in-situ detecting of
convective contributions to the interdiffusion transport by tracking the
time dependence of diffusion. Thus, the determination of the
interdiffusion coefficient can
be restricted to convection-free experiment times resulting in an
increased accuracy of the measured interdiffusion coefficient.
Additionally, quasielastic neutron scattering was used to measure
convection-free Ni and Cu self diffusion. Molecular dynamics simulations
were used to determine all diffusion coefficients and the thermodynamic
factor. The comparison between experiment and simulation shows an
excellent agreement of the interdiffusion coefficients as a function of
temperature. It was found that interdiffusion is enhanced by
thermodynamic forces with a maximum around the stoichiometric composition.
In recent years, the optimization of the imaging stations at research reactors and especially the further development of the neutron detectors allowed the measurement of hydrogen distributions with increasing spatial and temporal resolution at lower detection limits at the same time. Hydrogen has compared to iron a high total neutron cross section, which allows with basic radiographic methods the visualization of hydrogen in two and three dimensions inside the microstructure of components. This enables the in situ measuring of hydrogen mass flows inside cm thick steel samples with a temporal resolution of 20 s as well as the quantitative measurement of hydrogen accumulations at the crack’s inner surfaces in hydrogen embrittled iron samples. For the first time, we detected directly gaseous hydrogen in the crack cavities and we could measure the gas pressure. This new quality of the information on a micrometer scale allows new insights for the analysis of damage mechanisms, e.g. of hydrogen embrittlement. Further, this method is non-destructive and provides local information in situ and in three dimensions with a spatial resolution of 20-30 µm, which is not accessible with common methods as e.g. thermal desorption spectroscopy.
In this contribution, we show examples that demonstrate the spatial and temporal resolution of the neutron radiography and tomography method in order to visualize and quantify hydrogen accumulations at cracks. The measurements were performed at the research reactor BER II of HZB in Berlin and at the FRM II reactor of the neutron source Heinz Maier-Leibnitz in Garching.
Fast digital radiography was used to observe the crack development during single pass bead-on-plate gas tungsten arc welding with a minifocus X-ray source and a digital detector array with 75-micrometer pixel size. The sample material were 5 mm thick AlMgSi plates. An acquisition rate of 12 frames per second and an exposure time of 40 ms per frame were used for real-time observation of the hot crack propagation during welding. The basic spatial resolution of the images is about 80 µm. A 3D laminographic reconstruction of the acquired 2D radiographic images gives access to the volumetric extent of the cracks in the welded sample. The parallel use of a high-speed camera during welding allows the real-time inspection of surface cracks. The development of surface cracks was compared with the crack’s distribution in the sample volume and these results were correlated to the used welding parameters.
Cold neutrons are predominantly scattered at hydrogen when penetrating hydrogen-charged samples, resulting in a high image contrast between hydrogen and e.g. iron. The used radiographic set-ups consist of a neutron source, state-of-the-art scintillator screens and digital cameras. This allows monitoring diffusive hydrogen fluxes two-dimensionally with 20s temporal resolution. Such hydrogen fluxes can be quantified by using standards with known hydrogen content and similar sample thickness.
Neutron tomography generates three-dimensional models of the hydrogen distribution in steel. Such models gain new insight for damage analysis by showing the hydrogen accumulations around cracks and by enabling the hydrogen gas pressure estimation inside cracks.
The capabilities and limitations, as well as perspectives of this method will be discussed and illustrated with help of selected examples.
Die dehnungsinduzierte Martensitbildung wurde mittels Röntgen-Diffraktion und -Radiographie mit dem EDDI-Instrument am BESSY II in supermartensitischem Stahl mit Anlassaustenit untersucht. Die Bildung wird durch Wasserstoff im Gefüge unterdrückt. Die Duktilität von wasserstofffreiem SMSS korreliert mit dem Anteil an dehnungs-induziertem Martensit
Neutron imaging is a valuable tool for measuring hydrogen distributions qualitatively and quantitatively in metals. Time-resolved neutron radiography allows to measure hydrogen mass flow inside cm thick steel samples with ~10 s temporal resolution. Hydrogen accumulations around cracks in embrittled iron samples can be visualized three-dimensionally by neutron tomography. This quality of information allows new insights for the analysis of damage mechanisms on a micrometer scale, e.g. of hydrogen blistering. Even the gas pressure of molecular hydrogen in crack cavities has been measured from tomographic reconstructions to be in the range of 5 MPa to 15 MPa for technical iron. Further, this method is non-destructive and provides local information in situ and in all three dimensions with a spatial resolution of 20 - 30 µm. The combination with other methods gives a new quality of information, e.g. of the hydrogen allocation on fractured surfaces.
Hydrogen embrittlement (HE) is a widely known phenomenon and under investigation already for more than a century. This phenomenon, though thoroughly studied, is not yet completely understood, and so far, there are several suggested mechanisms that try to explain the occurrence of HE. One important factor of understanding the HE phenomenon and predicting hydrogen-assisted failure is the descent knowledge about the hydrogen transport behaviour in the material. Neutron radiography is a proven method for tracking hydrogen diffusion and it was applied successfully in various research studies. In the presented study, we examined the hydrogen effusion behaviour in duplex stainless steel by means of neutron radiography and calculated the effective diffusion coefficient from the obtained transmission images.