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This work explores the feasibility of producing bead-on-plate welds of a CrCoNi medium entropy alloy and a CrMnFeCoNi high entropy alloy using electron beam welding. The alloys were welded in two states: one in an as-cold-rolled condition and the other in an annealed condition. In addition, the materials are welded with two different parameters. The FCC microstructure of the welds is investigated using scanning electron microscopy assisted by energy-dispersive X-ray spectroscopy and electron-backscattered diffraction. The impact of the weld on the microstructure is discussed. The heat-affected zone is negligible for the annealed condition of both medium and high entropy alloys since there is no driving force for recrystallisation and the exposure time to high temperature is insufficient for grain coarsening. The texture formed in the fusion zone is also discussed and compared to the texture in the base metal and the heat-affected zone. Although the grain growth along the (100) crystallographic direction is preferential in all cases, the crystallographic texture type differs from each weld. Higher hardness values are measured in the medium entropy alloy’s base metal and fusion zone than in the high entropy alloy.
This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multiprincipal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong <001> θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123} <634> S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.
Das Kapitel beginnt mit einer kurzen Einführung über die Korrosion (Wechselwirkung zwischen einem Metall, einer korrosiven Umgebung und der der jeweiligen Konstruktion). Im zweiten Abschnitt werden die wichtigsten Formen der wässrigen elektrochemischen Korrosion (Flächenkorrosion, galvanische, selektive und interkristalline Korrosion sowie Loch- und Spaltkorrosion) betrachtet. Darüber hinaus wird die elektrochemische Korrosion unter mechanischer Belastung betrachtet (Spannungsrisskorrosion, wasserstoffunterstützte Rissbildung, Korrosionsermüdung), sowie Sonderformen der Korrosion (Erosion, Fretting und mikrobiologisch induzierte Korrosion). Der dritte Abschnitt befasst sich mit der chemischen und Hochtemperaturkorrosion (Oxidation, Aufkohlung, Hochtemperatur-Wasserstoffangriff, Aufschwefelung, Nitrierung, Halogenierung). Zusätzlich enthält das Kapitel Maßnahmen zur Vermeidung der Korrosion.
The chapter starts with a brief introduction about corrosion, which is defined as the interdependency between a metal, a corrosive environment, and the respective component design. The second section introduces the most important forms of aqueous electrochemical corrosion (uniform corrosion, galvanic corrosion, selective and intergranular corrosion, and finally pitting and crevice corrosion in the case of passive layer forming metals). In addition, electrochemical corrosion under applied mechanical load is introduced (stress corrosion cracking, hydrogen-assisted cracking, corrosion fatigue), as well as special forms of corrosion (erosion, fretting, and microbiologically induced corrosion). The third section of this chapter introduces (mostly dry) chemical corrosion and high-temperature corrosion (oxidation, carburization, high-temperature hydrogen attack, sulfurization, nitriding, halogenation). As in the case of electrochemical corrosion, chemical corrosion can also be superimposed by mechanical loads. Finally, general facts on the testing of corrosion are introduced.
Aircraft main landing gear (MLG) components are commonly manufactured from low-alloyed, martensitic, ultra-high strength steels (UHSS) that have to be coated for corrosion protection, representing an expensive and environmentally harmful production step. To avoid already partly banned corrosion protection plating, the new high-alloyed UHSS, Ferrium S53 (UNS S10500), has been designed to replace lowalloyed legacy materials and has been subjected to a limited field test over five years. As with the legacy alloys, UNS S10500 has a fully hardened martensitic microstructure known to be susceptible to hydrogen assisted cracking, per se. Containing about 10 wt% Cr, steels such as S10500 are at the lower limit for corrosion resistant alloys. Similar to super-martensitic stainless steels used in the oil and gas industry, a common failure sequence in marine environments represents pitting and subsequent hydrogen assisted stress corrosion cracking (HASCC). For addressing such phenomena quantitively, as required for respective lifetime assessments of MLG components and systems, the tolerance of such materials dependent on the absorbed hydrogen concentration must be evaluated quantitatively. However, there is a lack of such valuable materials data, as well as of the fractographic behavior dependent on the hydrogen concentration that might be absorbed during HASCC. To provide an improved understanding of the hydrogen dependent mechanical and fractographic behavior, samples of the legacy AISI 4340 and the new S10500 MLG steels have electrochemically been hydrogen-saturated and subjected to tensile testing. In contrast to a previous study, this contribution for the first time focuses on materials that have been salvaged from real service used landing gear components. In this study, it has been demonstrated that the service-applied S10500 steel has not only a higher strength, but also an improved ductility in comparison to the legacy AISI 4340 steel after similar service durations that provides a higher tolerance against hydrogen concentrations that might be absorbed during potential pitting and HASCC in marine environments. In addition, it has been found that the absorbed hydrogen concentration significantly affects the fracture behavior. Interestingly, hardening of the hydrogen charged low-alloyed AISI 4340 steel changes the fracture topography from trans- toward intergranular, while hardening of the S10500 steel turned the fracture topography from inter- to transgranular at respectively high hydrogen concentrations.
Creep-resistant steels such as the 13CrMoV9-10, used in the construction of thick-walled pressure vessels, are most commonly submerged arc welded (SAW). These steels can develop stress relief cracks (SRC) if the mandatory post weld heat treatment (PWHT) is performed improperly. Current PWHT parameters, such as heating rate and holding time at a specific holding temperature, are based on both empirical experience and conventional free shrinking welding experiments to characterize the SRC-susceptibility of the weld. These cannot adequately depict the higher residual stresses caused by the structurally induced stiffness of the surrounding construction. This study discusses the development of a repeatable, precise, and time-efficient methodology to study the effects of different stress levels and heating rates on the SRC susceptibility of the coarse grain heat-affected zone (CGHAZ). For that purpose, samples were thermically treated to simulate a coarse grain heat-affected zone (CGHAZ) and subsequently exposed to representative levels of stress during the heating phase of a PWHT. The recorded stress and heating rate–dependent strains were mathematically analyzed via curve tracing/calculus to identify interdependent effects. This procedure facilitates the measurement of material characteristics such as carbide growth on grain boundaries at the µm-scale via an integrated value over the entire sample volume. The first and second derivatives show a slight, precipitate-dependent, increase in hardness of the sample, depending on the heating rate and applied stress. This new methodology generates an improved assessment of the SRC susceptibility of SAW microstructures of creep-resistant CrMoV steels.
Multi-Principal-Element or High-Entropy Alloys (MPEAs/HEAs) have gained increasing interest in the past two decades largely due to their outstanding properties such as superior mechanical strength and corrosion resistance. However, research studies on their processability are still scarce. This work assesses the effect of different machining conditions on the machinability of these novel alloys, with the objective of advancing the introduction of MPEA systems into industrial applications. The present study focuses on the experimental analysis of finish-milling conditions and their effects on the milling process and resulting surface finish of CoCrFeNi, Al0.3CoCrFeNi and Al0.3CoCrFeNiMo0.2 alloys fabricated via Spark Plasma Sintering. Ball-nose-end milling experiments have been carried out various milling parameters such as cutting speed, feed per cutting edge, and ultrasonic assistance. In situ measurements of cutting forces and temperature on the tool edge were performed during the experiments, and surface finish and tool wear were analyzed afterwards. The results exhibited decreasing cutting forces by means of low feed per cutting edge and reduced process temperatures at low cutting speed, with the use of ultrasonic-assisted milling. It was shown that the machinability of these modern alloys through conventional, as well as modern machining methods such as ultrasonic-assisted milling, is viable, and common theories in machining can be transferred to these novel MPEAs.
Metals that are exposed to high pressure hydrogen gas may undergo detrimental failure by embrittlement. Understanding the mechanisms and driving forces of hydrogen absorption on the surface of metals is crucial for avoiding hydrogen embrittlement. In this study, the effect of stress-enhanced gaseous hydrogen uptake in bulk metals is investigated in detail. For that purpose, a generalized form of Sievert's law is derived from thermodynamic potentials considering the effect of microstructural trapping sites and multiaxial stresses. This new equation is parametrized and verified using experimental data for carbon steels, which were charged under gaseous hydrogen atmosphere at pressures up to 1000 bar. The role of microstructural trapping sites on the parameter identification is critically discussed.
Effect of Tensile Loading and Temperature on the Hydrogen Solubility of Steels at High Gas Pressure
(2023)
The hydrogen solubility in ferritic and martensitic steels is affected by hydrostatic stress, pressure, and temperature. In general, compressive stresses decrease but tensile stresses increase the hydrogen solubility. This important aspect must be considered when qualifying materials for high‐pressure hydrogen applications (e.g., for pipelines or tanks) by using autoclave systems. In this work, a pressure equivalent for compensating the effect of compressive stresses on the hydrogen solubility inside of closed autoclaves is proposed to achieve solubilities that are equivalent to those in pipelines and tanks subjected to tensile stresses. Moreover, it is shown that the temperature effect becomes critical at low temperatures (e.g., under cryogenic conditions for storing liquid hydrogen). Trapping of hydrogen in the microstructure can increase the hydrogen solubility with decreasing temperature, having a solubility minimum at about room temperature. To demonstrate this effect, the generalized law of the hydrogen solubility is parameterized for different steels using measured contents of gaseous hydrogen. The constant parameter sets are verified and critically discussed with respect to the high‐pressure hydrogen experiments.
Die Studie gibt einen kurzen Überblick über die jetzige Bedeutung der Fügetechnik in Wasserstofftechnologien und über zukünftige Forschungsbedarfe für die Branche in den einzelnen Technologiefeldern Wasserstofferzeu-gung, -speicherung, -transport und -nutzung. Fügetechnologien haben dabei wesentliche Bedeutung für die er-folgreiche Umsetzung von technischen Komponenten der Wasserstofftechnologien. Die Schwerpunkte bzw. For-schungsbedarfe ergeben sich bspw. durch die Erstellung neuer Infrastruktur für Wasserstoffspeicherung und -transport sowie durch Umnutzung der bestehenden Erdgasinfrastruktur. Bei der Wasserstofferzeugung und -anwendung wird z.B. die Entwicklung effiziente Massenproduktionsmethoden von Elektrolyseuren und Brennstoff-zellen einen wichtigen Meilenstein bilden und laserbasierte Fügetechnologien sind hier zum Teil schon etabliert. Die additive Fertigung nimmt dabei eine Querschnittsposition ein und besitzt hohes Anwendungspotential für die Zukunft z.B. für die Fertigung von Komponenten in Gasturbinen. Aus den technischen Fragestellungen und For-schungsbedarfen ergeben sich zudem Herausforderungen für die notwendige Neu- und Weiterentwicklung von technischen Regelwerken und Normen und den Eingang in die Aus- und Weiterbildung von fügetechnischem Fachpersonal.
The avoidance of failures during the fabrication or operation of petrochemical reactors made of creep-resistant, low-alloy steels as 13CrMoV9-10 requires still research despite over 60 years of international investigations in the field of stress relief cracking. The quality of modern base materials and filler metals leads to the fact that previously known crack causes, such as impurities of S or P, recede into the background. Rather, the causes are increasingly to be found in the fabrication process. Investigations on the influence of heat control on the stresses in welded components and thus on the stress relief cracking sensitivity under realistic manufacturing conditions are not yet available. This work is subdivided in two parts. Part 1 of this study focused on the effect of heat control during submerged arc welding on the stresses. For this purpose, a testing facility was applied, which allows to observe the forces and moments accumulating during welding or heat treatment in a component-like specimen under shrinkage restraint. The stress acting in the specimen increases with higher preheat/interpass temperatures and higher heat input. During the heat treatment, the stresses are relieved. Nevertheless, cracks are formed already during heating. The total crack length correlates with the heat input.
Welding of 13CrMoV9-10 vanadium steel requires care due to an increased susceptibility to stress relief cracking during post weld heat treatment. Previous research into the crack formation in creep-resistant steels has focused on thermal and metallurgical factors; however, little knowledge has been gathered regarding the crack formation during post weld heat treatment considering real-life restraint conditions. This work is subdivided in two parts. Part I showed that an increasing heat input during submerged arc welding under restraint led to an increasing stress level in the joint prior to the post weld heat treatment. The magnitude of stress relief cracking observed in the heat-affected zone after the post weld heat treatment is affected by the heat input. In Part II of this work, the cracks and the associated microstructure which occurred under restraint were studied. The application of a Special acoustic emission analysis indicated that the cracks formed in a temperature range between 300 and 500 °C during the post weld heat treatment. The toughness in the heat-affected zone of the restrained welds was affected by the welding heat input.
Microstructural analyses of all specimens revealed accelerated aging due to precipitation of carbides during post weld heat treatment under restraint.
Offshore wind turbines are an important goal in national energy strategies worldwide. Foundation structures are manufactured from submerged arc welded (SAW) plates with thicknesses up to 200 mm. In that connection, high-strength steels like the S420G2+M are more and more applied offering the possibility for increased stability and load-bearing capacity of the foundations. These offshore steel grades can show a susceptibility for delayed hydrogen assisted cold cracking of the weld joints. For that purpose, a minimum waiting time (MWT) of up to 48 h (dependent on applied standards) is recommended before non-destructive testing is allowed and conducted. But this concept is based on older steel grades that have been used for three or more decades. Nowadays, the metallurgical improvements (clean steels, proper rolling, and heat treatment) of base materials and well as welding consumables must be anticipated. Hence, the MWT concept should be critically discussed as it is assumed to be very conservative. For that reason, the focus of this study was to investigate the diffusion behavior in S420G2+M steel and its multi-layer SAW joint. Electrochemical permeation experiments were carried at room temperature. Boundary conditions were anticipated in terms of using different sample thicknesses. From the experimental data, hydrogen diffusion coefficients and absorbed diffusible hydrogen concentrations had been calculated. It was shown that hydrogen diffusion in the base material is increased compared to the weld metal. In addition, the sample thickness had a significant on the calculated diffusion coefficients. The minimum and maximum diffusion coefficients had been used for numerical modelling of the hydrogen diffusion in the welding joint. It became clear that a MWT must be always regarded together with a critical initial diffusible hydrogen concentration for the evaluation of a possible delayed cracking as diffusion times were mostly > 48 h due to the thick plates.
In vielen Bereichen des modernen Stahlbaus kommen geschweißte Vollwandträger, wie I-Träger zum Einsatz. Im Gegensatz zu Walzprofilen können die Bauhöhe, die Querschnittsform und die Blechdicken an die Beanspruchung angepasst werden, wodurch sich wirtschaftliche Vorteile erzielen lassen. Dabei werden heutzutage immer häufiger hochfeste Feinkornbaustähle mit (Mindest-) Fließgrenzen bis 690 MPa eingesetzt. Zur verbesserten Ausnutzung des Tragfähigkeitspotenzials geschweißter Profile aus diesen Stählen existieren keine geeigneten Modelle zur Erfassung der schweißfertigungsbedingten Eigenspannungen an realen Komponenten. Diese Daten werden durch konservative Annahmen angenähert. Die potentielle Tragfähigkeit geschweißter Konstruktionen wird daher in der Regel unterschätzt. Die Eigenspannungsannahmen in DIN EN 1993 basieren auf starken Vereinfachungen, bspw. im Rahmen sog. „Ersatzstabnachweise“. Diese setzen die Proportionalität der Eigenspannungsamplituden zur Fließgrenze voraus. Vergleichsrechnungen mit einem parametrisierten Eigenspannungsmodell zeigten mögliche Abweichungen in den berechneten Tragfähigkeiten über mehrere Knicklinien. Die Festlegungen hinsichtlich der anzuwendenden Knicklinie beeinflussen jedoch die Blechdicken und somit in erheblichem Maße auch die Fertigungskosten.
Als Optimierungsstrategie wurden in diesem Forschungsprojekt deswegen unterschiedliche anwendungsorientierte Modellansätze entwickelt, welche mithilfe nichtlinearer Traglastberechnungen, die höhere Ausnutzung von Tragfähigkeitsreserven ermöglichen sollen.
Forschungsziel war somit primär die Entwicklung und Bereitstellung entsprechender Berechnungsansätze. Experimentell wurde hierzu der Eigenspannungszustand von konventionell geschweißten Strukturen am Beispiel der im Stahlbau besonders häufig verwendeten I-Träger aus zwei gängigen Baustählen, einem unlegierten Baustahl S355J2+N und einem Feinkornbaustahl S690QL, erfasst und durch numerische Schweißstruktursimulationen ergänzt. Anschließend erfolgte die Ableitung eines vereinfachten Berechnungsmodells zur weiteren Berücksichtigung in nichtlinearen Tragfähigkeitsberechnungen. Die Erkenntnisse sollen in einen entsprechenden Anwenderleitfaden, herausgegeben durch das Technical Committee 8 der Europäischen Konvention für Stahlbau, einfließen und diesbezüglich als Orientierung einer möglichen zukünftigen Anpassung der Knicklinien in DIN EN 1993-1-1 dienen.
Im Projekt wurden die genannten Stähle in zwei typischen Dickenkombinationen von 25/15 mm und 20/10 mm (Gurt- und Stegbleche) zu Großprobekörpern „I-Träger“ unter industrienahen Fertigungsbedingungen mittels Metall-Aktivgasschweißen gefügt. Diese erzeugten die typischen Eigenspannungen, die in solchen Trägern nach der Fertigung vorliegen. Während des Schweißens wurden die Temperaturfelder durch Thermoelemente aufgezeichnet. Die Eigenspannungen wurden nach dem Schweißen experimentell durch eine Kombination aus mechanischem Zerlegen (Zerlegemethode) und anschließender mobiler Röntgendiffraktometrie (XRD) festgestellt. Der Einfluss der industriell üblichen Brennschnittkantenvorbereitung der Bleche auf die finalen
Eigenspannungszustände wurde experimentell zusätzlich an kleinmaßstäblichen Proben untersucht. Metallographische Analysen (Schliffbilder und Härtemessungen) komplettierten das experimentelle Programm der FE 2 (BAM, Berlin).
Die Ergebnisse zeigten, dass beim mechanischen Zerlegen bereits ein Großteil der Eigenspannungen herausgelöst wurde. Eine Aussage zu den nahtnahen Zugeigenspannungen war allerdings erst durch die Überlagerung mit der XRD möglich. Dabei zeigte der S690QL überwiegend ein besseres Verhalten als der S355J2+N, welches sich unter anderem in abgesenkten Zugeigenspannungen im Verhältnis zur jeweiligen Fließgrenze äußerte. Die maximalen Eigenspannungen wurden dabei zumeist in der Wärmeeinflusszone (WEZ) der Schweißnaht gemessen, unabhängig von der eingebrachten Streckenenergie oder der Festigkeit des Grundwerkstoffs. Die baupraktische Annahme, dass die maximalen Eigenspannungen in Höhe der Fließgrenze vorliegen können, wurde nur in Einzelfällen und nur für den S355J2+N bestätigt. Die experimentellen Ergebnisse zeigten für die Druckeigenspannungen ebenfalls reduzierte relative Eigenspannungswerte des S690QL. Dies zeigt die notwendige weitere Optimierung bei der Wahl der Bewertungsansätze für Eigenspannungen auf die Tragfähigkeit.
Die erzeugten Messwerte dienten im weiteren Verlauf als Eingangswerte für die Modellentwicklung und die numerischen Simulationen an I-Trägern durch die FE 1 (LSH, Cottbus). Die numerischen Schweißsimulationen erfolgten in Simufact.Welding® für die Simulationsmodelle mit „klassischem“ Schweißsimulationsansatz sowie in Ansys® für vereinfachte Simulationen und Näherungsansätze. Die Modelle lieferten insgesamt vergleichbare Ergebnisse für die Längseigenspannungen mit teils jedoch erheblichen Unterschieden in den Rechenzeiten. Durch eine angepasste Vernetzung konnte die Rechenzeit in allen Fällen deutlich reduziert werden. Aufgrund der verwendeten Bauteilgrößen ist die Anwendung der Modellierung mit Volumenelementen allerdings meistens nicht praktikabel. Für typische Trägerschweißungen bieten stattdessen Querschnittsmodelle mit der Annahme eines verallgemeinerten ebenen Verzerrungszustands eine mögliche Alternative. Der Abgleich der Ergebnisse mit den Experimenten lieferte zusammenfassend überwiegend gute bis befriedigende Übereinstimmungen. Die experimentell festgestellten Tendenzen wurden dabei in allen Fällen richtig vorhergesagt.
Eine umfassende Bewertung der erzielbaren Genauigkeiten war bisher nicht möglich. Dies lag einerseits daran, dass die Datendichte der mit der durch die Zerlegung gewonnen Ergebnisse gering war. Anderseits lieferten die überlagerten Ergebnisse mit der XRD in Teilen, insbesondere in den interessierenden Druckbereichen, widersprüchliche Aussagen zum Eigenspannungseinfluss, was bisher nicht abschließend geklärt werden konnte. Darüber hinaus entstanden etwaige Abweichungen in den Simulationen auch durch die zugrunde gelegten Materialdaten selbst. Diese entsprachen Standarddatensätzen aus Materialbibliotheken. Die Skalierung dieser erfolgte gleichmäßig über den gesamten Temperaturbereich. Insbesondere für den S690QL existierten zu wenige belastbare Materialkennwerte.
Entsprechende Versuche konnten im Rahmen dieses Vorhabens nicht mit durchgeführt werden.
Dennoch konnten hinsichtlich eines vereinfachten Berechnungsvorgehens zur Berücksichtigung der Schweißeigenspannungen in Tragfähigkeitsberechnungen einige wichtige Erkenntnisse gewonnen werden. Auf Basis der in den numerischen Schweißsimulationsmodellen ermittelten plastischen Dehnungsmuster, auch „Eigenspannungsquellen“, wurde eine vereinfachte Simulationsvorgehensweise vorgeschlagen und verifiziert. Die dabei eingeführten Vereinfachungen wurden im Hinblick auf die Berechnung ausschließlich der plastischen Längsdehnungen bzw. der Längseigenspannungen gewählt. Die Anwendbarkeit ist daher auf ausreichend lange Schweißträger mit Längsnähten beschränkt. Einige Modellmodifikationen zur weiteren Vereinfachung bzw. Modellimplementierung in die strukturmechanische Berechnung an großen Komponenten wurden vorgestellt. Abschließend erfolgt die Einstufung in eine mögliche Knicklinie auf Basis eines mittels GMNIA (geometrisch und materiell nichtlineare Analyse mit Imperfektionen) Berechnungen simulierten Streubands der Tragfähigkeiten mittels Abaqus® und Ansys®. Die anfängliche Hypothese einer günstigeren Knicklinie eines S690QL im Vergleich zum S355J2+N wurde dabei rechnerisch bestätigt. Der experimentelle Nachweis mittels entsprechender Trägerfähigkeitsversuche ist allerdings noch ausstehend.
With the introduction of a hydrogen-based energy and national economy, safety-relevant components for hydrogen technologies are becoming increasingly important. Characteristic of hydrogen technologies are, for example, harsh environmental conditions such as cryogenic or high-pressure storage, corrosion issues in fuel cells and electrolyzers, turbines, and many more. Additive manufacturing of components is becoming increasingly important and irreplaceable for the production of complex technical systems. Using the case studies of burners for gas turbines and electrodes and membranes for polymer (PEMFC) and solid oxide (SOFC) fuel cells, this article shows the potential of additive manufacturing of components. At the same time, however, the challenge of considering divergent mechanical properties depending on the direction of assembly in a "hydrogen-compatible" manner is also highlighted. Finally, the challenges posed by additive manufacturing and hydrogen for materials testing are highlighted under scenarios that are as realistic as possible.
Dieses Dokument fasst den Projekfortschritt des BAM-Projektes "Entwicklung von Wärmenachbehandlungsstrategien zur Vermeidung von Spannungsrelaxationsrissen" im Rahmen des Fachausschusses 1 "Schweißmetallurgie" des DVS e.V. für das Projektjahr 2021 zusammen und stellt die wichtigsten Ergebnisse in Kurzform vor.
Bewertungsmethode zur Interaktion von Metallurgie und Reaktionskräften beim PWHT warmfester Stähle
(2022)
Die Präsentation fasst die Entwicklung einer Ersatzprüfmethode für sogenannte Stress-Relief-Cracks (SRC) bei dickwandigen und kriechfesten geschweißten Stahlkomponenten zusammen. Über die Kombination von gezielter mechanischer Beanspruchung unter hoher Temperaturbeanspruchung werden die Effekte des Post Weld Heat Treatments auf SRC realistisch erstmal auf Laborskala nachgebildet.
Joining processes for components in hydrogen technologies: Current need and future importance
(2022)
This presentation gives an overview on the importance of joining processes for component fabrication in hydrogen technologies. For that reason, the current need and future research and developement activites are highlighted for the three technological fields: hydrogen storage, transport and use (in terms of the emerging field of additive manufacturing). Finally, some remarks are given for necessary changes in the standardization.
Die Vortrag gibt einen Überblick, warum Kaltrissprüfung von geschweißten Stählen ständig weiterentwickelt werden muss. Am Beispiel des Implant- und Tekken-Tests werden die Einsatz- u. Anwendungsgrenzen bewertet. Zu berücksichtigen ist, dass beim Schweißen eine äußere mechan. Beanspruchung durch bauteilspezifische Steifigkeitsverhältnisse wirksam ist. Zusätzlich werden weiterführende Prüfverfahren zur Bestimmung der Wasserstoffkonzentration und -diffusion in Schweißnähten vorgestellt, wie die Trägergasheißextraktion (TGHE) für die Ermittlung der Wasserstoffkonzentration (ISO 3690) oder Hochtemperaturdiffusionskoeffizienten. Diese Werte sind für die schweißtechnische Praxis von großer Bedeutung, um Haltezeiten z.B. für das Wasserstoffarmglühen abzuleiten.
Martensitic 9 %-Cr steels like P91 and P92 show susceptibility to delayed hydrogen assisted cracking depending on their microstructure. In that connection, effective hydrogen diffusion coefficients are used to assess the possible time-delay. The small number of available diffusion coefficients varies already at room temperature by several orders of magnitude (mostly regarded as result of present microstructure). Especially P91 weld metal diffusion coefficients are rare so far. For that reason, electrochemical permeation experiments had been conducted using P92 base metal and P91 weld metal (in as-welded and heat-treated condition) with different thicknesses. The diffusion coefficients are calculated by two methods (time-lag and inflection point method) were performed. The results show that, despite of microstructural effects, the sample thickness must be considered as it influences the calculated diffusion coefficients. Finally, the comparison of calculated and measured hydrogen concentrations (determined by carrier gas hot extraction) enables the identification of realistic diffusion coefficients.
This presentation summarizes the latest results on the BAM-Themenfeld project SURDIA on processing of high-entropy alloys (HEAs) at BAM. At first, the influence of machining by ultrasonic-assisted milling on the surface integrity is presented. Second, the weld processing by Tungsten Inert Gas (TIG) welding is presented and the results of the Friction Stir Welding (FSW), which is conducted at BAM for the first time.
This study gives an overview on the important field of joining processes for component fabrication in hydrogen technologies. For that reason, the current need and future research and developement activites are highlighted for the different technological field of hydrogen generation, storage, transport and use. In addition, the emerging field of additive manufacturing is included. Finally, some remarks are given for necessary changes in the standardization and its challenges.
Martensitic 9 %-Cr steels like P91 and P92 show susceptibility to delayed hydrogen assisted cracking depending on their microstructure. In that connection, effective hydrogen diffusion coefficients are used to assess the possible time-delay. The small number of available diffusion coefficients varies already at room temperature by several orders of magnitude (mostly regarded as result of present microstructure). Especially P91 weld metal diffusion coefficients are rare so far. For that reason, electrochemical permeation experiments had been conducted using P92 base metal and P91 weld metal (in as-welded and heat-treated condition) with different thicknesses. The diffusion coefficients are calculated by two methods (time-lag and inflection point method) were performed. The results show that, despite of microstructural effects, the sample thickness must be considered as it influences the calculated diffusion coefficients. Finally, the comparison of calculated and measured hydrogen concentrations (determined by carrier gas hot extraction) enables the identification of realistic diffusion coefficients.
Wasserstoff erfüllt die zentrale Rolle für die Umwandlung der bisherigen fossil-basierten Energieerzeugung und -nutzung auf eine dekarbonisierte, nachhaltige Form. Dazu muss der Wasserstoff erzeugt, gespeichert, transportiert werden, bevor er wieder der Nutzung zugeführt wird. Hierzu sind entlang der gesamtem Prozesskette Wasserstofftechnologien notwendig, die einen sicheren Betrieb erfordern. Hierbei kommt dem schweißtechnischen Anlagen- und Behälterbau wesentliche Bedeutung zu, insbesondere (aber nicht ausschließlich) für Speicherung und Transport des Wasserstoffes. Der vorliegende Beitrag gibt einen kurzen Überblick, wo und wie die konventionelle Schweißtechnik hierzu wichtige Beiträge leistet. Die additive Fertigung, also das „Drucken“ von Bauteilen wird dabei zunehmend wichtiger, entlang der gesamtem Prozesskette der Wasserstofftechnologien. Gleichwohl darf nicht unterschätzt werden, dass auch wesentlicher Bedarf an der Erweiterung und teilweiser Neufassung von bestehenden Regel- und Normenwerken besteht.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa are gaining popularity in civil engineering and construction of heavy vehicles. With increasing yield strength, the susceptibility for degradation of the mechanical properties in presence of diffusible hydrogen, i.e. hydrogen-assisted cracking (HAC) generally increases. HAC is a result of the critical interaction of local microstructure, mechanical load and hydrogen concentration. In existing standards for welding of HSLA steels, recommendations (preheating, interpass temperature and hydrogen removal heat treatment) are given to limit the amount of introduced hydrogen during welding. The recommendations are based on investigations with conventional arc welding processes. In the past decade, modern weld technologies were developed to enable welding of narrower weld seams with V-grooves of 30°, e.g. the modified spray arc process. In that connection, a reduced number of weld runs and weld volume are important technical and, hence, economic benefits. In the present study, the hydrogen distribution in S960QL multi-layer welds with thickness of 20 mm was analyzed depending on heat control. The influence of different weld seam opening angles (grooves), heat input, interpass temperature and hydrogen removal procedures was investigated. The results show that weldments with narrow groove contained increased diffusible hydrogen amount. Reasons had been longer diffusion path and higher wire feeding rate compared to conventional welds with wider 60° Vgroove. Hydrogen concentration has been reduced by decreasing both the heat input and interpass temperature. Hydrogen free weldments were achieved via hydrogen removal heat treating at 250 °C for 5 h subsequently after welding. Regarding the strength of the investigated steel, it is recommended to conduct a heat treatment after welding. For the first time, hydrogen concentration gradients were experimentally determined across the weld seam thickness in HSLA multi-layer welds.
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.
Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in welded joints using a thermal conductivity device (TCD) for quantitative measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries and factors that additionally influence hydrogen determination. They are namely: specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PID-furnace controller as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up the reach the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by evaluation of the recorded data. Generally, independent temperature measurement with calibration specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
A common approach to describe hydrogen traps is by their activation energy that is necessary to release hydrogen from the trap. In the present study, Cr-Mo-V steel T24 (7CrMoVTiB10-10) base material and TIG weld metal were investigated. Electrochemically hydrogen charged specimens were analyzed by thermal desorption analysis (TDA) with different linear heating rates. The results show two different effects. At first, the microstructure effect on trapping is evident in terms of higher hydrogen concentrations in the weld metal and increased activation energy for hydrogen release. Secondly, it is necessary to monitor the real specimen temperature. A comparison between the adjusted heating rate and the real specimen temperature shows that the calculated activation energy varies by factor two. Thus, the trap character in case of the base material changes to irreversible at decreased temperature. Hence, the effect of the experimental procedure must be considered as well if evaluating TDA results.
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.
Hochentropie-Legierungen (HEAs) zeichnen sich durch einen Mischkristall-System aus mindestens fünf und Mittelentropie-Legierungen (MEAs) durch mindestens drei Hauptlegierungselemente aus, in äquiatomarer Zusammensetzung. Sie zeigen außergewöhnliche Anwendungseigenschaften, wie z.B. hohe Festigkeit, Duktilität oder Korrosionsbeständigkeit. Zukünftige HEA/MEA-Komponenten aufgrund ihrer Eigenschaften für wasserstoffhaltige Umgebungen (wie Behälter für kryogene oder Hochdruckspeicherung) von Interesse. Daher ist die Bewertung der Wasserstoffabsorption und die Diffusion in diesen Materialien von großer Bedeutung. Dazu wurden in unserer Studie eine CoCrFeMnNi-HEA und eine CoCrNi-MEA untersucht. Die Proben wurden elektrochemisch mit Wasserstoff beladen. Für die Ermittlung des Wasserstoffdiffusionsverhaltens wurde die thermische Desorptionsanalyse (TDA) mit unterschiedlichen Heizraten bis zu 0,250 K/s angewandt. Die nachfolgende Peakentfaltung der Signale führte zu Hochtemperatur-Desorptionsspitzen und Wasserstofftrapping auch über 280°C. Eine resultierende Gesamtwasserstoffkonzentration > 40 ppm wurde für den MEA ermittelt und > 100 ppm für den HEA. Dies deutet auf zwei wichtige Effekte hin:
(1) verzögerte Wasserstoffdiffusion und (2) eine beträchtliche Menge an getrapptem Wasserstoff auch bei hoher Temperatur. Beide Effekte können hinsichtlich einer wasserstoffunterstützten Rissbildung kritisch werden, dies erfordert jedoch weitere Untersuchungen. Zusätzlich erfolgte die Bestimmung des lokalen Volta-Potentials mittels hochauflösender Kelvin-Sonden-Kraft-Mikroskopie (SKPFM). Die ermittelten Scans zeigen einen bestimmten Einfluss der Wasserstoffbeladung auf die Potentiale.
9 %-Cr steel P91 is widely used in power plants due to the excellent creep-resistance. Components of this steel are typically welded and demand for careful welding fabrication, whereas a so-called post weld heat treatment (PWHT), must be conducted to increase the toughness and decrease the hardness of the martensitic as-welded (AW) microstructure. Before the PWHT, a hydrogen removal (or dehydrogenation) heat treatment is necessary as hardened AW martensitic microstructure is generally prone to delayed hydrogen assisted cracking (HAC). The microstructure and temperature dependent hydrogen diffusion is an important issue as it determines how long a potential crack-critical hydrogen concentration could remain in the microstructure. In this context, reliable hydrogen diffusion coefficients of P91 weld metal are rare. Hence, the diffusion behavior of P91 multi-layer weld metal was investigated in two different microstructure conditions: AW and further PWHT (760 °C for 4 h). Two different experimental techniques were used to cover a wide range of hydrogen diffusion temperatures: the electrochemical permeation technique (PT) at room temperature and the carrier gas hot extraction (CGHE) for a temperature range from 100 to 400 °C. From both techniques typical hydrogen diffusion coefficients were calculated and the corresponding hydrogen concentration was measured. It was ascertained that both heat treatment conditions show significant differences in hydrogen diffusivity. The biggest deviations were identified for room temperature. In this case, the AW condition shows significant hydrogen trapping and up to seven times lower diffusion coefficients. Additionally, PT investigations showed a preferred diffusion direction of hydrogen in the weld metal expressed by the diffusion coefficients and the permeability for both heat treatment conditions. The CGHE generally revealed lower diffusion coefficients for the AW microstructure up to 200 °C. In addition, the AW condition showed hydrogen concentrations up to 50 ml/100 g (considering electrochemical charging). Nonetheless, this hydrogen was not permanently (reversibly) trapped. Nonetheless, this temperature is approximately 100 °C below recommended dehydrogenation heat treatment (DHT). This has two main consequences: (I) in case of welding is interrupted or no DHT is conducted, a HAC susceptibility of hardened martensitic P91 weld metal cannot be excluded and (II) DHT can be conducted at temperatures around 200 °C below the recommended temperatures.
Hydrogen was once called “the versatile embrittler” [1], which summarizes very well the effect on reduction of ductility and/or toughness in technical alloys like steel. In that connection, welding is one of the most important component fabrication technologies. During welding, hydrogen can be transferred to the weld pool from manifold sources (like contaminations, residuals at the surface, etc.). As hydrogen embrittles a material, the safety of welded components with hydrogen is always a critical issue. Weld heat input causes additional changes in the microstructure like grain growth or partial dissolution of precipitates and many more. All these things influence the mechanical properties and also represent hydrogen traps. These traps decrease the hydrogen diffusion compared to the ideal lattice. The result can be so-called delayed hydrogen assisted cracking (HAC) of the weld joint due to the significantly decreased diffusivity by trapped hydrogen. This is often an underestimated risk as those cracks can appear in the weld joint even after some days!
It is essential to know about hydrogen ingress during welding and the microstructure specific hydrogen diffusion. Both are depended on weld parameter influence and the chemical composition of the base material and weld metal. For that purpose, gas analytic methods like solid-state carrier gas hot extraction (CGHE) are useful tools to: (1) identify detrimental hydrogen concentrations from weld joints, (2) binding energies from hydrogen traps by thermal desorption analysis or (3) high-temperature diffusion coefficients. Those values are extremely important for welding practice in terms of recommendations on realistic hydrogen removal heat treatment (HRHT) after welding. Considering the increasing use of “digital” experiments, the data is also needed for reliable numerical simulations of HAC process or HRHT-effectiveness.
The present contribution gives an overview on the influence of hydrogen on weld joints, the necessity, methods and standards for hydrogen determination (CGHE) with the aim of fabrication of safe welded and crack-free components.
[1] R. A. Oriani (1987), Corrosion 43(7):390-397. doi: 10.5006/1.3583875
Die Energiewirtschaft ist bereits jetzt in massivem Umbruch. Herausforderungen der Energiewende sind u.a. die Einbindung von regenerativen Energiequellen und deren Speicherung. Dieses stellt auch die Fügetechnik, als zentrale Produktionstechnologie für die Komponentenfertigung, vor neue Aufgaben. Der Vortrag gibt zu ausgewählten Themen einen kurzen Überblick.
Der Vortrag gibt einen kurzen Überblick über die Bedeutung der Fügetechnik in Wasserstofftechnologien und über zukünftige FuE-Bereiche. Fügetechnologien haben dabei wesentliche Bedeutung für die Umsetzung von technischen Komponenten der Wasserstofftechnologien. Forschungsschwerpunkte ergeben sich insbesondere für die Erzeugung und den Transport des Wasserstoffs.
Offshore wind turbines (OWT) are a key factor of the sustainable energy generation of tomorrow. The continuously increasing installation depths and weight of the OWTs require suitable foundation concepts like monopiles or tripods. Typically, mild steels like the S420ML are used with plate thicknesses up to several hundreds of mm causing high restraints in the weld joints. Due to the large plate thickness, submerged arc welding (SAW) with multiple wires is the state-of-the-art welding procedure. As a result of the very high stiffness of the construction, a certain susceptibility for time-delayed hydrogen-assisted cracking (HAC) may occur. The evaluation of crack susceptibility is very complex due to the component size and stiffness of real offshore structures. For this purpose, a near-component test geometry was developed to transfer the real stiffness conditions to laboratory (i.e., workshop) scale. The investigated mock-up, weighing 350 kg, comprised heavy plates (thickness 50 mm, seam length 1,000 m) joined by a 22-pass submerged arc weld. Additional stiffeners simulated the effect of high restraint or shrinkage restraint of the weld. Extreme scenarios of hydrogen absorption during welding were simulated via the use of welding fluxes in dry (HD < 5 ml/100g Fe) and moisture condition (HD > 15 ml/100g Fe). The residual stresses were determined by a robot X-ray diffractometer. Areas of critical tensile residual stress (at the level of the yield strength) were found in the weld metal and heat affected zone. To identify possible delayed cracking, the welds were tested by phased array ultrasonic testing (PAUT) after 48 h. Summarized, no significant occurrence of HAC was detected, indicating the high crack resistance of the welded joint, i.e., a suitable combination of base material, welding consumable and welding parameters.
The study provides an overview of the aspects of joining and its importance in manufacturing of components for the more and more important field of hydrogen as key factor for the energy transition to a decarburized future. To this end, the fundamentals of the technology fields of hydrogen production, storage, transport, and application are presented and the state of the art of manufacturing of components for hydrogen technologies by joining is summarized. Based on representative examples from practice, research and development, the importance of joining technology in hydrogen technologies is clearly highlighted and perspectives for the future are derived. From a macroeconomic perspective, the focal points, or trends of joining technologies here include: the erection of new infrastructure for hydrogen storage and transport, and the safe conversion of existing natural gas infrastructure and its challenges for welded materials. In addition, we show the problems that are anticipated with in-service repair welding of hydrogen pipelines. In hydrogen applications, the efficient mass production of fuel cells and electrolysers is becoming increasingly important. For that reason, the importance of additive manufacturing is highlighted. Finally, the challenges for technical regulations and standardization by using hydrogen are shown.
Multi-principal element alloys (MPEAs) are innovative materials that have attracted extensive research attention within the last decade. MPEAs are characterized by a solid solution of equiatomic metallic elements. Depending on the number of elements, they are also referred as high entropy alloys (HEAs with n ≥ 4 elements like CoCrFeMnNi) and medium-entropy alloys (MEA with n = 3 elements CoCrNi). Depending on the alloy concept, MPEAs show exceptional properties in terms of mechanical performance or corrosion resistance at extreme environments. In that connection, hydrogen and its challenges for the most metallic materials gets more and more important. MPEAs are candidate materials for the substitution of conventional materials like austenitic stainless steels e.g., at very high-pressure up to 1000 bar. Those pressures are typically reached in valves or compressors for refueling of tanks with operational pressure of 700 bar. So far, the susceptibility of HEA/MEAs to hydrogen assisted cracking (if any) and the especially the underlying hydrogen uptake and diffusion was not within the scientific scope and not investigated in detail yet. For that reason, we focused on the hydrogen absorption the characterization of the hydrogen diffusion and trapping at elevated temperatures in a CoCrFeMnNi-HEA (each element with 20 at.-%) and CoCrNi-MEA, each element with 33.3 at.-%). As reference grade, the commercially available austenitic stainless steel AISI 316L was investigated. High-pressure hydrogen charging was conducted at different pressures in autoclave environment with maximum value of 1,000 bar. Thermal desorption analysis (TDA) via carrier gas hot extraction with coupled mass spectrometry was used with a max. heating rate of 0.5 K/s up to 650 °C. The measured desorption spectra of the different samples were deconvoluted into a defined number of individual peaks. The individually calculated peak temperatures allowed the definition of activation energies for predominant trap sites in the respective materials as well as the percentage share of the totally absorbed hydrogen concentration. The results present for the first time the complex interaction of both MPEAs and high-pressure hydrogen charging. A deconvolution of four peaks was selected and a main desorption peak was identified the dominant hydrogen trap containing the biggest share of the absorbed hydrogen concentration. The chemical composition an austenitic phase of both MPEAs is responsible for delayed hydrogen diffusion and strong, but mostly reversible, trapping. The comparison with the 316L samples showed significantly higher activation energies in the MPEAs, whereas hydrogen was also trapped at very high extraction temperatures. The absorbed maximum hydrogen concentration at 1,000 bar was 130 ppm for the CoCrFeMnNi-HEA, 50 ppm for the CoCrNi-MEA and 80 ppm for the 316L. It is interesting that the CoCrFeMnNi-HEA has obviously a way higher trapping capability compared to the conventional austenitic 316L, which could be a major advantage in terms of resistance to hydrogen assisted cracking.
In the course of tomorrow's hydrogen-based energy transition, the construction of the corresponding infrastructure will play a central role. The majority of materials used to date are typically welded for component fabrication. In that context, steels are widely applied and can be prone to hydrogen embrittlement. This includes the classical delayed cold cracking during welding processing as well as embrittlement phenomena during operation. For the evaluation of any hydrogen effect on, for example, the mechanical properties of a welded metallic material, the hydrogen content must be precisely determined. In the case of welds, for example, according to ISO 3690, this is the isothermal carrier gas hot extraction (CGHE). CGHE is based on accelerated hydrogen degassing due to thermal activation of hydrogen at elevated temperatures. In addition to the pure quantification of hydrogen, thermal desorption analysis (TDA) with varied heating rates can be used to determine and evaluate the bonding state at microstructural defects in the material. For both techniques, experimental and measurement influences have to be considered, which have a great effect on the result. For CGHE, for example, ISO 3690 suggests different sample geometries as well as minimum extraction times. The present study summarizes results and experiences of numerous investigations with different sample temperatures and geometries (ISO 3690 type B and cylindrical TDA samples) regarding: the influence of the sample surface (polished/welded), measurement accuracies depending on the sample volume as well as the insufficient monitoring of the effect of the PI controller on the extraction temperature. In particular, a deviating extraction temperature to the set temperature, can significantly falsify the measurement results. Based on the results, methods are shown to quickly reach the desired extraction temperature without having to physically interfere with the measurement equipment. This serves to substantially improve the reliability of hydrogen measurement through increased signal stability and accelerated hydrogen desorption. In general, an independent temperature measurement with dummy samples for the selected heating procedure is advisable to exclude possible unwanted temperature influences already before the measurement. In addition (and way more important), the methods described can be transferred directly to industrial applications.
Der Vortrag gibt einen tiefen Überblick über die Bedeutung der Füge- und Schweißtechnik in Wasserstofftechnologien. Dazu gliedert sich der Vortrag in die Komplexe H2-Erzeugung, Speicherung, Transport, Anwendung auf und gibt jeweils repräsentative Industriebeispiele für den heutigen Anwendungsstand. Insbesondere werden hier Fertigungstechnologien für Brennstoffzellen vorgestellt, sowie Herausforderungen beim Reparaturschweißen von Wasserstoffpipelines. Zeitgleich wird in einem eigenen Kapitel die Bedeutung der additiven Fertigung ebenso erläutert, wie die aktuellen und umfassenden Tätigkeiten auf dem Gebiet der Normung zu H2-Technologien.
Die vorliegende Präsentation fasst die Ergebnisse von drei laufenden bzw. beendeten AiF/IGF-Projekten zusammen, die über die Forschungsvereinigung Stahlanwendung FOSTA e.V. an der BAM bearbeitet wurden zum Thema: Bewertung und Vermeidung von schweißverarbeitungsbedingten Rissen in hochfesten Baustählen.
Im Zuge der wasserstoffbasierten Energiewirtschaft von morgen, kommt der Errichtung der dementsprechenden Infrastruktur die zentrale Rolle zu. Der überwiegende Teil bisher eingesetzter und meist geschweißter Werkstoffe, wie Stähle, kann unter bestimmten Bedingungen zur Wasserstoffversprödung neigen. Dies umfasst die klassische verzögerte Kaltrissbildung während der Schweißverarbeitung als auch Versprödungsphänomene während des Betriebs. Für die Bewertung eines jeden Wasserstoffeffektes auf bspw. die mechanischen Eigenschaften eines geschweißten metallischen Werkstoffes, muss der Wasserstoffgehalt präzise bestimmt werden, Im Fall von Schweißnähten ist die beispielsweise nach der ISO 3690, die klassische isotherme Trägergas-Heißextraktion (TGHE). Die TGHE basiert dabei auf der beschleunigten Wasserstoffentgasung aufgrund der thermischen Aktivierung des Wasserstoffs bei erhöhten Temperaturen. Neben der reinen Quantifizierung des Wasserstoffs, kann über die thermische Desorptionsanalyse (TDA) mit variierten Heizraten, der Bindungszustand an mikrostrukturellen Defekten im Material festgestellt und bewertet werden. Für beide Techniken sind experimentelle und messtechnische Einflüsse zu beachten, die großen Effekt auf das Ergebnis haben. Für die TGHE schlägt die ISO 3690 schlägt beispielswiese verschiedene Probengeometrien sowie Mindestextraktionszeiten vor. Die vorliegende Studie fasst Ergebnisse und Erfahrungen zahlreicher Untersuchungen an der Bundesanstalt für Materialforschung und -prüfung (BAM) mit unterschiedlichen Probentemperaturen und Geometrien (ISO 3690 Typ B und zylindrische TDA-Proben) zusammen hinsichtlich: Einfluss der Probenoberfläche (poliert/geschweißt), Messgenauigkeiten in Abhängigkeit des Probenvolumens sowie die unzureichende Überwachung des Effektes des PI-Reglers auf die Extraktionstemperatur. Insbesondere eine abweichende Extraktionstemperatur zur eingestellten Temperatur, kann die Messergebnisse wesentlich verfälschen. Basierend auf den Ergebnissen werden Methoden aufgezeigt, um schnell die gewünschte Extraktionstemperatur zu erreichen, ohne physisch in das Messequipment eingreifen zu müssen. Dies dient der wesentlichen Verbesserung der Zuverlässigkeit der Wasserstoffmessung durch erhöhte Signalstabilität und beschleunigte Wasserstoffdesorption. Im Allgemeinen ist eine unabhängige Temperaturmessung mit Dummy-Proben für die gewählte Heizprozedur angeraten, um mögliche unerwünschte Temperatureinflüsse bereits vor der Messung auszuschließen. Die beschriebenen Methoden können ohne großen Aufwand direkt in die industrielle Anwendung überführt werden.
Die Wasserstoffinfrastruktur ist eine zentraler Bestandteil der erfolgreichen Umsetzung der wasserstoffbasierten Energiewende. Dabei bilden Ferngasleitungen quasi das "Rückgrat" der Transportinfrastruktur großer Gasmengen. Dabei müssen unter Umständen Reparaturen an den Pipelines durchgeführt werden. Dabei ist in der Erdgasinfrastruktur das Schweißen an in Betrieb befindlichen (d.h. von Gas durchflossenen) Pipelines Stand der Technik. Es ist jedoch vollkommen offen, inwieweit diese Technik auf die reine Wasserstoffpipelines übertragbar sind. Hierzu leistet das Projekt H2SuD wichtige Beiträge zur Aufklärung des Einfluss eines Reparaturschweißprozesses auf die Wasserstoffaufnahme und Degradation im Betrieb befindlicher H2 Gasleitungen.
Der Vortrag gibt einen kurzen Überblick über die Bedeutung der Fügetechnik in Wasserstofftechnologien und über zukünftige Herausforderungen. Fügetechnologien, insbesondere die Schweißtechnik, haben dabei wesentliche Bedeutung für die Umsetzung von technischen Komponenten der Wasserstofftechnologien. Forschungsschwerpunkte ergeben sich insbesondere für die Erzeugung und den Transport des Wasserstoffs.
This study gives an overview on the important field of joining processes for component fabrication in hydrogen technologies. The current main issues and future perspectives are highlighted for the different technological field of hydrogen generation, storage, transport and use. In addition, the emerging field of additive manufacturing is included. Some remarks are given for standardization and regulations.
Large scale hydrogen assisted cracking test for thick walled SAW joints for offshore applications
(2023)
Offshore wind turbines continuously increase in size and weight and demand adequate offshore foundations concepts like monopiles, tripods or jackets. These components are typically submerged arc welded (SAW) high-strength thick steel plates like the S420ML. During welding, the occurrence of delayed hydrogen assisted cracking (HAC) must be anticipated. HAC is a critical combination of the local hydrogen concentration within a susceptible microstructure under certain mechanical load, i.e., the occurring (welding) residual stresses. The welding sequence of the thick-walled plates complicate the residual stress distribution due to the necessary repeated thermal cycling, i.e., welding seam / layer deposition to fill the joint. For that purpose, SAW with two-wire-technique was used to weld a specially designed and prototype-like mock-up of a real component with a thickness of 50 mm, filled with 22 passes and a seam length of 1,000 mm. Additional welded stiffeners simu-lated the effect of a high restraint, to stimulate critical HAC conditions. The necessity of a minimum waiting time (MWT) before the NDT can be conducted (to exclude HAC) was critically verified by the application of ultrasonic testing of the welded joint at different time-steps of the NDT of 48 h after welding. The residual stresses were determined by a robot XRD goniometer. Tensile residual stresses up to the yield limit are found both in the weld metal and in the heat-affected zone. Numerical modelling allowed the qualitative estimation of the hydrogen diffusion in the weld. A remarkable HAC occurrence was not identified and proves both, a certain resistance of the weld joint to HAC and the (questionable) duration of the MWT.
In the course of tomorrow's hydrogen-based energy transition, the construction of the corresponding infrastructure will play a central role. In that context, large diameter long-distance transmission pipelines for hydrogen will be the backbone in the European Union with service pressures from 70 to 90 bar (e.g., de-pending on national regulations). It is a major goal to repurposing the existing natural gas (NG) infrastructure despite the necessity of new pipelines. From that point of view repair welding or further welding of branch pipe etc. can be necessary during in-service, i.e., permanent flow of pressurized hydrogen. The reason is that a shut-down of large diameter pipelines is not easy or sometimes merely impossible. At the moment, it is entirely open if current repair welding procedures for NG pipe-lines can be transferred to pure hydrogen pipelines. For that reason, a collaborative project between BAM, DVGW (German Association for Gas and Water Professions) and a large number of gas grid operators, pipeline manufacturers and construction companies was initiated in 2023 to answer questions on: (1) How many hydrogen is additionally absorbed during the preheating and maintaining at interpass temperature under remaining operational pressures? (2) Is the hydrogen concentration sufficient to reach a critical condition? (3)Which material and weld microstructure are the most susceptible? (4) Is there a difference in the repair welding behavior of NG pipelines with materials in “used” condition? (5) Which welding parameters and joint dimensions must be ensured for safe repair welding? The final aim of this project is the publication of a recommended practice for repair welding of in-service hydrogen pipelines. For that reason, the present study gives an overview on: (A) current practice in repair welding of in-service pipelines and (b) plans for hydrogen pipelines and first results of international research projects.
This contribution briefly summarizes the standardization activities in accordance with the "Nationale Roadmap Wasserstofftechnologien" and presents selected results on the activities in Germany with scope on hydrogen transport in pipelines. The talk was given during a panel discussion to set-up a steering committee for standardization for hydrogen pipelines and welding , coordinated by the International Institute of Welding.