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Carrier gas hot extraction (CGHE) is a commonly applied technique for determination of hydrogen in weld joints using a thermal conductivity detector (TCD) for hydrogen measurement. The CGHE is based on the accelerated hydrogen effusion due to thermal activation at elevated temperatures. The ISO 3690 standard suggests different specimen geometries as well as necessary minimum extraction time vs. temperature. They have the biggest influence on precise hydrogen determination. The present study summarizes the results and experience of numerous test runs with different specimen temperatures, geometries (ISO 3690 type B and small cylindrical samples), and factors that additionally influence hydrogen determination. They are namely specimen surface (polished/as-welded), limited TCD sensitivity vs. specimen volume, temperature measurement vs. effects of PI-furnace controller, as well as errors due to insufficient data assessment. Summarized, the temperature is the driving force of the CGHE. Two different methods are suggested to increase the heating rate up to the desired extraction temperature without changing the experimental equipment. Suggestions are made to improve the reliability of hydrogen determination depended on the hydrogen signal stability during extraction accompanied by Evaluation of the recorded data. Generally, independent temperature measurement with dummy specimens is useful for further data analysis, especially if this data is used for calculation of trapping kinetics by thermal desorption analysis (TDA).
Risssicherheit von Schweißnähten an Pipelines der Offshore-Industrie aus supermartensitischem Stahl
(2005)
Up to the present, the thermomechanical loads during welding and subsequent cooling under design-specific shrinkage restraint resulting from the interaction between the materials and the structure have often not been taken into consideration approperiately for weldability assessment of components. As compared to previous investigations using small specimens this report presents component weld tests under varying intensities of restraint. Online records of the reaction forces and moments demonstrate in which way the intensity of restraint affects the reaction stresses and moments and their meaning for the evaluation of the load level in welded components.
Düring the past decades, weldability studies have predominantly been concentrated on the material and the welding process. In order to achieve a closer insight into the structural effects, component weldability tests have been performed by online monitoring of the reaction forces and moments. As a particular item, the effects of different strengths of the base and filier material on the reaction force and stress build up transverse to the welding direction have been studied at a specific structural restraint. As a most relevant result for practical welding it is demonstrated that the final reaction force and also the reaction stress level in the multilayer butt welds decreases with overmatch of the weld metal.
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metallic crystal lattice. For the investigations reported in this contribution, hydrogen thermal desorption analysis has been carried out to observe the influence of hydrogen desorption on the lattice of super martensitic stainless steel during continuous heating. The lattice expansion parameter and the phase transformations have been monitored during the thermal desorption process, and the influence of hydrogen on such characteristics has been evaluated. It was found that hydrogen has a significant influence on both the lattice parameter and on the thermal expansion. However, hydrogen has no influence on phase transformation during thermal desorption. The hydrogen's desorption behavior in this process was also observed and it turned out that hydrogen desorbs in two stages, i.e., firstly diffusible hydrogen and trapped hydrogen afterward.
A 20 mm thick heat resistant 13CrMoV9-10 steel was welded using a multi-pass submerged arc tandem setup on a 3-D testing facility designed to prevent shrinkage both during the welding and the subsequent cooling to room temperature. The effects of variations in pre-heating and interpass temperatures on the local stresses were monitored by in situ measurement of reaction forces and bending moments during welding and the subsequent dehydrogenation heat treatment. The austenite to bainite transformation and its effects on the overall loads were monitored through the use of a high sampling rate. Increasing the pre-heating and interpass temperatures led to a rise in reaction forces and bending moments. The bending moments originated from the eccentric force transmission over the backing, with a concomitant risk of fracture, especially in the critical root zone. Although the pre-heating and interpass temperatures had a strong effect on the reaction forces, their effect on the bending moments upon cooling to room temperature was negligible.
This study provides an in-depth survey of current technologies and applications for assessing the cold cracking susceptibility of welded joints. From the large variety of existent cold cracking test procedures, the most important and internationally established tests are presented and evaluated in terms of their usefulness and application limits. According to the type of loading, the test procedures are classified into self-restraint and externally loaded tests. Apart from the merely metallurgical weldability tests for determining the cracking susceptibility of base materials, filler materials and weld metals, advanced test methods are presented for evaluating the cold cracking susceptibility of welded components. A salient point brought out in this respect is the fact that the level of external loading in such component weld tests can be applied independently of the welding parameters, reproducing as realistically as possible the practical application case, i.e. the level of the restraint intensity. This study includes a summarized comparison of the cold cracking procedures. It is emphasized that highly accurate consideration and reproduction of the design-specific stiffness conditions is essential in the tests for assessing the cold cracking behaviour of welded joints. Therefore, various numerical analyses are presented in a final chapter for calculating the restraint intensity as a definitive factor affecting cold cracking.
Effects of Preheating and Interpass Temperature on Stresses in S 1100 QL Multi-Pass Butt-Welds
(2008)
In situ analysis of hydrogen behaviour in stainless steels by high energy synchrotron radiation
(2011)
Hydrogen generally causes lattice distortions and phase transformations when introduced into a metal matrix. The present contribution provides a report of first in situ investigations of hydrogen effects on the lattice of the austenite and the martensite phase in a supermartensitic stainless steel using hard synchrotron radiation for respective energy dispersion diffraction measurements. Lattice distortions, such as the planar spacing expansion, have been monitored during the complete hydrogen effusion process over 24 h and are directly correlated to the dissolved hydrogen in the metal matrix. Among other results, it turned out that hydrogen at the introduced concentration level causes a reversible lattice expansion and after hydrogen effusion the lattice nearly regained its original dimensions. Hydrogen much more significantly affects the austenite phase with respect to lattice expansion than the martensite phase, but no phase transformations occur during the hydrogen desorption process.
Hydrogen interaction with residual stresses in steel studied by synchrotron X-ray diffraction
(2014)
The residual stress state in a material has an important role in the mechanism of cracking, induced or assisted by hydrogen. In this contribution, the beamline EDDI in BESSY II instrument in Berlin was used in order to investigate the influence of hydrogen upon the residual stresses state existing in a Supermartensitic stainless steel sample. The method used for investigating the residual stresses is the sinus square ψ method. This method involves the usage of high energy X-ray diffraction in order to measure the residual stress state and magnitude. It was found that hydrogen presence has a significant influence upon the magnitude of the residual stresses, as its value decreases with high hydrogen content. This effect is reversible, as hydrogen desorbs from the sample the residual stress magnitude gains its initial value before hydrogen charging.
In einer speziellen 3D-Prüfanlage zur wirklichkeitsnahen Simulation von komplexen Verhältnissen an Großproben wurden UP-Mehrlagenschweißungen am warmfesten Stahl 13CrMoV9-10 durchgeführt. Während der schweißtechnischen Fertigung und der Abkühlung auf Raumtemperatur wurden die Proben an der freien Ausdehnung und Schrumpfung gehindert. Die daraus resultierenden Reaktionsspannungen führten während der Wärmenachbehandlung zu Relaxationsrissen in der Wärmeeinflusszone. Das Spannungsarmglühen wurde bei einer Temperatur von 705°C und einer Haltezeit von 8 h durchgeführt. Während der Aufheiz- und Abkühlphasen wurde die thermische Ausdehnung der Probe über die 3D-Prüfanlage kompensiert und damit das Spannungsniveau bis zum Erreichen der maximalen Glühtemperatur aufrechterhalten. Nach Erreichen der Glühtemperatur konnte die Probe relaxieren. Der Einsatz der Schallemissionsmessung ermöglichte es die Entstehung von Spannungsrelaxationsrisse, erstmalig online, nachzuweisen. Durch Synchronisierung der Schallemissionsereignisse mit den Messdaten der 3D – Prüfanlage wurde der kritische Temperaturbereich für das Auftreten von Spannungsrelaxationsrissen bestimmt. Die Rissinitiierung findet im Temperaturbereich zwischen 350°C - 500°C statt. Aussagen über das Risswachstum können auf Grund der ungünstigen Versuchsbedingungen, mit Emission von Störgeräuchen, nicht getroffen werden.
Various test methods are available for assessing the susceptibility of materials to solidification cracking during welding. In the widely used Varestraint test, the crack length is selected as a criterion as a function of the applied bending strain.
Unfortunately, the crack length does not characterize the material behavior alone but depends to varying degrees on the individual test parameters used, which makes the interpretation of the results difficult. In addition, the crack length is not comparable under different test conditions. To overcome these disadvantages, we have developed a novel evaluation methodology that decouples the machine influence from the material behavior. The measured crack length is related to the maximum possible value specified by welding speed and deformation time. This relative crack length is calculated numerically, considering the orientation of the cracks. Experiments on two high-alloy martensitic welding consumables show that, in contrast to the conventional evaluation, a comparison of different welding parameters becomes possible. Furthermore, the strain rate proved to be a suitable crack criterion in agreement with Prokhorov's hot cracking model.
While the problem of the identification of mechanisms of hydrogen assisted damage has and is being thoroughly studied, the quantitative analysis of such damage still lacks suitable tools. In fact, while, for instance, electron microscopy yields excellent characterization, the quantitative analysis of damage requires at the same time large field-of-views and high spatial resolution. Synchrotron X-ray refraction techniques do possess both features. In this work, we show how synchrotron X-ray refraction computed tomography (SXRCT) can quantify damage induced by hydrogen embrittlement in a lean duplex steel, yielding results that overperform even those achievable by synchrotron X-ray absorption computed tomography. As already reported in literature, but this time using a non-destructive technique, we show that the hydrogen charge does not penetrate to the center of tensile specimens. By the comparison between virgin and hydrogen-charged specimens, we deduce that cracks in the specimen bulk are due to the rolling process rather than hydrogen-assisted. We show that (micro)cracks propagate from the surface of tensile specimens to the interior with increasing applied strain, and we deduce that a significant crack propagation can only be observed short before rupture.
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.
The article "Influence of welding stresses on relief cracking during heat treatment of a creep-resistant 13CrMoV steel: Part I - Effect of heat control on welding stresses and stress relief cracking" was originally published in the journal "Welding in the world" Online First without Open Access. After publication in volume 64, issue 5, page 807-817 the authors decided to opt for Open Choice and to make the article an Open Access publication. Therefore, the copyright of the article has been changed to The Author(s) 2020 and the article is forthwith distributed under the terms of the Creative Commons Attribution Attribution 4. 0 International License.
The article "Influence of welding stresses on relief cracking during heat treatment of a creep-resistant 13CrMoV steel: Part II - Mechanisms of stress relief cracking during post weld heat treatment" was originally published in the journal "Welding in the world" Online First without Open Access. After publication in volume 64, issue 5, page 819-829 the authors decided to opt for Open Choice and to make the article an Open Access publication. Therefore, the copyright of the article has been changed to The Author(s) 2020 and the article is forthwith distributed under the terms of the Creative Commons Attribution Attribution 4. 0 International License.
Modern arc processes, such as the modified spray arc (Mod. SA), have been developed for gas metal arc welding of high-strength structural steels with which even narrow weld seams can be welded. High-strength joints are subjected to increasingly stringent requirements in terms of welding processing and the resulting component performance. In the present work, this challenge is to be met by clarifying the influences on hydrogen-assisted cracking (HAC) in a high-strength structural steel S960QL. Adapted samples analogous to the self-restraint TEKKEN test are used and analyzed with respect to crack formation, microstructure, diffusible hydrogen concentration and residual stresses. The variation of the seam opening angle of the test seams is between 30° and 60°. To prevent HAC, the effectiveness of a dehydrogenation heat treatment (DHT) from the welding heat is investigated.
As a result, the weld metals produced at reduced weld opening angle show slightly higher hydrogen concentrations on average. In addition, increased micro- as well as macro-crack formation can be observed on these weld metal samples. On all samples without DHT, cracks in the root notch occur due to HAC, which can be prevented by DHT immediately after welding.
Für eine nachhaltige und ressourceneffiziente Fertigung von Windkraftanlagen ist der Einsatz moderner hochfester Feinkornbaustähle vielfach notwendig. Bei der Montage hochfester Stahlkonstruktionen kommt es trotz anforderungsgerechter schweißtechnischer Fertigung vereinzelt zur Detektion von unzulässigen Unregelmäßigkeiten im Schweißbereich. Derzeitige Normen und Verarbeitungsrichtlinien geben hierbei kaum Informationen zu adäquaten Reparaturkonzepten, insbesondere für hochfeste Stähle. Vor diesem Hintergrund werden an der Bundesanstalt für Materialforschung und -prüfung (BAM) bauteilrelevante Untersuchungen zu den schweißbedingten Beanspruchungen und Gefügeveränderungen bei der schweißtechnischen Reparatur durch lokales thermisches Ausfugen und erneutes Schweißen durchgeführt.
Hierzu werden einige wesentliche Ergebnisse an Beispielen des Ingenieurbaus, mit Blick auf die mechanisch-technologischen Eigenschaften und Eigenspannungen erörtert. In einem aktuellen Vorhaben (FOSTA P1311, IGF 20162N) sind darauf aufbauend die Einflüsse durch die Schrumpfbehinderung und Wärmeführung beim Reparaturschweißen hochfester Feinkornbaustähle (S500MLO, S960QL) sowie die Reparaturzyklenanzahl Fokus systematischer Untersuchungen. Mittels strukturmechanischer Berechnungen ließen sich hierzu selbstbeanspruchende Probenformen identifizieren, die unterschiedliche definierte Steifigkeitsverhältnisse von Reparaturschweißnähten realer Bauteile abbilden. Schweißnähte an diesen bauteilähnlichen Proben weisen signifikant erhöhte schweißbedingte Beanspruchungen mit zunehmenden (Biege-)Einspanngraden auf. Im Vergleich mit bisherigen Erkenntnissen wird gezeigt, dass eine sichere Reparatur solcher Schweißnähte nur mittels optimaler Reparaturkonzepte und Wärmeführung gelingt, um diesen hohen Beanspruchungen und auch den speziellen Gefügen hochfester Stähle Rechnung zu tragen.
Ziel der Untersuchungen ist es letztlich, Empfehlungen für beanspruchungs- und werkstoffgerechte Reparaturkonzepte gerade für KMU abzuleiten, um Schäden und zumeist teure Nacharbeiten zu vermeiden.
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.
In modernen Schweißkonstruktionen des Stahl- und Ingenieurbaus werden zunehmend hochfeste Feinkornbaustähle eingesetzt. Grundsätzlich sind während der schweißtechnischen Verarbeitung hochfester Stahlgüten aufgrund ihres speziellen Gefüges und wegen ihres vergleichweise hohen Streckgrenzenverhältnisses engere Verfahrensgrenzen einzuhalten, als bei Stählen mit niedrigerer Festigkeit. Die Sicherheit von hochfesten Schweißverbindungen erfordert präzise Kenntnisse über die Höhe und Verteilung der schweißbedingten Beanspruchungen, da sie die erreichbare Tragfähigkeit unmittelbar beeinflussen. In den Untersuchungen wurde der Einfluss der Wärmeführung in Form der Zwischenlagentemperatur und Streckenenergie auf die Eigenspannungsausbildung und Gesamtbeanspruchung in hochfesten Schweißverbindungen ermittelt. Zur Berücksichtigung einer äußeren Bauteilsteifigkeit wurden die Proben in Nahtquerrichtung unter einem definierten Einspanngrad in einer speziellen Prüfanlage (CTW-Test) mehrlagig geschweißt. Es zeigte sich beispielsweise, dass insbesondere die Zwischenlagentemperatur einen direkten Einfluss auf die globalen Reaktionsspannungen hat. Ferner bedingt die Erhöhung der Wärmeführung ein deutliches Anheben der Zugeigenspannungen im Bereich des Schweißgutes.
Große Materialstärken und komplexe Strukturen bewirken eine erhebliche Schrumpfbehinderung der Schweißnaht.
Die aus dem Schweißprozess resultierenden multiaxialen Belastungen tragen maßgeblich zur Gesamteigenbeanspruchung einer Schweißkonstruktion bei und lassen sich nur unter realen Fertigungsbedingungen analysieren.
Zur wirklichkeitsnahen Simulation der komplexen Steifigkeitsverhältnisse realer Bauteile wurden hierzu in einer speziellen 3D-Prüfanlage UP-Mehrlagenschweißungen am warmfesten Stahl 13CrMoV9-10 durchgeführt. Während der schweißtechnischen Fertigung sowie der Abkühlung auf Raumtemperatur wurden die Proben an der freien Ausdehnung und Schrumpfung gehindert. Der Einfluss der Wärmeführung, d.h. Streckenenergie (E) und Vorwärm-/Zwischenlagentemperatur (T(Pli>), auf die resultierende Bauteilbeanspruchung wurde durch in-situ Messungen der Reaktionskräfte (Fy) und Biegemomente (Mx) während des Schweißens und der anschließenden Wasserstoffarmglühung untersucht. Sowohl die Reaktionskräfte als auch die Biegemomente steigen mit zunehmender Vorwärmtemperatur an und somit erhöht sich die Beanspruchung des Wurzelbereiches. Während die Vorwärm-/Zwischenlagentemperatur einen signifikanten Einfluss auf die resultierenden Reaktionskräfte hat, ist der Einfluss auf die Biegemomente nach Abkühlung auf Raumtemperatur vernachlässigbar. Eine Erhöhung der Streckenenergie wirkt sich nur geringfügig auf die Höhe der Endreaktionskraft aus, beeinflusst aber signifikant das resultierende Biegemoment nach erfolgter Abkühlung auf Raumtemperatur.
Hot crack prevention in materials production and processing is an essential prerequisite for welded component safety. The causes of hot cracking can ultimately be attributed to the occurrence of metallurgical effects and to structural loads. More than 140 hot cracking test procedures have hitherto been developed for determining the hot cracking resistance. In principle, they are divided in self-restraint and externally loaded hot cracking tests with diverse process variants. Only some of the hot cracking tests are international standardized. Although various factors are known that encourage or prevent hot cracking, it is often not possible even with defined welding conditions to draw immediate conclusions about the hot cracking resistance of a welded component alone from a metallurgical composition of the base and filler materials. Based on an evaluation of the existing theories relating to hot cracking susceptibility assessment, this study summarizes the major hot cracking test procedures and highlights the application limits of the test procedures by presenting overviews along with explanations. It shows that weld hot cracking tests can generally be used to rank materials, welding consumables, and welding conditions. The evaluation of hot cracking test results and of their transferability among one another and to real components always requires consideration of the close relationships between metallurgy, welding process, and parameters, respectively, and prevailing restraint conditions.
Beside quenched and tempered (QT) high strength steels advanced technologies in steel manufacturing provide steels produced by the thermo-mechanical controlled process (TMCP) with yield strength of 960 MPa. These steels differ in the carbon and micro-alloying element content. With variation of heat control TIG-welded dummy seams on both steel types were performed. Analyses concerning microstructure and residual stress evolution due to welding showed typical stress distributions according to common concepts. Yet, the TMCP-steel shows higher residual stresses than the QT-steel.