Degradation von Werkstoffen
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Paper des Monats
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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.
The reduction of harmful emissions to the environment is one of the most urgent challenges of our time. To achieve this goal, it is inevitable to shift from using fossil fuels to renewable energy sources. Within this transition, hydrogen can play a key role serving as fuel in transportation and as means for energy storage.
The storage and transport of hydrogen using austenitic stainless steels as the infrastructure, as well as the use of these grades in hydrogen containing aggressive environments, remains problematic. The degradation of the mechanical properties and the possibility of phase transformation by ingress and accumulation of hydrogen are the main drawbacks. Advanced studies of the behaviour of hydrogen in austenite is necessary to fully understand the occurring damage processes. This knowledge is crucial for the safe use of components in industry and transportation facilities of hydrogen.
A powerful tool for depicting the distribution of hydrogen in steels, with high accuracy and resolution, is time-of-flight secondary ion mass spectrometry (ToF-SIMS). We here present a comprehensive research on the hydrogen degradation processes in AISI 304L based on electrochemical charging and subsequent ToF-SIMS experiments. To obtain furthermore information about the structural composition and cracking behaviour, electron-backscattered diffraction (EBSD) and scanning electron microscopy (SEM) were performed afterwards. All the gathered data was treated employing data fusion, thus creating a thorough portrait of hydrogen diffusion and its damaging effects in AISI 304L.
Specimens were charged with deuterium instead of hydrogen. This necessity stems from the difficulty to separate between artificially charged hydrogen and traces existing in the material or adsorbed from the rest gas in the analysis chamber. Similar diffusion and permeation behaviour, as well as solubility, allow nonetheless to draw onclusions from the experiments.
Hydrogen absorption and diffusion in T24 steel weld joints and effect on mechanical properties
(2018)
The present contribution summarizes results obtained from experiments with low-alloyed boiler steel grades T24 (CrMoV alloy), T22 (CrMo) and thermally simulated T24 HAZ microstructure and pure weld metal. The hydrogen absorption and diffusion behavior was investigated by permeation experiments and thermal desorption analysis (TDA). The results showed significant weld microstructure influence on diffusion by hydrogen trapping. Trapped hydrogen was determined in the T24 BM at temperatures up to 120 °C compared to 75 °C in the T22. In addition, the T24 HAZ showed decreased diffusion coefficients (at room temperature) of approximately one magnitude compared to the BM. For the mechanical properties, tensile tests were conducted with electrochemically hydrogen charged specimens and compared to results obtained from slow strain rate tests (SSRT) in high-temperature water up to 200 °C. The as-welded HAZ had remarkably increased susceptibility compared to the BM already at low hydrogen concentration of 1-2 ppm. The SSRT were conducted for free corrosion and acidic environment and confirmed this behavior at elevated temperatures for both HAZ microstructures of T24 and T22. Summarized, each weld microstructure has specific diffusion coefficients and shows different susceptibility to degradation of the mechanical properties, i.e. delayed hydrogen assisted cracking or stress corrosion cracking.
Hydrogen absorption and diffusion in T24 steel weld joints and effect on mechanical properties
(2018)
The present contribution summarizes results obtained from experiments with low-alloyed boiler steel grades T24 (CrMoV alloy), T22 (CrMo) and thermally simulated T24 HAZ microstructure and pure weld metal. The hydrogen absorption and diffusion behavior was investigated by permeation experiments and thermal desorption analysis (TDA). The results showed significant weld microstructure influence on diffusion by hydrogen trapping. Trapped hydrogen was determined in the T24 BM at temperatures up to 120 °C compared to 75 °C in the T22. In addition, the T24 HAZ showed decreased diffusion coefficients (at room temperature) of approximately one magnitude compared to the BM. For the mechanical properties, tensile tests were conducted with electrochemically hydrogen charged specimens and compared to results obtained from slow strain rate tests (SSRT) in high-temperature water up to 200 °C. The as-welded HAZ had remarkably increased susceptibility compared to the BM already at low hydrogen concentration of 1-2 ppm. The SSRT were conducted for free corrosion and acidic environment and confirmed this behavior at elevated temperatures for both HAZ microstructures of T24 and T22. Summarized, each weld microstructure has specific diffusion coefficients and shows different susceptibility to degradation of the mechanical properties, i.e. delayed hydrogen assisted cracking or stress corrosion cracking.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing im-portance in steel construction and civil engineering. However, weld processing of those steels is a major challenge. The susceptibility for degradation of mechanical properties of weld joints sig-nificantly increases in presence of hydrogen and can result in hydrogen assisted cracking (HAC). Generally, risk for HAC increases with increasing yield strength of HSLA steels. To min-imize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydro-gen amount and its distribution in the weld seam and (2) options to lower the amount of intro-duced hydrogen. Existing standards recommend heat treatment procedures (interpass tempera-ture or post weld heat treatment) to reduce the diffusible hydrogen concentration in weldments. In this context, different weld seam geometries should be considered. For HSLA steel fabrication weld processing with seam opening angles of 45° to 60° is typical. Modern weld technologies allow welding with seam opening angles of 30° - reduced welding time and costs. In the present study, the hydrogen distribution in multi-layer welds of a 960 MPa HSLA steel was analysed. Influence of different seam opening angles as well as heat input, interpass temperature and post weld heat treatments were investigated. The welded samples were quenched in ice water imme-diately after welding and subsequently stored in liquid nitrogen. After defined warming up, small specimens were machined from the weld seam by water jet cutting. The diffusible hydrogen concentration was measured by carrier gas hot extraction with coupled mass spectrometer. The results showed, that low heat input and post weld heat treatment procedures can lower hydrogen concentrations in welds. Furthermore, a gradient of the hydrogen concentration was identified with increasing weld pool depth. By varying the seam opening angles different hydrogen concen-trations were measured.
During the energy transformation from fossil fuels to renewable energy sources, the use of hydrogen as fuel and energy storage can play a key role. This presents new challenges to industry and the scientific community alike. The storage and transport of hydrogen, which is nowadays mainly realized by austenitic stainless steels, remains problematic, which is due to the degradation of mechanical properties and the possibility of phase transformation by hydrogen diffusion and accumulation. The development of materials and technologies requires a fundamental understanding of these degradation processes. Therefore, studying the behavior of hydrogen in austenitic steel contributes to an understanding of the damage processes, which is crucial for both life assessment and safe use of components in industry and transportation. As one of the few tools that is capable of depicting the distribution of hydrogen in steels, time-of-flight secondary ion mass spectrometry was conducted after electrochemical charging. To obtain further information about the structural composition and cracking behavior, electron-backscattered diffraction and scanning electron microscopy were performed. Gathered data of chemical composition and topography were treated employing data fusion, thus creating a comprehensive portrait of hydrogen-induced effects in the austenite grade AISI 304L. Specimens were electrochemically charged with deuterium instead of hydrogen. This arises from the difficulties to distinguish between artificially charged hydrogen and traces existing in the material or the rest gas in the analysis chamber. Similar diffusion and permeation behavior, as well as solubility, allow nonetheless to draw conclusions from the experiments.
Hydrogen determination in weld seams is standardized in ISO 3690. In accordance to this standard, a defined time for hydrogen collection has to be anticipated for different extraction temperatures. In other words, the temperature is the most important value that has to be monitored in addition to the aimed hydrogen determination.
The specimen geometry has influence on the real sample temperature during CGHE vs. the adjusted furnace temperature. This presentation gives a short summary on possible influences on the "correct" hydrogen determination temperature during carrier gas hot extraction (CGHE) using infrared radiation driven furnace. The main findings are: (1) specimen surface is important in terms of polished or oxidized condition, (2) specimen geometry is important for fast heating, (3) PID-values of control software are a considerable influence to accelerate the heating process depite thick specimens and (4) independent sample temperature determination before CGHE is strongly recommended.
In order to satisfy the growing requirements towards lightweight design and resource efficiency in modern steel constructions, e.g., mobile cranes and bridges, high-strength steels with typical yield strength ≥ 690 MPa are coming into use to an increasing extent. However, these steels require special treatment in welding. The susceptibility for degradation of the mechanical properties
in the presence of hydrogen increases significantly with increasing yield strength. In case of missing knowledge about how and the amount of hydrogen that is uptaken during welding, hydrogen-assisted cracking (HAC) can be a negative consequence.
Moreover, modern weld technology like the modified spray arc process enables welding of narrower weld seams. In this context,
a reduced number of weld beads, volume, and total heat input are technical and economical benefits. This work presents the influence of welding parameters on the diffusible hydrogen content in both (1) single-pass and (2) multi-layer welds. Different
hydrogen concentrations were detected by varied contact tube distance, wire feed speed, arc length, and varied arc type (transitional
arc and modified spray arc). The results show that all welding parameters have significant influence on the diffusible hydrogen concentration in the single-pass welds. By increasing the number of weld beads in case of multi-layer welding, the
hydrogen concentration has been reduced. Whereby, differences in hydrogen concentrations between both arc types are present.