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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.
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.
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.