Filtern
Dokumenttyp
- Vortrag (40)
- Zeitschriftenartikel (25)
- Beitrag zu einem Tagungsband (20)
- Buchkapitel (2)
- Forschungsbericht (2)
- Posterpräsentation (1)
Schlagworte
- Hydrogen (25)
- Welding (23)
- Wasserstoff (18)
- Schweißen (13)
- Diffusion (12)
- Creep-resistant steel (9)
- High-entropy alloy (9)
- Fügetechnik (8)
- Mechanical properties (7)
- Forschung (6)
Organisationseinheit der BAM
- 9.0 Abteilungsleitung und andere (90) (entfernen)
Paper des Monats
- ja (6)
Eingeladener Vortrag
- nein (40)
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.
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.
High-entropy alloys (HEAs) and compositionally complex alloys (CCAs) represent new classes of materials containing five or more alloying elements (concentration of each element ranging from 5 to 35 at. %). In the present study, HEAs are defined as single-phase solid solutions; CCAs contain at least two phases. The alloy concept of HEAs/CCAs is fundamentally different from most conventional alloys and promises interesting properties for industrial applications (e.g., to overcome the strength-ductility trade-off). To date, little attention has been paid to the weldability of HEAs/CCAs encompassing effects on the welding metallurgy. It remains open whether welding of HEAs/CCAs may lead to the formation of brittle intermetallics and promote elemental segregation at crystalline defects. The effect on the weld joint properties (strength, corrosion resistance) must be investigated. The weld metal and heat-affected zone in conventional alloys are characterized by non-equilibrium microstructural evolutions that most probably occur in HEAs/CCAs. The corresponding weldability has not yet been studied in detail in the literature, and the existing information is not documented in a comprehensive way. Therefore, this study summarizes the most important results on the welding of HEAs/CCAs and their weld joint properties, classified by HEA/CCA type (focused on CoCrFeMnNi and AlxCoCrCuyFeNi system) and welding process.
HEAs represent a relatively new class of materials. The the alloy concept is fundamentally different from the most conventional materials and alloys that are used today. Recently, the focus of HEA designs is more application-based. For that purpose, the elements of interest are carefully selected and multiple phases as well as micro-structures are deliberately adjusted. Currently, only limited attention has been paid to weldability of HEA. This encompasses possible effects on metallurgy and its influence on the desired properties. It remains open if welding causes e.g. considerable number of intermetallic phases or segregations and their effect on weld joint properties. For that reason, the scope of this study is to summarize already available studies on welding of HEAs with respect to the HEA-type, the applied welding process and its influence on the weld joint properties.
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.
Wasserstoff kann eine Rissbildung u.U. auch noch nach Tagen in der Schweißnaht bewirken. Dabei stellen höherfeste Stähle etablierte Kaltrissprüftests vor Herausforderungen. Generell nimmt die zur Rissvermeidung tolerierbare Wasserstoffkonzentration mit zunehmender Festigkeit der Werkstoffe ab. Zudem verändern weiterentwickelte Schweißverfahren die Nahtgeometrie und Wärmeeinbringung und die Ausbildung risskritischer Mikrostrukturen (z.B. Wärmeeinflusszone). 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.
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.
Offshore wind turbines (OWT) are a major goal of the energy strategy of Germany encompassing the increase of the installed wind power. OWT components are manufactured from welded steel plates with thicknesses up to 200 mm. The underlying standards and technical recommendations for construction of OWTs encompass specifications of so-called minimum waiting time (MWT) before non-destructive testing of the weld joints is allowed. Reason is the increased risk of time-delayed hydrogen assisted cold cracking as hydrogen diffusion is very slow due to the very thick plates. The strict consideration of those long MWT up to 48 h during the construction of OWTs leads to significant financial burden (like disproportionately high costs for installer ships as well as storage problems (onshore)). In this study, weld joints made of S355 ML were examined in comparison with the offshore steel grade S460 G2+M. The aim was to optimize, i.e., reduce, the MWT before NDT considering varied heat input, hydrogen concentration and using self-restraint weld tests. This would significantly reduce the manufacturing time and costs of OWT construction. To quantify the necessary delay time until hydrogen-assisted cold cracks appear, acoustic emission analysis was applied directly after welding for at least 48 h.
Druckbehälter mit großen Wandstärken (250+ mm) aus hochwarmfesten, niedriglegierten Stählen werden hauptsächlich Mehrdraht-Unterpulver-(UP) geschweißt. Der damit verbundene hohe Energieeintrag führt unter anderem zu hohen lokalen Schweißeigenspannungen, welche bei unsachgemäßer Prozessführung der notwendigen Wärmenachbehandlung (PWHT) zu Spannungsrelaxationsrissen (SRR) führen. In den Bewertungskriterien zur Spannungsrelaxationsrissanfälligkeit finden weder die, mit klassischen, freischrumpfenden Schweißversuchen nicht abbildbaren, erhöhten Eigenspannungen aus der Schrumpfbehinderung der Bauteilsteifigkeit Beachtung, noch die damit einhergehenden metallurgischen Effekte wie bspw. das Ausscheidungswachstum während der PWHT und damit einhergehende Härteunterschiede an den Korngrenzen bzw. die vorzeitige Alterung durch Bildung differenter Sonderkarbide. Die Aufheizrate des PWHT als Einflussfaktor für die SRR-Bildung bleibt derzeit ebenfalls unbeachtet.
Daher wurde eine Versuchsmethodik entwickelt, die eine mögliche SRR-Anfälligkeit durch die Kopplung von thermischer und mechanischer Beanspruchung untersucht. Dazu wurden Proben mit unterschiedlichen Gefügen thermisch simuliert und während des PHWT repräsentativ mit sehr hohen Lasten/Eigenspannungen mechanisch beansprucht.
Dazu wurde an Kleinzugproben für das as-welded UP-Schweißgut und die thermisch simulierte GKZ eines 13CrMoV9-10 die Dehnung des Prüfbereichs bei variablen Querspannungen und Aufheizraten gemessen. Diese aufheizraten- und gefügeabhängige Längenänderung der Probe wurde durch mathematische Prozeduren analysiert. Damit war es möglich, singuläre werkstoffliche Effekte auf μm-Ebene (Ausscheidungswachstum) als summarische Längen- bzw. Volumenänderung per klassischer Kurvendiskussion zu beschreiben. Die erste und zweite Ableitung zeigten eine ausscheidungsabhängige Härtezunahme, welche von der Spannung und Aufheizrate im geringen Maß linear abhängig war. Gleichzeitig sind Volumenänderungen bei Bildung und Wachstum differenter Sonderkarbide bekannt, welche zusammen mit gefügespezifischen Untersuchungen und der neuen Versuchsmethodik zu einer verbesserten Beurteilung der SRR-Anfälligkeit von geschweißten CrMoV-Stählen beitragen sollen. Prinzipiell ist die vorgestellte Methodik jedoch werkstoff- und zustandsoffen, d.h. sowohl für Grundwerkstoff e als auch definierte Wärmebehandlungsbedingungen geeignet. Dies ermöglicht erstmals die Schaffung einer Transfergröße zwischen Labor und realen Schweißungen, unter Beibehaltung der Bauteilsteifigkeit, d.h. vereinfachte, aber realistische Eigenspannungsabbildung als Grundlage für weitergehende gefügespezifische metallurgische Effekte während des PWHT.
Medium and High Entropy Alloys (MEA/HEA) are recently developed material classes, providing manifold applications, e.g., due to extraordinary structural properties. In that connection, the machinability as important issue for the processing of these materials was not in the scientific focus. This study focusses on experimental analysis of milling process conditions including ultrasonic assisted milling (USAM) and their effects on the resulting surface integrity of equiatomic CoCrFeNi-MEA specimens. For that reason, milling parameters (cutting speed, feed per cutting edge) were systematically varied for both conventional milling and USAM. The surface integrity was analyzed in terms of topography, defects, and residual stresses. Especially USAM leads to a decrease of occurring cutting forces and, hence, to an improvement of the surface integrity. Beneficial effects were observed in terms of lower tensile residual stresses at high cutting speed and feed per cutting edge.
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. Limited data on room temperature diffusion coefficients reported in literature vary widely by several orders of magnitude (mostly attributed to variation in 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. From the results obtained, diffusion coefficients were calculated using to different methods, time-lag, and inflection point. Results show that, despite 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.
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.
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.
For a significant increase in the strength of high-strength fine-grained structural steels with a nominal yield strength ≥690 MPa, the addition of microalloying elements such as Nb and Ti is required. The standard specifications for the chemical composition of these steels (e.g., in EN 10025-6) often only give the manufacturer limit contents to achieve the defined properties. The effect of the alloying elements in the heat affected zone (HAZ) is sometimes completely contrary.
This makes it difficult to adequately predict the batch dependency regarding weldability and the load-bearing behaviour of the welded joint. Three different micro-alloyed steels of the grade S690QL were produced on a laboratory scale, focusing on different Nb and Ti contents. To investigate the tempering effect, these were gas metal arc welded in three layers. In addition to metallographic investigations of individual HAZ areas, thermodynamic phase calculations were carried out using Thermo-Calc, following variations in the chemical composition. This provides an understanding of phase transformation, precipitation growth, and dissolution during welding as a function of temperature and cooling conditions. The results show
a divergent metallurgical behaviour in the HAZ of the three different micro-alloyed steels. Thereby, the Ti micro-alloyed grade showed a strong softening of the HAZ in contrast to the Nb micro-alloyed grade. This can be attributed to a contrary precipitation behaviour during welding.
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.
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.
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.
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.
Hoch- und Mittelentropie-Legierungen (engl. High/Medium Entropy Alloys–HEA/MEA) sind relativ neue Werkstoffklassen. Im Gegensatz zu herkömmlichen Le-gierungen bestehen HEA aus fünf bzw. MEA aus drei oder vier Legierungselementen im äquiatomaren Gleichgewicht. Viele HEA weisen aufgrund hervorragender struktu-reller Eigenschaftskombinationen von sehr tiefen bis zu hohen Temperaturen enor-mes Anwendungspotential auf. Die Anwendung für reale Bauteile stellt jedoch die zentrale Frage der Eignung für die fügetechnische Verarbeitung. Dieser Aspekt findet in der weltweiten Materialforschung bisher kaum Beachtung. Anforderungen an die zuverlässige und sichere fügetechnische Verarbeitung dieser Werkstoffe sind von wesentlicher Bedeutung hinsichtlich wirtschaftlicher Bauteilfertigung für spätere po-tenzielle Anwendungsbereiche, bspw. in der Energietechnik. Das Rührreibschweißen (engl. Friction Stir Welding–FSW) stellt hierbei einen vielversprechenden Fügepro-zess dar, dessen Hauptvorteil in der Schweißprozesstemperatur unterhalb der Schmelztemperatur der zu fügenden Werkstoffe liegt. Dies bedingt die Vermeidung der Ausbildung von harten, versprödenden intermetallischen Phasen, wie sie z. B. beim konventionellen Schmelzschweißen von Eisen-Aluminium-Verbindungen auf-treten. Die vorliegende Arbeit stellt grundlegende Untersuchungen zur Schweißbar-keit einer CoCrFeMnNi-Legierung (HEA) und einer CoCrNi-Legierung (MEA) mit dem FSW-Prozess vor. Die Versuchsschweißungen werden mittels mikroskopischer Analysen hinsichtlich des Schweißnahtaufbaus sowie Schweißnahtfehlern charakte-risiert. FSW zeigt sich als grundlegend geeignet zur Verbindungsschweißung der HEA bzw. MEA. Anhand dieser Ergebnisse kann eine weitere Bewertung der Schweißbar-keit erfolgen.
The new alloying concept of multi-element systems with defined entropy (HEA—high-entropy alloy; MEA—medium-entropy alloy) is gaining increasing importance in materials research. Significantly improved properties or combinations of properties are shown by some HEA/MEA systems. Thus, primarily the production and resulting microstructures of HEA, as well as its properties, have been investigated so far. Furthermore, processing is a main issue in transferring HEA systems from the laboratory to real components. Since welding is the most important joining process for metals, it is crucial to investigate the influence of welding to guarantee component integrity. Welding leads to residual stresses, which significantly affect the component integrity. Hence, the focus of this study is the residual stress formation and distribution in a CoCrFeMnNi HEA and ternary CoCrNi MEA using two different welding processes: tungsten inert gas (TIG) welding and solid-state friction stir welding (FSW). As a pathway for the application of HEA in this investigation, for the first time, residual stress analyses in realistic near-component specimens were performed. The residual stresses were determined by X-ray diffraction (XRD) on the surfaces of top and root weld side. The results were correlated with the local welding microstructures. The results show that both FSW and TIG generate significant tensile residual stresses on the weld surfaces in, and transverse to, the welding direction. In the case of FSW of the CoCrFeMnNi HEA, the longitudinal residual stresses are in the range of the yield strength of approx. 260 MPa in the weld zone.
Submerged arc welded (SAW) components of creep-resistant low-alloyed Cr-Mo-V steels are used for thick-walled heavy petrochemical reactors (wall-thickness up to 475 mm) as well as employed in construction of modern high-efficient fossil fired power plants. These large components are accompanied by significant restraints during welding fabrication, especially at positions of different thicknesses like welding of nozzles. As a result, residual stresses occur, playing a domi-nant role concerning so-called stress relief cracking (SRC) typically during post weld heat treat-ment (PWHT). Besides specific metallurgical factors (like secondary hardening due to re-precipitation), high tensile residual stresses are a considerable influence factor on SRC. For the assessment of SRC susceptibility of certain materials mostly mechanical tests are applied which are isolated from the welding process. Conclusions regarding the influence of mechanical factors are rare so far. The present research follows an approach to reproduce loads, which occur during welding of real thick-walled components scaled to laboratory conditions by using tests designed on different measures. A large-scale slit specimen giving a high restraint in 3 dimensions by high stiffness was compared to a medium-scale multi-pass welding U-profile specimen showing a high degree of restraint in longitudinal direction and a small-scale TIG-re-melted specimen. The small-scale specimens were additionally subjected to mechanical bending to induce loads that are found during fabrication on the real-scale in heavy components. Results show for all three cases compa-rable high tensile residual stresses up to yield strength with high gradients in the weld metal and the heat affected zone. Those high tensile stresses can be significant for cracking during further PWHT.
As energy carrier of the future, green hydrogen can make a decisive contribution to a sustainable energy supply. Particularly important is the decarbonization of heavy industry. Therefore, a reliable supply of hydrogen must be guaranteed. A hydrogen pipeline grid can achieve this purpose. In Europe concrete plans for a hydrogen pipeline grid already exist, as the so called “European Hydrogen backbone”. As building a completely new pipeline grid is economically not justifiable, a dual-way strategy of repurposing natural gas (NG) pipelines and newly built pipelines is intended. Long distance pipeline grids require regular maintenance and repairs. For natural gas (NG) pipelines in-service welding is state of the art. Concepts such as hot-tapping (drilling into pressurized pipelines) and “stoppling” (sealing off pipeline sections after installing bypasses or pressure relieves) allow the repair of damaged pipeline sections or the installation of new branches without shutting the pipeline down, which in most cases isn’t possible from an economic standpoint. The EIGA 121/14 guideline already pointed out in 2014 that “a hydrogen hot-tap shall not be considered a routine procedure”. This shows that the determination of the transferability of these repair concepts to hydrogen pipelines is of utmost importance. Due to the degrading effects of hydrogen on mechanical proper-ties, commonly referred to as “hydrogen embrittlement” it is necessary to understand and investigate the occurring effects. In the special case of repair welding on in-service hydrogen pipelines an increased hydro-gen uptake due to a large heat input and microstructural changes in the material needs to be considered. Therefore, material degradation must be investigated to determine whether modifications of repair procedures are necessary to ensure a reliable and safe hydrogen transportation via pipelines. For this reason, this paper gives an overview on existing weld repair concepts and its risks and limitations in terms of hydrogen absorption and possible embrittlement effects when transferred to hydrogen pipelines. The complexity lies in a large variety of materials (low alloyed steels), pipeline diameters and corresponding welded joints. The material compatibility of materials in used (repurposed NG-pipelines) and new condition must be investigated. Particularly for repurposed/used pipelines the effect of a pre-matured/corroded inner surface and sometimes insufficient documentation of the material “history” needs to be considered. An overview on ongoing and completed R&D-projects with respect to repair concepts for hydrogen pipelines and hydrogen effects on pipeline materials is given. The focus hereby lies on possible methods of material testing and modeling. Its current difficulties, limits and possible solution will be discussed.
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.
Mikrolegierungselemente, wie Nb und Ti, sind für die signifikante Festigkeitssteigerung von vergüteten, hochfesten Feinkornbaustählen mit einer Nominalstreckgrenze ≥ 690 MPa unerlässlich. Normvorgaben zur chemischen Zusammensetzung geben dabei nur obere Grenzwerte für die Hersteller vor. Weiterhin wirken sich bereits kleine Abweichungen in der Legierungsroute teilweise drastisch auf die mechanischen Eigenschaften aus. Somit wird eine adäquate Vorhersage der Schweißeignung und der Integrität der Schweißverbindung aufgrund der variierenden Mikrogefüge erschwert bis unmöglich. Unerwünschte Nebeneffekte sind die mögliche Erweichung der Wärmeeinflusszone (WEZ) als auch der gegenteilige Effekt der Aufhärtung. Vor diesem Hintergrund werden erstmals systematisch die unterschiedlichen Mikrolegierungsrouten mit variierenden Ti und Nb-Gehalten an Versuchsschmelzen untersucht. Die Stahlgüte S690QL bildet dabei die Grundlage der chemischen Zusammensetzung sowie der entsprechenden Wärmebehandlung. Zur Untersuchung der jeweiligen Schweißeignung wurden Dreilagen-Schweißungen mittels moderner MAG-Hochleistungsschweißprozesse durchgeführt und kritische Gefügebereiche mit hoher Erweichung/Aufhärtung identifiziert. Der Fokus der analytischen Betrachtungen lag hier auf der Identifikation der Phasenumwandlungen beim Abkühlen und auf dem metallurgischen Ausscheidungsverhalten. Zusätzlich wurden isotherme und nicht-isotherme Phasenberechnungen mit der Software Thermo-Calc durchgeführt. Mechanisch-technologische Untersuchungen zur Kerbschlagzähigkeit mittels Kerbschlagbiegeversuchen durchgeführt wurden, bestätigen dabei die Ergebnisse der thermodynamischen Simulation bezüglich des Ausscheidungsverhaltens während der Temperatur-Zeit Schweißzyklen. Daraus lässt sich der Einfluss der Wärmeeinwirkung beim Schweißen auf die Gefügeausbildung in der WEZ und der korrespondierenden mechanischen Eigenschaften qualitativ beschreiben.
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.
Microalloying elements, such as Nb and Ti, are essential for the targeted mechanical strength of quenched and tempered, high-strength fine-grained structural steels with a nominal yield strength ≥ 690 MPa. Current specifications for chemical composition only provide upper limits for manufacturers. But even small deviations in the alloying route can have a drastic effect on the mechanical properties. Thus, an adequate prediction of the weldability and the integrity of the welded joint becomes difficult or even impossible due to the varying composition and, hence, the microstructures. Undesirable side effects are the possible softening of the heat-affected zone (HAZ) as well as the opposite effect of hardening. Against this background, different microalloying routes with varying Ti and Nb contents are systematically investigated for the first time on specially designed lab-cast alloys. The basis of each alloy route was the common S690QL in terms of both the chemical composition as well as the heat treatment. To investigate the weldability, three-layer welds were performed using metal active gas welding (MAG) and critical microstructural areas with high softening/hardening were identified. The scope was here on the identification of phase transformations during cooling and on the respective metallurgical precipitation behavior. Isothermal and non-isothermal phase calculations were performed using Thermo-Calc® and showed that the prediction of the non-equilibrium precipitation characteristics during welding is not trivial, especially for this relatively complex chemical composition. The mechanical properties of the welded joints were identified by both Charpy tests (toughness) and tensile tests (strain and strength). During the test, the local straining behavior of the welded joints, was identified using a digital image correlation (DIC) system, see Figure 1. Despite the generally good weldability of the materials, the results show a significant influence of the microalloying route as well as the welding heat input on the different precipitation kinetics.
Multiple principal element alloys (MPEA) encompass the well-known high entropy alloys (HEAs). MPEA/HEA represent a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.-%. This alloying concept thus differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, and the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. In the last 20 years, however, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on pro-cessing issues such as joining and welding processes. The weldability of MPEA has received very little atten-tion so far. Experience with dissimilar metal welds (DMWs) is completely lacking but is essential for the appli-cation of these materials in combination with conventional materials. The present study presents, comprehen-sive experimental results on the weldability of MPEA-DMWs. For that purpose, a Co20Cr20Fe20Mn20Ni20 HEA in cold-rolled and heat-treated condition was joined by means of tungsten inert gas welding (TIG) with the austenitic Cr-Ni steel AISI 304. The DMWs resulted in interesting mechanical properties. They were obtained by instrumented tensile tests as well as the local deformation in the weld area by using digital image correlation (DIC) technique. A significant softening in the heat-affected zone (HAZ) of the MPEAs as well as a slightly reduced tensile strength with a significant decrease of the elongation at fracture were found. The experiments provided proof in principle of the weldability of the MPEAs for DMWs with conventional materials that ensure a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
Multiple principal element alloys encompass the well-known high entropy alloys (HEA). The alloy system represents a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.%. Thus, this alloying concept differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. However, in the last 20 years, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on processing issues such as joining and welding processes. The weldability of HEAs has received very little attention so far. The experience with dissimilar metal welds is completely lacking but is essential for the application of these materials in combination with conventional materials. The present study presents comprehensive experimental results on the weldability of an equimolar CoCrFeMnNi-HEA in cold-rolled and heat-treated condition, which was joined by tungsten inert gas welding to an austenitic steel AISI 304. The mechanical properties of the dissimilar metal welds were characterized by cross-weld tensile samples, whereas the local deformation in the weld of the different welding zones was measured by digital image correlation. In accordance with the respective initial HEA condition (cold-rolled vs. heat-treated), the local strain behavior was divergent and influenced the global mechanical properties of both DMW types. Nonetheless, the experiments provided proof in principle of the weldability for dissimilar joints of the CoCrFeMnNi-HEA welded to conventional materials like austenitic stainless steels ensuring a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
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.
Komponenten aus dickwandigen, hochwarmfesten CrMoV-Stählen erfordern eine anspruchsvolle UP-Schweißverarbeitung. Dabei muss die notwendige Wärmenachbehandlung der Schweißnaht (PWHT) sicher beherrscht werden.
Hier kann u.U. Bauteilversagen infolge von Spannungsrelaxationsrissbildung (SRR) auftreten. Bislang erfolgt die Beurteilung der SRR-Neigung primär über Ersatzgrößen, wie temperaturbedingte Änderung der Härte und Duktilität.
Die Wirkung der konstruktiven Schrumpfbehinderung einer Schweißnaht auf die SRR ist bislang völlig unbekannt.
Hier setzt das Forschungsprojekt an, hinsichtlich der Gewinnung und Optimierung der Parameter für die PWHT unter realitätsnahen Bauteilsteif igkeiten (def inierte Einspannbedingung) mit dem Ziel der SRR-Vermeidung. Am Beispiel des niedriglegierten, hochwarmfesten Stahls 13CrMoV9-10 wurden UP- Schweißungen unter industriepraktischen Bedingungen freischrumpfend bzw. unter definierter Schrumpfbehinderung durchgeführt. Diese wurden mit unterschiedlichen PWHT-Szenarien (Variation der Auf heizrate und Endtemperatur) beaufschlagt. Die Ergebnisse des Projektes DVS 01.2261 / IGF 20171 N werden in diesem Beitrag vorgestellt, wie Aussagen zur SRR-Suszeptibilität in Abhängigkeit von Mikrostruktur und vorliegender mehrachsiger mechanischer Beanspruchung der Schweißnaht vor/während des PWHT. Für einen kritischen Zustand sind hohe Kräfte / Spannungen notwendig, die in freischrumpfenden Schweißversuchen u.U. nicht abbildbar sind. Die Berücksichtigung des mikrostrukturspezifischen Ausscheidungsverhaltens (in Schweißgut und Wärmeeinflusszone) unter realitätsnahen Einspannbedingungen ist für die Bewertung einer SRR-Suszeptibilität notwendig. Instrumentierte Schweißversuche unter definierter Schrumpfbehinderung ermöglichen hier das Monitoring der entstehenden Reaktionskräfte, -momente bzw. -spannungen. Ausgehend von diesem Referenzzustand erfolgte die Applikation der PWHT-Szenarien bzw. deren praktische Überprüfung. Zusätzlich wurde eine Ersatzprüfmethodik entwickelt, die die Betrachtung der mikrostrukturspezifischen SRR-Suszeptibilität unter Beibehaltung der äußeren Schrumpfbehinderung der Schweißnaht während des PWHT ermöglicht. Hierdurch wird die Bauteilübertragbarkeit ermöglicht und eine Transfergröße geschaffen, welche die Bauteilbewertung hinsichtlich der SRR-Neigung umfasst.
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.
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.
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.
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.
Multielement-Legierungen (MPEA – multiple pricipal element alloys) stellen eine neue Klasse von Werkstoffen dar, die aus mindestens drei äquiatomaren Legierungselementen mit jeweils einem Gehalt von 5–35 Atom-% bestehen. Somit unterscheidet sich das Legierungskonzept fundamental von konventionellen bspw. Stahl- oder Al-basierten Legierungen. In jüngster Zeit lag der Schwerpunkt auf dem Legierungs-Design der MPEA und der Überprüfung bzw. Erreichung der gewünschten Eigenschaften bzw. -kombinationen. Hierzu werden die egierungselemente gezielt ausgewählt und die Mikrostrukturen ein- und zum Teil auch mehrphasig eingestellt. Ziel sind hochinnovative MPEA mit angepassten Eigenschaften für die industrielle Anwendung. Dabei sollen insbesondere die bei konventionellen Legierungen wesentlichen Zielkonflikte, wie bspw. der Trade-off zwischen Festigkeit und Duktilität, überwunden werden. Darüber hinaus sind mechanische Eigenschaften bei sehr hohen sowie kryogenen Temperaturen bei gleichzeitig höchster Korrosionsbeständigkeit von hohem Interesse. Die Herstellung von Bauteilen erfordert dazu werkstoff- und beanspruchungsgerechte Füge- bzw. Schweißverfahren. Der Schweißeignung von MPEA wurde bis Ende 2021 nur äußert wenig Aufmerksamkeit zuteil. Die Auswirkungen durch Schweißprozesse auf die Metallurgie und die gewünschten Eigenschaften sind bisher kaum bekannt und limitieren die potenzielle Anwendbarkeit als Funktions- oder Konstruktionswerkstoffe.
In einem interdisziplinären BAM-Forschungsprojekt werden aktuell grundlegende Untersuchungen zur Schweißverarbeitung und resultierender Mikrostruktur-Anwendungseigenschafts-Beziehung verschiedener MPEA durchgeführt. Insbesondere Wärmeeinflusszone und Schweißgut sind geprägt durch eine Mikrostruktur, die aus Ungleichgewichtszuständen resultiert. Im Falle von MPEA wurde dies bisher nicht systematisch untersucht. Dazu erfolgte im Rahmen des Projektes eine umfassende Literaturauswertung zum Schweißen von MPEA und Zusammenfassung in einer umfassenden, systematischen Datenbasis. Aus diesem Grund sollen in der vorliegenden Studie die bisher bedeutsamsten Erkenntnisse zur Schweißbarkeit von MPEA systematisch nach MPEA-Typ und Schweißverfahren geordnet und mit eigenen aktuellen Forschungsergebnissen verglichen werden. Durch WIG- und Rührreibschweißversuche an den MPEA-Systemen CoCrNiFeMn bzw. CoCrNi konnten im Projektverlauf bereits wesentliche Fragestellungen beantwortet werden. Dazu zählen Auftreten bzw. Verhinderung intermetallischer Phasen/Seigerungen oder deren Auswirkung auf die Eigenschaften der Schweißverbindung (bspw. Festigkeit oder Korrosionsbeständigkeit).
For higher operational temperatures and pressures required in petrochemical plants, the modified 13CrMoV9-10 steel was developed providing high resistance against creep and compressed hydrogen. Extreme care during the welding procedure is necessary for this steel, attributed to low toughness, high strength in as-welded state, and increased susceptibility to stress relief cracking (SRC) during post-weld heat treatment (PWHT). Previous research of SRC in creep-resistant steels discussed mainly thermal and metallurgical factors. Few previous findings addressed the influences of welding procedure on crack formation during PWHT considering real-life manufacturing conditions. These investigations focus on effects of welding heat control on stresses during welding and subsequent PWHT operations close to realistic restraint and heat dissipation conditions using a special 3D testing facility, which was presented in parts I and II of this contribution. Part III addresses investigations on residual stress evolution affecting crack formation and discusses the transferability of results from large-scale testing to laboratory-scale. Experiments with test set-ups at different scales under diverse rigidity conditions and an assessment of the residual stresses of the weld-specimens using X-ray (surface near) and neutron diffraction analysis (bulk) were performed. This study aims to provide a way of investigating the SRC behaviour considering component-specific residual stresses via small-scale testing concepts instead of expensive weld mock-ups.
Influence of Surface Preparation on Cracking Phenomena in TIG-Welded High and Medium Entropy Alloys
(2022)
Multi-element systems with defined entropy (HEA—high entropy alloy or MEA—medium entropy alloy) are rather new material concepts that are becoming increasingly important in materials research and development. Some HEA systems show significantly improved properties or combinations of properties, e.g., the overcoming of the trade-off between high strength and ductility. Thus, the synthesis, the resulting microstructures, and properties of HEA have been primarily investigated so far. In addition, processing is crucial to achieve a transfer of potential HEA/MEA materials to real applications, e.g., highly stressed components. Since fusion welding is the most important joining process for metals, it is of vital importance to investigate the weldability of these materials. However, this has rarely been the subject of research to date. For that reason, in this work, the weldability depending on the surface preparation of a CoCrFeMnNi HEA and a CoCrNi MEA for TIG welding is investigated. The fusion welding of longer plates is described here for the first time for the CoCrNi alloy. The welds of both materials showed distinct formation of cracks in the heat affected Zone (HAZ). Optical and scanning electron microscopy analysis clearly confirmed an intergranular fracture topography. However, based on the results, the crack mechanism cannot be conclusively identified as either a liquid metal embrittlement (LME) or hot cracking-like liquid film separation.
High Entropy Alloys (HEAs) are a recent class of materials. In contrast to conventional alloys, HEAs consist of five alloying elements in equiatomic equilibrium. The high entropy effect is due, among other things, to the increased configuration entropy, which promotes solid solution formation. Many HEAs have enormous application potential due to excellent structural property combinations from very low to high temperatures. For the introduction of HEAs in real components, however, the question of the applicability of machining production technologies for component manufacture is of central importance. This has so far received Little attention in global materials research. Reliable and safe processing is essential for the demand of economical component production for potential areas of application, e.g. in power plant technology.
For metals, milling is the standard machining process. This article presents the results of machining analyses. It focuses on the surface integrity resulting from the milling process on a Co20Cr20Fe20Mn20Ni20-HEA. For this purpose, investigations were carried out using ball nose end milling tools for conventional milling process in comparison to an innovative hybrid process available at BAM Berlin, Ultrasonic-Assisted Milling (USAM). USAM promises a lower Degradation of the surface properties due to lower loads on the workpiece surface during machining.
For this purpose, basic milling parameters (cutting speed and tooth feed) were systematically varied and cutting forces were measured during the milling experiments. The subsequent Analysis of these forces allows an understanding of the mechanical loads acting on the tool and component surface. These loads cause topographical, mechanical and microstructural influences on the surface and consequently on the surface integrity. For their characterization, light and scanning electron microscopy were used, and the roughness and residual stresses via X-ray diffraction were measured. The results indicate significant advantages using USAM, especially due to reduced cutting forces compared to the conventional milling process. This causes lower mechanical loads on the tool and surface, combined with lower tensile residual stresses on and below the surface, and ultimately results in a significantly enhanced surface integrity.
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.
High- and medium-entropy alloys (HEAs) are a quite new class of materials. They have a high potential for applications from low to high temperatures due to the excellent combination of their structural properties.
Concerning their application as components; processing properties, such as machinability, have hardly been investigated so far. Hence, machinability analyses with a focus on the influence of the milling process and its basic parameters (cutting speed, feed per cutting edge) on the resulting surface integrity of specimens from an equiatomic high- (CoCrFeMnNi) and a medium- (CoCrNi) entropy alloy have been carried out. A highly innovative milling process with ultrasonic assistance (USAM) was compared to conventional milling processes.
Recent studies have shown that USAM has a high potential to significantly reduce the mechanical load on the tool and workpiece surface during milling. In this study, the basic machining and ultrasonic parameters were systematically varied. After machining, the surface integrity of the alloys was analyzed in terms of topography, defects, subsurface damage, and residual stresses. It was observed that USAM reduces the cutting forces and increases the surface integrity in terms of lower tensile residual stresses and defect density near the surfaces for the CoCrFeMnNi alloy. It was shown that the cutting forces and the metallurgical influence in the sub surface region are reduced by increasing the cutting speed and reducing the feed rate per cutting edge. With the CoCrNi alloy, the tool revealed severe wear. As a result, for this alloy no influence of the parameters on the machinability could be determined.
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 the presence of diffusible hydrogen, i.e., hydrogen-assisted cracking (HAC), generally increases. HAC is a result of the critical interaction between local microstructure, mechanical load, and hydrogen concentration. In existing standards for welding of HSLA-steels, recommendations including working temperatures and dehydrogenation heat treatment (DHT) are given to Limit the amount of introduced hydrogen during welding. These recommendations are based on investigations into 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, economic benefits. In the present study, the hydrogen distribution in S960QL multi-layer welds with thickness of 20 mm was analyzed. The influence of different weld seam opening angles, heat input, working temperature and DHT were investigated. The results show that weldments with narrow grooves contained an increased amount of diffusible hydrogen. Hydrogen concentration has been reduced by decreasing both the heat input and working temperature. Hydrogen-free weldments were only achieved via subsequent DHT after welding. Furthermore, hydrogen distribution was experimentally determined across the weld seam thickness in HSLA gas metal arc welded multi-layer welds for the first time.
High-strength structural steels are used in machine, steel, and crane construction with yield strength up to 960 MPa. However, welding of these steels requires profound knowledge of three factors in terms of avoidance of hydrogen-assisted cracking (HAC): the interaction of microstructure, local stress/strain, and local hydrogen concentration. In addition to the three main factors, the used arc process is also important for the performance of the welded joint. In the past, the conventional transitional arc process (Conv. A) was mainly used for welding of high-strength steel grades. In the past decade, the so-called modified spray arc process (Mod. SA) has been increasingly used for welding production. This modified process enables reduced seam opening angles with increased deposition rates compared with the Conv. A. Economic benefits of using this arc type are a reduction of necessary weld beads and required filler material. In the present study, the susceptibility to HAC in the heat-affected zone (HAZ) of the high-strength structural steel S960QL was investigated with the externally loaded implant test. For that purpose, both Conv. A and Mod. SA were used with same heat input at different deposition rates. Both conducted test series showed same embrittlement index “EI” of 0.21 at diffusible hydrogen concentrations of 1.3 to 1.6 ml/100 g of arc weld metal. The fracture occurred in the HAZ or in the weld metal (WM). However, the test series withMod. SA showed a significant extension of the time to failure of several hours compared with tests carried out with Conv. A.
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 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
Martensitic 9 %-Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen assisted cracking. The focus of this study was the microstructure and heat treatment effect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition.
Tensile tests with hydrogen free reference samples and electrochemically hydrogen charged samples were carried out, the mechanical properties were assessed and supported by dedicated fractographic analysis. In addition, hydrogen and microstructure dependent fracture criteria were calculated. All investigated microstructures showed a hydrogen influenced degradation of the mechanical properties compared to the hydrogen free reference samples. In that connection, the as welded martensitic P91 weld metal had the highest degree of degradation in presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a significantly increased risk for hydrogen assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.
Martensitic 9% Cr steels like P91 and P92 can show an increased susceptibility to delayed hydrogen-assisted cracking. The focus of this study was the microstructure and heat treatment efect on the mechanical properties of P92 base material and P91 multi-layer weld metal in both as-welded and post weld heat treated (PWHT) condition. Tensile tests with hydrogen free reference samples and electrochemically hydrogen charged samples were carried out; the mechanical properties were assessed and supported by detailed fractographic analysis. Finally, a hydrogen and microstructure-dependent fracture criterion is established. All investigated microstructures showed a hydrogen-infuenced degradation of the mechanical properties compared to the hydrogen-free reference samples. The as-welded martensitic P91 weld metal had the highest degree of degradation in the presence of hydrogen. The P91 PWHT weld metal and the P92 base material had comparable properties. From that point of view, a signifcantly increased risk for hydrogen-assisted cold cracking during welding fabrication of P91 weld joints must be considered before any heat treatment is conducted.
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.
Welded components of P91 9% Cr steel demand for careful welding fabrication with necessary post weld heat treatment (PWHT). Before the PWHT, a hydrogen removal heat treatment is necessary for avoidance of hydrogen assisted cracking (HAC). In this context, the microstructure and temperature-dependent hydrogen diffusion is important, and reliable diffusion coefficients of P91 weld metal are rare. For that reason, the diffusion behavior of P91 multi-layer weld metal was investigated for as-welded (AW) and PWHT condition by electrochemical permeation experiments at room temperature and carrier gas hot extraction (CGHE) from 100 to 400 °C. Hydrogen diffusion coefficients were calculated, and the corresponding hydrogen
concentration was measured. It was ascertained that both heat treatment conditions show significant differences. At room
temperature the AW condition showed significant hydrogen trapping expressed by to seven times lower diffusion coefficients. A preferred diffusion direction was found in perpendicular direction expressed by high permeability. The CGHE experiments
revealed lower diffusion coefficients for the AW condition up to 400 °C. In this context, a hydrogen concentration of approximately 21 ml/100 g was still trapped at 100 °C. For that reason, a certain HAC susceptibility of as-welded P91 weld metal cannot
be excluded, and hydrogen removal should be done before PWHT.