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A x-ray radioscopy technique for measuring in situ chemical diffusion coefficients in metallic melts
is presented. The long-capillary diffusion measurement method is combined with imaging
techniques using microfocus tubes and flat panel detectors in order to visualize and quantitatively
analyze diffusive mixing of two melts of different chemical composition. The interdiffusion
coefficient as function of temperature and time is obtained by applying Ficks diffusion laws.
Tracking the time dependence of the mean square penetration depth of the mixing process allows to
detect changes in the mass transport caused by convective flow. The possibility to sort out
convective mass transport contributions from analysis enhances significantly the accuracy compared
to the conventional long-capillary diffusion measurement method with postmortem analysis. The
performance of this novel diffusion measurement method with x-ray radiography technique is
demonstrated by a diffusion experiment in an Al-Ni melt.
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.
Neutron radiography and tomography have been used for a time resolved in situ analysis and a 3D mapping of hydrogen diffusion in iron and steel. Samples were electrochemically charged with hydrogen and afterwards neutron transmission images were taken. Hydrogen diffusion coefficients in duplex stainless steel were determined at 623 K by measuring and comparing the sample's mean intensity with a hydrogen-free reference sample and subsequent normalisation to standards with known hydrogen content. In technical iron and in supermartensitic stainless steel the hydrogen distributions have been investigated. The radiographic images in iron show blisters, cracks and the distribution of molecular hydrogen inside cracks. The analysis of the diffusion behaviour of hydrogen out of a blister illustrates the capabilities of the method with respect to time and spatial resolution. The neutron tomography of supermartensitic tensile stressed samples illustrates the capability to visualise hydrogen distributions three-dimensionally. -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
On a utilisé la radiographie neutronique et la tomographie pour une analyse in situ à résolution temporelle et une cartographie 3D de la diffusion de l'hydrogène dans le fer et l'acier. On a chargé des échantillons par électrochimie avec de l'hydrogène et ensuite on a pris des images par émission de neutrons. On a déterminé les coefficients de diffusion de lhydrogène dans l'acier inoxydable duplex à 623 K en mesurant et en comparant l'intensité moyenne de l'échantillon avec un échantillon de référence sans hydrogène et une normalisation subséquente à des échantillons références à teneur connue en hydrogène. On a examiné la distribution d'hydrogène dans le fer technique et dans l'acier inoxydable supermartensitique. Les images radiographiques du fer montrent des soufflures, des fissures, et la distribution de l'hydrogène moléculaire à l'intérieur des fissures. L'analyse du comportement de diffusion de l'hydrogène hors d'une soufflure illustre les possibilités de la méthode par rapport à la résolution temporelle et spatiale. La tomographie neutronique des échantillons supermartensitiques chargés en traction illustre la capacité de visualiser les distributions d'hydrogène en trois dimensions.
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.
Neutron radiography (NR) is compared with the commonly used carrier gas hot extraction (CGHE) technique. We performed isothermal hydrogen effusion experiments at 623 K to study the mass transport kinetics. The investigated material was technical iron. The quantification of the hydrogen mass flow is done for NR by using concentration standards. The temporal hydrogen concentration evolution in the sample coincides well for both methods, i.e. NR and CGHE, and is in good agreement with literature. The advantages of the NR method are the non-destructive nature of measuring and the in-situ determination of hydrogen concentrations with high spatial and temporal resolution. Remaining hydrogen inside the sample can be identified directly by the NR method.
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.
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.
Neutron Bragg-edge imaging was applied for the visualization of a γ-Austenite to α'-martensite phase transformation. In the present study, a super martensitic
stainless steel sample was heated until complete austenitization and was subsequently cooled down to room temperature. The martensitic phase Transformation started at Ms = 190 °C. Using a monochromatic neutron beam with λ = 0.390 nm, the transmitted intensity was significantly reduced during cooling below Ms, since the emerging martensitic phase has a higher attenuation coefficient than the austenitic phase at this wavelength. The phase Transformation process was visualized by filming the transmission images from a scintillator screen with a CCD camera with a temporal resolution of 30 s and a spatial
resolution of 100 µm.
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.
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.
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.
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.
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 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.
Gravity has significant effects on alloy solidification, primarily due to thermosolutal convection and solid phase buoyancy. Since 2004, the European Space Agency has been supporting investigation of these effects by promoting in situ X-ray monitoring of the solidification of aluminium alloys on microgravity platforms, on earth, and in periodically varying g conditions. The first microgravity experiment-investigating foaming of liquid metals - was performed on board a sounding rocket, in 2008. In 2012 the first ever X-ray-monitored solidification of a fully dense metallic alloy in space was achieved: the focus was columnar solidification of an Al-Cu alloy. This was followed in 2015 by a similar experiment, investigating equiaxed solidification. Ground reference experiments were completed in all cases. In addition, experiments have been performed on board parabolic flights-where the effects of varying gravity have been studied. We review here the technical and scientific progress to date, and outline future perspectives.
We investigated the hydrogen distribution spatially and temporally in technical iron at room temperature. Samples were charged electrochemically and subsequently analysed by means of neutron radiography and tomography. The radiographic images allowed for a time-resolved analysis of hydrogen fluxes. The three-dimensional distribution of hydrogen measured by neutron tomography delivered valuable information for the damage analysis of hydrogen-induced cracks. For the first time hydrogen concentration gradients inside the material could be detect directly together with the cracks. The neutron radiography and tomography results were gained at the Research Reactor BER II of the HZB in Berlin.
We investigated hydrogen embrittlement and blistering in electrochemically hydrogen-charged technical iron samples at room temperature. Hydrogen-stimulated cracks and blisters and the corresponding hydrogen distributions were observed by neutron tomography. Cold neutrons were provided by the research reactor BER II to picture the sample with a spatial resolution in the reconstructed three-dimensional model of ~25 µm. We made the unique observation that cracks were filled with molecular hydrogen and that cracks were surrounded by a 50 µm wide zone with a high hydrogen concentration. The zone contains up to ten times more hydrogen than the bulk material. The hydrogen enriched zone can be ascribed to a region of increased local defect density. Hydrogen also accumulated at the sample surface having the highest concentration at blistered areas. The surfaces of the brittle fractured cracks showed micropores visualized by scanning electron microscopy. The micropores were located at grain boundaries and were surrounded by stress fields detected by electron backscattered diffraction. The cracks clearly originated from the micropores.
Neutron tomography has been applied to investigate the mechanism of hydrogen assisted cracking in technical iron and supermartensitic steel. Rectangular technical iron block samples showed blistering due to intense hydrogen charging and the tomographic method revealed in situ the spatial distribution of hydrogen and cracks. Hydrogen accumulated in a small region around cracks and the cracks are filled with hydrogen gas. Cracks close to the surface contained no hydrogen. Hydrogenous tensile test samples of supermartensitic steel were pulled until rupture and showed hydrogen accumulations at the notch base and in the plastically deformed region around the fracture surface.
Neutron imaging
(2016)
Neutron imaging methods are appropriate to investigate hydrogen distributions in several metallic systems. The large total neutron cross section of hydrogen compared to those of elements or isotopes, respectively, in usual structural materials like steels or zirconium alloys allows the detection even of small amounts of hydrogen in such materials. The dependence of the total macroscopic neutron cross section of the sample or component on the hydrogen concentration can be determined experimentally by means of calibration specimens with known hydrogen concentrations. Such a calibration allows for a full quantitative determination of the local hydrogen concentration with a spatial resolution better than 20 μm. Because neutron radiography is fast and non-destructive, in situ investigations of time-dependent processes like hydrogen absorption and release or hydrogen bulk diffusion can be performed. This chapter gives an introduction into the main neutron imaging methods, radiography and tomography, and gives as examples results of neutron imaging investigations of hydrogen in different steels and in zirconium alloys,respectively.
MSL compatible isothermal furnace insert for high temperature shear-cell diffusion experiments
(2011)
For long-time diffusion experiments shear-cell techniques offer more favourable terms than the traditional long capillary techniques. Here, we present a further developed shear-cell that enables the measurement of diffusion coefficients up to temperatures of 1600 °C. Hence, diffusion experiments can be carried out at temperatures not accessible until now by conventional capillary or shear-cell techniques. The modified shear-cell, which can contain up to six samples of a total length of 90mm and a diameter of 1.5 mm, is built of 30 shear discs of 3mm thickness each. It is operated in an isothermal furnace insert which can be accommodated in the Materials Science Laboratory of the International Space Station. This provides the opportunity that the shear-cell can be applied to microgravity and to ground-based experiments, respectively. The heater insert with an overall length of 518mm and a diameter of 210mm consists of four heating zones with a total power of 3.5 kW. Temperature homogeneity along the graphite sample compartment is better than 2K at 1600°C. Details of the new design are discussed and results of first successfully performed heating and shearing cycles are presented.
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.
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.
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.
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 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 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.
The purpose of the present study was to show the feasibility of measuring hydrogen effusion in austenitic stainless steel (1.4301) using neutron radiography at the facility ANTARES of the research reactor FRM II of the Technische Universität München. This method is appropriate to measure in-situ hydrogen effusion for hydrogen concentrations as small as 20 ppmH. Experiments were carried out in the temperature range from room temperature up to 533 K. The measurement principle is based on the parallel comparison of electrochemically hydrogen charged specimen with hydrogen-free reference specimen at the same temperature. This allows the determination of the hydrogen concentration in the specimens as a function of time and temperature. Separate hot carrier gas extraction experiments using the same temperature–time profiles as the radiography experiments have been used to calibrate the grey values of the neutron transmission images into hydrogen concentrations. It can be stated that the hydrogen effusion correlates with the specimen temperature.
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.
In situ crack detection in the mushy zone and the solid weld of a gas tungsten arc (GTA) weld using X-ray imaging during welding is a new research area for NDT inspection. Usually, NDT flaw detection is done after the complete solidification of the weld seam. In this paper, we present the use of real-time radiography with a minifocus X-ray source (YXLON X-ray tube Y.TU 225-D04) and a 75μm pixel size digital detector array (Dexela 1512) for the acquisition of 2D radiographic images by a sequence of exposures with time intervals of 80 ms for hot crack detection during single pass bead-on-plate GTA welding of 3 mm thick plates of aluminium alloy AlMgSi (6060). An analysis of the crack distribution in the weld sample is conducted from the acquired 2D radiographs and its corresponding 3D volumetric reconstruction achieved by linear coplanar digital laminography.
This in situ approach opens new possibilities in the field of hot crack research by having the direct information of both the crack initiation and growth and its correlation to the welding parameters.
A long list of criteria determining the hot cracking susceptibility already exists. A main influence on solidification cracking can result from the design of the welded construction, i.e. from the influence of external loads. Using the Controlled Tensile Weldability (CTW) test, an external load hot cracking test, the influence of constant pre-load and different extension rates on the solidification cracking behavior of GTA (Gas Tungsten Arc) welds in an austenitic (AISI 309) and a ferritic (AISI 441) steel were investigated. Compared to specimens welded allowing free shrinkage and welded with an applied constant tensile pre-load, the specimens welded during the application of increasing tensile load show solidification cracks. In the weld seams, a characteristic angle α between the predominantly columnar grains and the fusion line can be observed. Specimens showing solidification cracks show a significantly larger angle α compared to the crack-free specimens. Based on these observations, the characteristic angle α is proposed as a new hot cracking criterion.
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.
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.