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Effect of cooling rate on microstructure and properties of microalloyed HSLA steel weld metals
(2015)
Two high strength Nb/Ti microalloyed S690QL steels were welded with identical filler material, varying welding parameters to obtain three cooling rates: slow, medium and fast cooling. As cooling rate increased, the predominantly acicular ferrite in Nb weld metal (WM) is substituted by bainite, with a consequence of obvious hardness increase, but in Ti WM, no great variation of acicular ferrite at all cooling rates contributed to little increment of hardness. The transition between bainite and acicular ferrite has been analysed from the point view of inclusions characteristics, chemical composition and cooling rate. Excellent Charpy toughness at 233 K was obtained with acicular ferrite as predominantly microstructure. Even with bainite weld of high hardness, the toughness was nearly enough to fulfill the minimal requirements. WM for Ti steel showed to be markedly less sensitive to the variations of cooling rate than that for Nb steel.
Quantitative investigations of hydrogen dependent properties of aircraft landing gear materials have only scarcely been carried out in the past. They are essential for respective component life time assessments. To better understand the behavior of these landing gear materials in a hydrogen rich environment, specimens were charged in the condition as delivered with known concentrations of hydrogen and then mechanically tested to evaluate the degradation effects. The present contribution is focused on evaluating the hydrogen concentration dependent material properties and the respective fracture topographies of the two investigated steels, and then continued by the comparison of the results with other previously investigated martensitic steels in order to better understand the thresholds for these materials to maintain structural integrity.
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.
High-strength low-alloyed (HSLA) steels with yield strength ≥ 690 MPa have increasing importance in steel construction and civil engineering. However, weld processing of those steels is a major challenge. The susceptibility for degradation of mechanical properties of weld joints significantly increases in presence of hydrogen and can result in hydrogen assisted cracking (HAC). Generally, risk for HAC increases with increasing yield strength of HSLA steels. To minimize the incidence of HAC, it is essential to gain knowledge about both the (1) absorbed hydrogen amount and its distribution in the weld seam and (2) options to lower
the amount of introduced hydrogen. Existing standards recommend heat treatment procedures (interpass temperature or post weld heat treatment) to reduce the diffusible hydrogen concentration in weldments. In this context, different weld seam geometries should be considered. For HSLA steel fabrication weld processing with seam opening angles of 45° to 60° is typical. Modern weld technologies allow welding with seam opening angles of 30° - reduced welding time and costs. In the present study, the hydrogen distribution in multi-layer welds of a 960 MPa HSLA steel was analysed. Influence of different seam opening angles as well as heat input, interpass temperature and post weld heat treatments were investigated. The welded samples were quenched in ice water immediately after welding and subsequently stored in liquid nitrogen. After defined warming up, small specimens were machined from the weld seam by water jet cutting. The diffusible hydrogen concentration was measured by carrier gas hot extraction with coupled mass spectrometer. The results showed, that low heat input and post weld heat treatment procedures can lower hydrogen concentrations in welds. Furthermore, a gradient of the hydrogen concentration was
identified with increasing weld pool depth. By varying the seam opening angles different hydrogen concentrations were
measured.
In order to satisfy the growing requirements towards lightweight design and resource efficiency in modern steel constructions, e.g. mobile cranes and bridges, high-strength steels with typical yield strength ≥ 690 MPa are coming into use to an increasing extent. However, these steels require special treatment in welding. The susceptibility for degradation of the mechanical properties in presence of hydrogen increases significantly with increasing yield strength. In case of missing knowledge about how and which amount of hydrogen is uptaken during welding, hydrogen assisted cracking can be a negative consequence. Moreover, modern weld technology like the modified spray arc process enables welding of narrower weld seams. In this context, a reduced number of weld beads, volume and total heat input are technical and economic benefits. This work presents the influence of welding parameters on the diffusible hydrogen content in both (1) single-pass and (2) multi-layer welds. Different hydrogen concentrations were detected by varied contact tube distance, wire feed speed, arc length as well as varied arc type (transitional arc and modified spray arc). The results showed, that all welding parameters had significant influence on the diffusible hydrogen concentration in the single-pass welds. By increasing the number of weld beads in case of multi-layer welding, the hydrogen concentration have been substantially reduced. Whereby, differences in hydrogen concentrations between both arc types are present.
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.
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.
High entropy alloys (HEA) are a new class of materials. In contrast to conventional alloys, HEA are single-phase alloys with at least five alloying elements. HEAs have enormous application potential due to (postulated) excellent structural property combinations from low to high temperatures. For HEA-application as structural materials in real components, a key issue is the suitability for joining processing. Requirements for the reliable and safe joining of these materials are crucial regarding economical component manufacture for future applications. In this context, friction stir welding (FSW) is a promising joining process due to the welding process temperature below the material melting point avoiding major issues, e. g. formation of (hard and brittle) intermetallic phases, which may have detrimental influences on the weld joint properties.
This study presents elementary research about the FSW process influences on a CoCrFeMnNi-HEA with focus on the microstructure and mechanical properties. For that purpose, the FSW joint of the HEA is compared to that of an austenitic stainless steel AISI 304. The microstructures of the welds were investigated and characterized by means of light microscopy, SEM, EBSD and XRD. Hardness and tensile testing were applied to determine influences on the mechanical properties. Generally, a comparable weldability of HEA and AISI 304 in terms of metallurgical characteristics and resulting mechanical properties exhibited. For the weld joints of both materials typical characteristics regarding FSW were observed within the weld metal and thermo-mechanically influenced zone: fine-grained stirred zone with increased hardness and reduced fracture elongation compared with the respective base material.
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).
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.
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 overcome 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 up 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. But based on the results, the crack mechanism cannot be conclusively clarified as either a liquid metal embrittlement (LME) or hot cracking like liquid film separation occurred.
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.
High-entropy alloys (HEAs) are characterized by a solid solution of minimum five and medium-entropy alloys (MEAs) of minimum three principal alloying elements in equiatomic proportions. They show exceptional application properties, such as high-strength and ductility or corrosion resistance. Future HEA/MEA-components could be exposed to hydrogen containing environments like vessels for cryogenic or high-pressure storage where the hydrogen absorption and diffusion in these materials is of interest. In our study, we investigated the HEA Co20Cr20Fe20Mn20Ni20 and the MEA Co33.3Cr33.3Ni33.3. For hydrogen ingress, cathodic charging was applied and diffusion kinetic was measured by high-resolution thermal desorption spectros-copy using different heating rates up to 0.250 K/s. Peak deconvolution resulted in high-temperature desorption peaks and hydrogen trapping above 280 °C. A total hydrogen concentration > 40 ppm was identified for the MEA and > 100 ppm for HEA. This indicates two important effects: (1) delayed hydrogen diffusion and (2) considerable amount of trapped hydrogen that must be anticipated for hydrogen assisted cracking phenomenon. Local electrochemical Volta potential maps had been measured for the hydrogen free condition by means of high-resolution Scanning Kelvin Probe Force Microscopy (SKPFM).
High-entropy alloys (HEAs) are innovative high-performance materials that have attracted more and more research attention. HEAs are characterized by a solid solution of typically five equiatomic metallic elements. In addition, medium-entropy alloys (MEA, with three elements) are of interest and become more and more important. Depending on the alloy concept, HEAs and MEAs show exceptional mechanical properties, especially high-strength and ductility combinations at both cryogenic and elevated temperatures combined with excellent corrosion resistance. Future structural HEA/MEA components can be exposed to potential applications with hydrogen containing environments like high-temperature water in pressurized nuclear reactors or aerospace structures. Other potential applications could be in materials for vessel walls in the field of cryogenic and high-pressure hydrogen storage. So far, the susceptibility of HEAs/MEAs to hydrogen assisted cracking (if any) and the hydrogen diffusion is not investigated in detail yet and can limit or extend possible applications of HEA/MEA as structural materials. In our work, we focused on the hydrogen absorption, diffusion, and distribution in a HEA (CoCrFeMnNi the original Cantor-alloy) and a MEA (CoCrNi). Cathodic hydrogen charging was carried out for the hydrogen ingress, and thermal desorption analysis (TDA) revealed complex hydrogen trapping in both alloy types up to 300 °C. The absorbed total hydrogen concentrations were > 100 ppm for the HEA and > 40 ppm for MEA. In addition, the assessment of the peak deconvolution is not trivial and must consider both experimental and microstructure influences.
Offshore-Windenergieanlagen bilden einen zentralen Bestandteil der zukünftigen Energieerzeugung. Hierzu notwendige Gründungs- und Turmstrukturen werden vorrangig aus UP-geschweißten Blechen aus hochfesten niedriglegierten Stählen mit Dicken bis zu 200 mm hergestellt. Die großen Blechdicken begünstigen hohe Schweißeigenspannungen und lange Diffusionswege für z. B. über den Schweißprozess eingebrachten Wasserstoff. Damit steigt das Risiko für eine wasserstoffunterstützte Kaltrissbildung (WKB) an. Zum sichereren Ausschluss von WKB in den geschweißten Komponenten wird in Abhängigkeit der Nahtgröße, Wärmeeinbringung und Stahlsorte eine Mindestwartezeit (MWZ) von bis zu 48 h empfohlen, bevor die zerstörungsfreie Prüfung durchgeführt wird. Es ist allerdings offen, ob die empfohlene MWZ zu konservativ ist. Einflüsse sind hier bspw. die schwierige Bewertung des Wasserstoffdiffusionsverhaltens im heterogenen UP-Mehrlagen-Schweißgut (SG) im Vergleich zum Grundwerkstoff (GW).
Zudem sind nur sehr begrenzt H-Diffusionskoeffizienten für UP-Mehrlagen-SG als Grundlage zur Abschätzung des Zeitintervalls einer möglichen verzögerten Kaltrissbildung (somit MWZ) oder auch für Nachwärmprozeduren zur Wasserstoffreduktion (keine MWZ) verfügbar. Verlässliche H-Diffusionskoeffizienten sind daher ein adäquates Tool zur Bewertung des Risikos einer verzögerten WKB. Dieser Beitrag beschreibt Untersuchungen zur Charakterisierung der Wasserstoffdiffusionsverhalten in UP-Mehrlagen-SG eines 60 mm dicken Bleches einer Offshore-Stahlgüte.
Dazu wurden Proben unterschiedlicher Dicke aus dem reinen Schweißgut extrahiert und über elektrochemische Permeation und Trägergasheißextraktion im Temperaturbereich bis 400 °C sehr interessante Ergebnisse zu den korrespondierenden Diffusionskoeffizienten erarbeitet.
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.
Literature provides a wide range of hydrogen diffusion coefficients for low alloyed steels used in power plants. In fact, experimental boundary conditions and calculation methods have influence on the determination of these coefficients. The diffusion and trapping behavior in creep-resistant steel 7CrMoVTiB10-10 has been studied. Based on experimental carrier gas hot extraction (CGHE) data, a numerical model has been developed to describe the hydrogen transport and respective hydrogen distribution at elevated temperatures. The numerical results suggest that common calculation methods for diffusion coefficients are limited for experimental data analysis. The sample preparation time before CGHE experiment influences the determined diffusion coefficients with the consequence that non-homogeneous hydrogen concentration profiles have to be considered in the simulations.
For the hydrogen-based energy economy of tomorrow, the construction of the necessary infrastructure will play a central role. Most materials used to date, such as welded steels, can be prone to hydrogen embrittlement under certain conditions. This includes the classic delayed cold cracking during welding as well as degradation phenomena during service of components in hydrogen-containing environment. For the evaluation of any hydrogen effect, for example, on the mechanical properties of a welded metallic material, the hydrogen content must be precisely determined. In the case of weld seams, the carrier gas hot extraction (CGHE) according to ISO 3690 is meanwhile state-of-the-art. CGHE is based on accelerated hydrogen degassing due to the thermal activation of hydrogen at elevated temperatures. In addition to the quantification of hydrogen, thermal desorption analysis (TDA) with varying heating rates can be used to determine and evaluate the hydrogen trapping at microstructural defects in the material. For both techniques, experimental and metrological influences must be considered, which have a major effect on the result. For example, ISO 3690 suggests different sample geometries and minimum extraction times for CGHE. This study summarizes the results and experiences of numerous investigations at the Federal Institute for Materials Research and Testing (BAM) with different sample temperatures and geometries (ISO 3690 type B and cylindrical TDA samples) regarding the influence of the sample surface (polished/welded), measurement accuracy depending on the sample volume and the insufficient monitoring of the effect of PI control on the extraction temperature. A deviating extraction temperature from the target temperature can significantly falsify the measurement results. Based on the results, methods are shown which allow the desired extraction temperature to be reached quickly without physically interfering with the measuring equipment. This serves to significantly improve the reliability of the hydrogen measurement through increased signal stability and accelerated hydrogen desorption. In general, an independent temperature measurement with dummy samples is recommended for the heating procedure of choice to exclude possible undesired temperature influences before the measurement. The methods described can be transferred directly to industrial applications
Modeling of Hydrogen-Assisted Cracking (HAC) in Duplex Stainless Steels (DSS):
- Hydrogen causes significant degradation in DSS and consequently HAC
- Mechanisms of HAC have not been fully understood in two-phase microstructures
- Mesoscale numerical modeling for HAC in DSS was created, coupled to the macroscale, enabling simulation of HAC in both phases independent from predefined crack paths
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.