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As onshore installation capacity is limited, the increase in the number of offshore wind turbines (OWT) is a major goal. In that connection, the OWTs continuously increase in size and weight and demand adequate foundations concepts like monopiles or tripods. These components are typically manufactured from welded mild steel plates with thickness up to 200 mm. The predominant welding technique is submerged arc welding (SAW). In accordance with the standards, the occurrence of hydrogen-assisted cracking is anticipated by either a minimum waiting time (MWT, before non-destructive testing of the welded joint is allowed) at ambient or a hydrogen removal heat treatment (HRHT) at elevated temperatures. The effectiveness of both can be estimated by calculation of the diffusion time, i.e., diffusion coefficients. In this study, these coefficients are obtained for the first time for a thick-walled S420G2+M offshore steel grade and its multi-layer SAW joint. The electrochemical permeation technique at ambient temperature is used for the determination of diffusion coefficients for both the base material and the weld metal. The coefficients are within a range of 1025 to 1024 mm2/s (whereas the weld metal had the lowest) and are used for an analytical and numerical calculation of the hydrogen diffusion and the related MWT. The results showed that long MWT can occur, which would be necessary to significantly decrease the hydrogen concentration. Weld metal diffusion coefficients at elevated temperatures were calculated from hydrogen desorption experiments by carrier gas hot extraction. They are within a range of 1023 mm2/s and used for the characterization of a HRHT dwell-time. The analytical calculation shows the same tendency of long necessary times also at elevated temperatures. That means the necessary time is strongly influenced by the considered plate thickness and the estimation of any MWT/HRHT via diffusion coefficients should be critically discussed.
The increasing application of supermartensitic steels for welded pipelines is an economical alternative to the hitherto used higher-alloyed materials in the North Sea oil and gas indus-try. Failure in such constructions must at any rate be excluded for economical and ecologi-cal reasons. The application of these steels for the transport of corrosive mixtures may, however, involve hydrogen pickup with subsequent hydrogen-assisted stress corrosion cracking. It is therefore necessary not only to assure the weldability, but particularly to have best possible knowledge of the service behaviour and of the failure risk. In order to ensure the transferability of test results to real joined components, innovative test methods are in-creasingly required to be incorporated into a closed test sequence. It will be demonstrated how it is possible to gain significant advantages from the direct comparison between ex-perimentally determined results from component weld tests on the one hand and material-specific data from small-scale tests on the other hand and numerical simulations. These data that have now been made available are of major importance for industrial applications and are considered to provide a sound basis for realistic lifetime assessments.
Energy dispersive synchrotron diffraction for in-situ analyses of hydrogen behavior in steels
(2013)
The extent of hydrogen embrittlement phenomenon is dependent on the interaction of hydrogen with the material i.e. diffusion behavior, lattice distortions, phase transformations and residual stresses developments. For the better understanding of the hydrogen interaction with the metal, possibilities of performing in-situ observations of the mentioned factors has to be accomplished. The EDDI (Energy Dispersive Diffraction) beamline at the BESSY II facility in Berlin provides this possibility and was used in order to investigate the aspects of this phenomenon in in-situ scale.
The energy dispersive method entails a synchrotron X-ray white beam with a wide energy range (20–120 keV) which allows a fixed position of the sample and the detector, and combined with the high photons flux it allows a very short measurement time for obtaining a full scale X-Ray Diffraction patterns.
In this contribution, the successful usage of high energy X-rays, applied for the first time for hydrogen behavior observations, provided the ability for evaluating in-situ the hydrogen diffusion behavior and the hydrogen interaction with stresses fields in the material.
Steel samples were charged with hydrogen and examined by Synchrotron radiation.
Two different methods were used in this research; the first is diffusible hydrogen concentration evaluation extracted from the lattice distortions, i.e. changes of the lattice parameter. The second is calculation of the residual stresses present in the sample and the interaction of hydrogen with them, using the known sin2ψ method.
High-energy synchrotron study of the stress-strain behavior of hydrogen-charged high strength steel
(2012)
In general, hydrogen assisted cracking is a result of a critical combination of local microstructure, mechanical load and hydrogen concentration. In that connection, welded microstructures of low-alloyed creep-resistant steels can show different hydrogen trapping kinetics. That influences the adsorbed hydrogen concentration as well as the diffusion itself in terms of moderate or strong trapping. A common approach to describe trapping is by the activation energy that is necessary to release hydrogen from a specific trap site. In the present study, T24 base material and weld metal were investigated. For that purpose, electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis(TDA) with linear heating using a mass spectrometer. The results showed a microstructure effect on hydrogen trapping kinetics at elevated temperatures. Additionally, it is necessary to monitor the specimen temperature. A comparison between idealized temperature profile and real specimen temperature showed that the calculated activation energy varied up to a factor of two. Thus, the assigned trap character(moderate or strong) changed. In case of high temperature peaks, this effect could be more important compared to the microstructure effect itself.
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.
In situ analysis of hydrogen behaviour in stainless steels by high energy synchrotron radiation
(2011)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metal matrix. The present contribution provides a report of first in situ investigations of hydrogen effects on the lattice of the austenite and the martensite phase in a supermartensitic stainless steel using hard synchrotron radiation for respective energy dispersion diffraction measurements. Lattice distortions, such as the planar spacing expansion, have been monitored during the complete hydrogen effusion process over 24 h and are directly correlated to the dissolved hydrogen in the metal matrix. Among other results, it turned out that hydrogen at the introduced concentration level causes a reversible lattice expansion and after hydrogen effusion the lattice nearly regained its original dimensions. Hydrogen much more significantly affects the austenite phase with respect to lattice expansion than the martensite phase, but no phase transformations occur during the hydrogen desorption process.
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.
Hydrogen-assisted cracking (HAC) represents a significant failure risk for (high strength) creep resistant low-alloyed steel components in fossil-fired power plant applications at temperatures of up to and above 200 °C. This particularly applies to respective start-up and shut-down processes associated with alternating service-conditions in terms of load flexible power plants. For quantitative determination of localized crack critical hydrogen concentrations, the temperature dependent hydrogen diffusion coefficients have to be determined as exactly as possible.
However, available literature provides a wide range of hydrogen diffusion coefficients for low alloyed steels with similar microstructures. Additionally, the available diffusion data seem not to be very reliable and their scatter increases with decreasing temperature. One reason is that the experimental boundary conditions can have a major impact on the determination of respective effective diffusion coefficients. Hence, the scope of this study is to evaluate the influence of the experimental boundary conditions on the derived diffusion coefficients. In addition, different methods for calculating diffusion coefficients are discussed. To elucidate such influences and to draw a line to practical application, the diffusion and trapping behavior in the creep resistant steel 7CrMoVTiB10 10 has been studied.
For such purpose, hydrogen charged specimens were isothermally degassed at different temperatures using carrier gas hot extraction (CGHE). Based on experimental data, a numerical model has been developed by which the hydrogen transport behavior and the respective hydrogen distribution during CGHE can be assessed.
It is demonstrated that the specimen heating rate has a large influence on the calculated diffusion coefficients under assumption of isothermal degassing which elsewhere has been underestimated in the assessment of diffusion data in creep-resistant steels. The numerical results suggest that calculation methods for diffusion coefficients are limited if compared to experimental results. It also turned out that the sample preparation time before CGHE can enormously influence determined diffusion coefficients. Consequently, non-homogeneous hydrogen concentration profiles have to be anticipated in the simulations to arrive at characteristic effusion curves consistent to respective CGHE experiments. In turn, validated diffusion coefficients are now available for the low-alloyed Cr-Mo-V steel which might be helpful to calculate appropriate hydrogen removal heat treatment procedures, for instance.
Literature provides a wide range of hydrogen diffusion coefficients for low alloyed steels used in power plants. In fact, experimental boundary conditions and calculation methods have influence on the determination of these coefficients. The diffusion and trapping behavior in creep-resistant steel 7CrMoVTiB10-10 has been studied. Based on experimental carrier gas hot extraction (CGHE) data, a numerical model has been developed to describe the hydrogen transport and respective hydrogen distribution at elevated temperatures. The numerical results suggest that common calculation methods for diffusion coefficients are limited for experimental data analysis. The sample preparation time before CGHE experiment influences the determined diffusion coefficients with the consequence that non-homogeneous hydrogen concentration profiles have to be considered in the simulations.
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.
Industry faces a growing demand for high-strength structural steels with yield strengths of up to 1,300 MPa in order to cope with increasingly higher strength requirements in engineering. Higher strength levels are achieved by a special coordinated production process and an adapted chemical composition. Nevertheless, disastrous damage cases with high-strength steels have occurred in the past. The sensitivity to mechanical property degradation by hydrogen increases dramatically with strength. This phenomenon leads to hydrogen-assisted cold cracking. T-joints with fillet welds made from one side with an included angle of 60° were examined for their cold cracking behavior. Based on the T-joint, a modified heat input, even interpass temperature, plate thickness, and length ones were examined. The diffusion behavior and the effectiveness of different post-weld heat treatments in joints were simulated. The results of post-weld heat treatments are illustrated in practical hydrogen removal heat treatment diagrams. It is noticed that the T-joint is subject to a very high risk of hydrogen-assisted cold cracking (HACC). Contrary to other joints, its most critical area for cracking is not the weld metal but the heat-affected zone surrounding area of the root pass. The simulation shows that HACC in the T-joint can only be avoided by applying a sufficient post-weld heat treatment.
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.
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.
Der Stahlbedarf in Deutschland wird maßgeblich neben dem Automobilsektor vom Maschinenbau und allgemeinen Bauwesen geprägt. In diesen Segmenten werden qualitativ hochwertige Stähle mit höchsten Ansprüchen an Festigkeit, Verformungsfähigkeit, schweißtechnische Verarbeitung und sicherheitsrelevante Aspekte gestellt. Wichtige Vertreter, welche diesen Ansprüchen gerecht werden, sind die heutigen modernen höherfesten FKB. Aus der Entwicklung dieser Stähle kristallisierten sich in den letzten Jahrzehnten verschiedene Legierungskonzepte und Herstellungsrouten heraus. Dem liegt neben essentiellen Eigenschaften, z.B. Streck- und Zugfestigkeit, noch weitere Anforderungen, bspw. Kaltumformbarkeit, Kerbschlagzähigkeit und Verschleißfestigkeit, zugrunde. Zunehmend werden im genormten Bereich mit Streckgrenzen bis 700 MPa neben den vergüteten Stählen (Q) auch thermomechanische Stähle (M) eingesetzt. Ein immerwährender paralleler Begleiter während der Stahlherstellung und -verarbeitung ist Wasserstoff.
Wasserstoff wird in den nächsten Jahren als Schlüsselelement für eine nachhaltige Energiewirtschaft angesehen. Aus heutiger Sicht ist Wasserstoff ein Hoffnungsträger für eine klimafreundliche Energiewirtschaft und zukunftsfähige Industrie. Forschung und Industrie arbeiten intensiv an der Erschließung und Weiterentwicklung des enormen Potentials, um eine höhere Nutzbarkeit zu erreichen. Die Gründe liegen zum einen darin, dass Wasserstoff als Brennstoff unproblematisch (Umweltverträglichkeit und Verfügbarkeit) ist und zum anderen ein hervorragender Energieträger ist.
Wasserstoff ist durch seine gebundene Form erst nach dem Lösen aus chemischen Verbindungen zugänglich. Dies geschieht für eine Nutzbarmachung in einer zukunftsfähigen Energiewirtschaft gezielt. Demgegenüber stehen Prozesse, wodurch Wasserstoff aus seiner chemischen Verbindung gelöst wird und aufgrund seiner Größe bzw. geringsten Atommasse von Werkstoffen aufgenommen wird. Damit verbunden interagiert der aufgenommene Wasserstoff mit dem Gefüge und kann zu einer negativen Beeinflussung der Eigenschaften des Werkstoffs führen.
Wasserstoff kann Degradationsprozesse in Stählen verursachen, die sich insbesondere auf die mechanischen Eigenschaften auswirken. Diese Mechanismen können wasserstoffunterstützte Risse in höherfesten Stählen während der Herstellung oder im industriellen Einsatz verursachen.
Elektrochemisch beladene Zugproben zeigen ein unterschiedliches Degradationsverhalten in ihren Eigenschaften. Die vorliegende Arbeit beschreibt die Wechselwirkungen zwischen Wasserstoff und Gitterdefekten in unterschiedlichen mikrolegierten Systemen und wärmebeeinflussten Zonen in den schweißbaren Feinkornbaustählen. Die Ergebnisse zeigen eine klare Abhängigkeit zwischen Mikrolegierung und Herstellungsprozess dieser Stahlsorten, respektive ihrer simulierten wärmebeeinflussten Bereiche.