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Die hier vorgestellte Großanlage zur Prüfung und Simulation von Bauteil schweißungen (GAPSI 16) mit einer Maximalprüfkraft von ±16 MN erlaubt wirklichkeitsnahe Analysen des Bau teilverhaltens während des Schwei ßens. Ein fundamentaler Vorteil ge genüber konventionellen Universal prüfmaschinen ist, Momente während des Schweißens mittels acht unabhän giger Hydraulikzylinder in zwei Prüf tischen aufbringen bzw. registrieren zu können.
Derzeitige Forschungsvorhaben sind zum einen Spannungs- und Verformungsvorgängen während des Schweißens von Blechen und zum anderen Modellen gewidmet, womit Kenngrößen für Flammrichtarbeiten ermittelt werden können. Simulations versuche an Großproben tragen we sentlich zu höherer Aussagesicherheit bei, zumal konventionelle Prüfmetho den mit einfachen Kleinproben und auch Berechnungen nach der Finite Elemente-Methode (FEM) stets Rand bedingungen und zumeist größere Idealisierungen enthalten. Dagegen weist die Großprüfanlage GAPSI 16 mit angepaßten Probengeometrien und Spannkräften einen neuen Weg.
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.
In der Beurteilung der Schweißbarkeit für die Sicherheit und Zuverlässigkeit von Bauteilen fanden bisher die aus der Interaktion zwischen den Werkstoffen und der Konstruktion resultierenden thermomechanischen Belastungen während des Schweißens und der nachfolgenden Abkühlung unter konstruktiv bedingter Schrumpfbehinderung häufig nur wenig Beachtung. Gegenüber bisherigen Untersuchungen an Kleinproben wurden daher im Rahmen dieser Arbeit bauteilrelevante Schweißverbindungen unter realistischen Spannungsverteilungen geprüft. Diese Bauteilschweißversuche wurden in einer speziellen Großprüfanlage unter Variation der Schrumpfbehinderung, des Grund- und Zusatzwerkstoffes sowie der Streckenenergie durchgeführt. Dabei erweist sich das Konzept des Einspanngrades als geeignet, um die reale konstruktionsbedingte Schrumpfbehinderung auf die Versuchsanlage zu transferieren. Mittels online-Aufzeichnungen der Reaktionskräfte und -momente konnte gezeigt werden, wie sich diese Einzeleinflussgrößen jeweils auf die Höhe und den Verlauf der Reaktionskräfte und -momente auswirken und welche Bedeutung sie für die Einschätzung des Belastungsniveaus in Form von Spannungen und Dehnungen in Bauteilen besitzen. Unter anderem stellte sich heraus, dass unter relativ geringen Einspanngraden eine unkritische Eigenbelastung des Bauteiles vorliegt, jedoch bei hoher Steifigkeit die thermomechanische Beanspruchung bis zum Riss führen kann. In dieser Arbeit wurde erstmalig die Wirkung der Festigkeit von Zusatzwerkstoffen auf die Spanungs-Dehnungsverteilung unter definierter Schrumpfbehinderung am Bauteil untersucht. Es zeigte sich, dass unabhängig vom Grundwerkstoff ein hochfester Schweißzusatz zu geringeren Reaktionskräften führt als beim Schweißen mit einem Schweißzusatz mit vergleichsweise niedrigerer Festigkeit. Weiterhin wurde der Dehnungsverlauf beim und nach dem Schweißen unter definierter Schrumpfbehinderung mittels Dehnungsmessstreifen und einem Wegmesssystem bestimmt. Abhängig vom Schweißnahtabstand und dem Einspanngrad traten charakteristische Verformungsverläufe auf. Anhand dieser Ergebnisse konnte eine bisher ungebräuchliche jedoch notwendige Aufteilung der Schweißung in Nah- und Fernfeld erfolgen. Mit der inkrementellen Bohrlochmethode wurden die Eigenspannungen im Nahtbereich unter Einspannung, nach dem Entlasten und nach dem Wiederbelasten auf das Niveau der ursprünglich wirkenden Reaktionskräfte der Proben ermittelt und mit röntgenografischen Werten verglichen. Der Zusammenhang zwischen Einspanngrad und den Nahteigenspannungen lässt sich durch einen einfachen funktonalen Ansatz beschreiben. Den Untersuchungen wurde aufgrund der auf diesem Gebiet vorliegenden sehr unzusammenhängenden Literatur ein detaillierter Kenntnisstand vorangestellt.
Nickel base alloys are frequently applied in safety-relevant fields such as chemical plant construction or power plant engineering. Particularly on account of their cubic face-centered solidification characteristic, these materials are frequently susceptible to metallurgy-specific hot cracking during fusion welding. A classification of these materials according to their hot cracking resistance in the MVT-Test (Modified Varestraint Transvarestraint Test) is carried out by the example of a series of base and welding filler materials. Moreover, it has been proven by the MVT Test that the hot cracking resistance of nickel base alloy (Alloy 602 CA) can be improved by selecting appropriate shielding gases.
To the revision of ISO 3690 regarding the acceptance of the carrier gas hot extraction method
(2004)
Improving HAZ Toughness of High Heat Input Welded Joints by Using Boron Diffusion from Weld Metal
(2004)
Is it reasonable to restrict ISO 3690 any longer to the determination of the weld metal hydrogen content of ferritic steel?
In what respect is the determination of the content of the diffusible and of the total hydrogen content, respectively, (still) merely a research task (supermartensite, duplex materials, austenite, magnesium, aluminium), or is there actually a demand in industry and is there already a first cause for monitoring in view of damage prevention, respectively?
Is there a need, arising from the development of materials and filler materials and from their intended applications, for more extensive standards dealing with hydrogen analyses for these materials?
Which parts of ISO 3690 could also be applied to any other metallic material/filler material, if necessary, and where would a complete alteration be required, respectively?
As an approach towards more rough industrial laser welding applications, the Nd:YAG laser welding process has been coupled to the plasma powder welding process for the first time. Process stabilities have been achieved by such hybrid welding which are even better than those known from previous coupling of the laser process with gas metal arc welding (GMAW), for instance.
The present contribution focuses on the technology and potentials of the novel laser hybrid welding technique and addresses the realisation and the perspectives for practical applications. The experimental results demonstrate that the new process coupling enables welding of austenitic stainless steels in the thickness range between 3 mm and 8 mm at various weld geometries and joint configurations and at welding speeds exceeding 2 m/min. In addition it has been proven that the laser plasma powder arc welding (LPPAW) technique can also be applied to out-of-position conditions. Continuous advancement of the welding equipment ensures a high gap bridging ability and levelling of misalignment due to higher powder feeding rates. The recent development of a new plasma powder torch even provides a higher efficiency of powder feeding and guarantees smoth weld surfaces. Up to the present, the investigations demonstrated that the novel laser plasma powder hybrid welding technique offers all perspectives for application in pipe fabrication, ship building and offshore as well as transportation industry.
Risssicherheit von Schweißnähten an Pipelines der Offshore-Industrie aus supermartensitischem Stahl
(2005)
Anwendung des neuen Controlled Tensile Weldability (CTW) Tests zur Untersuchung der Heißrissneigung
(2006)
The increasing application of supermartensitic steels for welded pipelines is an economical alternative to the hitherto used higher-alloyed materials in the North Sea oil and gas indus-try. Failure in such constructions must at any rate be excluded for economical and ecologi-cal reasons. The application of these steels for the transport of corrosive mixtures may, however, involve hydrogen pickup with subsequent hydrogen-assisted stress corrosion cracking. It is therefore necessary not only to assure the weldability, but particularly to have best possible knowledge of the service behaviour and of the failure risk. In order to ensure the transferability of test results to real joined components, innovative test methods are in-creasingly required to be incorporated into a closed test sequence. It will be demonstrated how it is possible to gain significant advantages from the direct comparison between ex-perimentally determined results from component weld tests on the one hand and material-specific data from small-scale tests on the other hand and numerical simulations. These data that have now been made available are of major importance for industrial applications and are considered to provide a sound basis for realistic lifetime assessments.
Aspects of positional laser-GMA-hybrid welding using high alloyed tubular cored wire electrodes
(2008)
Effects of Preheating and Interpass Temperature on Stresses in S 1100 QL Multi-Pass Butt-Welds
(2008)
CT-01 Formation of welding residual stresses in low transformation temperature (LTT) materials
(2008)
In the last decade, high-strength fine grained steels and welding consumables have gained a strong raise of application ranging from mobile cranes to bridge constructions. However, the cracking susceptibility of these steels increases significantly in correspondence with the achieved improvements in yield strength and the loss in plastic deformation reserves.
In order to determine this behavior a series of different standardized cold cracking tests has been developed. One remaining major problem of these tests is the uncertainty about the quantitative intensity of the restraint conditions as well as the corresponding welding residual stresses.
Consequently, the comparison of different tests and welding conditions as well as the transferability of the results onto real parts is difficult at best. The main topic of this paper is the analysis of the restraint conditions and their link with the welding induced residual stresses. The importance of the given standardized selfrestrained tests and first results about the transferability of
results onto real parts are discussed. The influence of the test specimen geometry on the restraint conditions of the test is investigated for a selected test with numerical Simulation using commercial FEA software. Additionally, the residual stresses caused by the welding process are measured and linked with the restraint conditions which are defined mainly by the geometry parameters.
Finally the transferability of the selected cold cracking test results is validated experimentally. The test results of a multilayer weld on high-strength fine grained steel of real size weldments are investigated. For these experiments a 16 MN large scale testing facility is used which is capable of applying the high reaction forces and clamping conditions found at large scale demonstrator parts.
The results show the importance of the quantitative knowledge of the restraint conditions and the welding residual stresses on the cold cracking resistance.
Crack and fatigue resistance are relevant evaluation criteria for welded joints and are decreased by tensile residual
stresses resulting from the welding and cooling process, while compressive residual stresses can have a positive
influence on the characteristics mentioned. In order to generate compressive residual stresses, a set of post weld
treatment procedures is available, like shot peening, hammering, etc. These procedures have the disadvantage that
they are time and cost extensive and have to be applied after welding. As another point, such technologies can only
produce compressive stresses at the top surface, i.e. can only contribute to the reduction of the risk of cracks initiated
at the surface, like fatigue cracks. A chance to generate compressive stresses over the complete weld joint during the
welding procedure is offered by the so-called Low Transformation Temperature (LTT -) filler wires. Compared to
conventional wires, these materials show lower phase transformation temperatures, which can work against coolingrelated
tensile stresses, resulting from respective shrinkage restraint. In consequence, distinct compressive residual
stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially
applicable to high-strength steel welding. Welds produced with different LTT filler wires have shown different levels
and distributions of the resulting residual stresses depending on the specific transformation temperature. The
transformation temperatures are determined by temperature measurement. Classical X-ray diffraction as well as
diffraction methods using high energy synchrotron radiation have been used for residual stress analysis. By means of
high energy synchrotron diffraction in reflection mode residual stress depth gradients can be determined nondestructively.
The phase selective nature of the diffraction measurements enables the simultaneous determination of
the phase specific residual stresses of all contributing crystalline phases within one experiment. The application of
white beam diffraction implies recording of a multitude of diffraction lines within the energy range of the provided
energy spectrum of the white beam. By this means phase specific residual stress depth distributions up to distances of
150 ìm below the surface can be analysed for steel using the energy dispersive set-up of the HMI-beamline EDDI at
the Bessy site, Berlin, providing an energy range between 20-150 keV. As a side effect quantitative phase analysis can
be carried out using white energy dispersive diffraction e.g. the determination of the content of retained austenite in the
weld.
Novel martensitic filler materials with specially adjusted martensite start temperatures (Ms) can counteract the cooling specific shrinkage due to expansion effects of the weld metal associated with phase transformations. That can be exploited to create compressive residual stresses in the weld and adjacent areas, i.e. beneficial for increasing fatigue strength. The Ms temperature is shifted via the chemical composition, mainly by the alloying elements nickel and chromium, resulting as well in different retained austenite contents. Investigations were made using different Low Transformation Temperature (LTT) alloys with varying nickel content. The resulting phase transformation temperatures were - for the first time - detected using high energy synchrotron diffraction and Single Sensor Differential Thermal Analysis (SS-DTA). Compared to angle dispersive diffraction, energy dispersive diffraction offers the possibility to measure residual stresses of the martensite and austenite phase parallel fast in one experiment up to depths of 100 µm. The residual stresses show significant distributions dependent on Ms temperature. The effect on the cold cracking behaviour of these alloys was investigated using the Tekken test. Results show that cold cracking can be avoided when appropriate contents of retained austenite are existent.
Stress build up in low transformation temperature filler material under defined restraint conditions
(2009)
For the safety and cost efficiency of welded high-strength steel structures, precise knowledge of the level and distribution of welding- and cooling-specific stresses and residual stresses is essential, since they exert a decisive influence on strength, crack resistance, and finally on the bearable service load. This paper presents innovative filler materials, of which the phase transformation temperature was deliberately adjusted via the chemical composition. The transformation behaviour of these martensitic Low Transformation Temperature (LTT-) filler materials shows direct effects on the local residual stresses in the weld and the HAZ. These effects can purposefully be exploited to counteract the thermally induced shrinkage of the material and to produce significant compressive residual stresses in the weld. Comparative welding experiments were carried out on 690 MPa high-strength base materials using various LTT-filler materials. High energy synchrotron radiation was used for residual stress measurement. Particularly the use of high energy synchrotron radiation makes it possible to detect the residual stress condition fast without destruction of material. Thereby, residual stress depth gradients can be determined simultaneously without removing material. In steel, gradients of up to 150 µm can be resolved in such a way. Furthermore, the application of high energy radiation permits determination of residual stresses of any available residual austenite contents. Results show significant dependence of transformation temperatures on the resulting residual stress level and distribution.
Cold cracking resistance is a relevant evaluation criterion for welded joints and affected by residual stresses which result from the welding procedure. Compressive residual stresses can thereby have a positive influence on preventing cracking. A unique possibility of generating compressive residual stresses already during the welding procedure is offered by the so-called Low Transformation Temperature (LTT) filler wires. Compared to conventional wires, these materials show decreased phase transformation temperatures which can work against the cooling-specific contraction. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Investigations were carried out to determine the phase transformation behaviour of different LTT-filler materials. Transformation temperatures were identified using Single Sensor Differential Thermal Analysis (SS-DTA). Additionally Synchrotron radiation was used to measure the transformation kinetics of all involved crystalline phases during heating and cooling of a simulated weld thermal cycle.
This international round robin test served to scrutinize the procedures specified in ISO/DIS 3690:2009 for determining the diffusible hydrogen content in weld metals with bcc-lattice structure. It was specifically intended to check in what respect the specifications defined in the indicated standards for specimen preparation, storage and hydrogen analysis provide comparable measurement results. The round robin test is presented comprising comparative measurements at various degassing temperatures using hot extraction techniques and a thermal conductivity detector (TCD). A major focus of this investigation was the examination of the maximum degassing temperature for analysing the diffusible hydrogen in materials with bcc-lattice structure. The analyses were performed using two different stick electrodes and three different filler wires. As a significant result it was found that no deviations or increases, were detected in the measured contents of diffusible hydrogen for the investigated degassing temperatures ranging between 45 degrees C and 400 degrees C. Hydrogen analyses for contents below HD = 1.5 ml/100 g with the hot extraction techniques in conjunction with TCD applied in this study led to considerable relative standard deviations.
Eine Anzahl verschiedener Effekte ergibt sich aus der Anwesenheit von Wasserstoff während des Schweißens hochlegierter Stähle. Die Betriebsdauer von geschweißten Bauteilen ist außerdem stark von der Anwesenheit von Wasserstoff im Umgebungsmedium und der Anfälligkeit der verschiedenen Schweißnahtgefüge für eine Degradation ihrer Eigenschaften durch Wasserstoff abhängig. Als eine relative neue Werkstoffgeneration finden supermartensitische hoch legierte Stähle (Supermartensitic Stainless Steels - SMSS) zunehmend als Ersatz für teuere Legierungen insbesondere in der Öl- und Gasindustrie Verwendung. Als Konsequenz ihres martensitischen Gefüges sind diese Legierungen anfällig für eine wasserstoffunterstützte Rissbildung (Hydrogen Assisted Cracking - HAC). Der Widerstand von supermartensitischen Stählen gegen wasserstoffunterstützte Spannungsrisskorrosion (Hydrogen Assisted Stress Corrosion Cracking - HASCC) unter Sauergasbedingungen wurde vor allem für industrielle Einsatzzwecke extensiv untersucht. Solche Studien vornehmlich an Grundwerkstoffen basieren überwiegend auf Standard-Prüfverfahren. Dem gegenüber würde das grundsätzliche Verhalten von Wasserstoff in den Gefügen geschweißter supermartensitischer Stähle wenig untersucht. Die zentralen Gründe für die diesem Beitrag zugrunde liegende Studie waren daher, die Effekte des Wasserstoffs auf das Gefüge von Wolfram Inert Gas (WIG)-Schweißungen supermartensitischer Stähle und die entsprechenden Wasserstoff-Trapping-Mechanismen zu untersuchen. Die Wirkungen des Wasserstoffs auf die verschiedenen WIG-geschweißten Gefüge wurden mittels Röntgendiffraktometrie, Lichtmikroskopie und Rasterelektronenmikroskopie untersucht. Eine Anzahl von Verfahren wurde außerdem angewendet, um den absorbierten Wasserstoff quantitativ zu bestimmen. Die Wechselwirkung zwischen Wasserstoff mit den mikrostrukturellen Defekten und die Charakteristika der Wasserstoffdesorption wurden mittels Thermischer Desorptionsspektroskopie (TDS) und Trägergas-Heißextraktionen des Wasserstoffs (LECO Analyse) untersucht. Die Wirkung des Gefüges auf die Absorption und Desorption von Wasserstoff werden im Detail diskutiert.
Cold cracking resistance is a relevant evaluation criterion for welded joints and affected by residual stresses which result from the welding procedure. Compressive residual stresses can thereby have a positive influence on preventing cracking. A unique possibility of generating compressive residual stresses already during the welding procedure is offered by the socalled Low Transformation Temperature (LTT) filler wires. Compared to conventional wires, these materials show decreased phase transformation temperatures which can work against the cooling-specific contraction. In consequence, distinct compressive residual stresses can be observed within the weld and adjacent areas. The strength of these fillers makes them potentially applicable to high-strength steel welding. Investigations were carried out to determine the phase transformation behaviour of different LTT-filler materials. Transformation temperatures were identified using Single Sensor Differential Thermal Analysis (SS-DTA). Additionally Synchrotron radiation was used to measure the transformation kinetics of all involved crystalline phases during heating and cooling of a simulated weld thermal cycle.
Zugeigenspannungen, wie sie beim Schweißprozess durch inhomogene Temperaturverteilungen und Schrumpfungen hervorgerufen werden, können die Lebensdauer geschweißter Verbindungen signifikant herabsetzen. Eine neue und außerordentlich attraktive Methode, um Druckeigenspannungen bereits während des Schweißens gezielt einzustellen, gelingt mit sogenannten LTT (Low Transformation Temperature)-Legierungen. LTT-Legierungen weisen eine martensitische Phasenumwandlung bei relativ niedrigen Temperaturen auf, wobei die damit verbundene Volumenexpansion zu einer Reduktion der Schrumpfeigenspannungen bzw. Erzeugung von Druckeigenspannungen führt. Zum direkten Nachweis der Phasenumwandlungen und der damit verbundenen resultierenden Schweißeigenspannungen wurden erstmals In-situ-Schweißexperimente unter Nutzung hoch energetischer, polychromatischer Synchrotronstrahlung (Weißstrahl) realisiert, um die Umwandlungskinetik während eines realen Schweißprozesses und die daraus resultierenden Schweißeigenspannungen zu analysieren. Es wird gezeigt, dass mit LTT-Legierungen signifikante Druckeigenspannungen in der Schweißnaht erreicht werden.
In this study, hydrogen absorption and storage was investigated for various high-alloyed ferritic-austenitic duplex stainless steels. On account of the specific transformation and solidification behaviour, respectively, of duplex stainless steels as compared to single-phase ferritic and austenitic steels, special conditions have to be considered concerning hydrogen absorption which may ultimately lead to microstructure-dependent hydrogen-assisted weld metal cracking. Hydrogen absorption during welding may occur via the shielding gas, moisture from the surroundings or via the welding filler material. As a contribution to the interpretation and prediction of hydrogen-induced cracking in welded duplex stainless steels, the actual hydrogen absorption via the arc as well as the weld metal hydrogen diffusion was investigated in a duplex stainless steel DSS (1.4462) and in a lean-duplex stainless steel LDS (1.4162). Isothermal heat treatment using carrier gas hot extraction enabled quantification of the amounts of hydrogen trapped in the respective microstructures. The total hydrogen concentrations were found to be nearly identical. Trapped hydrogen was however observed to be dependent on the material and on the microstructure condition. The influence of hydrogen on the mechanical properties of the weld metal was characterized with the help of tensile tests. In addition, hydrogen embrittlement was detected in scanning electron microscopic analyses.
Laser-gas metal arc (GMA)-hybrid welding enables a high deposition rate and a stable welding process. Process stability was found to be profoundly dependent on the type of the applied filler wire (solid or tubular cored). Whereas solid wires are deposited in pulsed arc mode, tubular cored wire electrodes are weldable by a stable spray-arc process within a wide operating range. This fact applies above all to a high alloyed metal cored wire electrode, of which the achievable deposition rates exceed the upper limits recommended by the manufacturer. A further interesting aspect is the application of a rutile tubular cored wire for positional laser-GMA-hybrid welding. A modern 20 kW-fibre laser source was available for the experiments. This study is concerned with the weldability of high alloyed tubular cored electrodes for high power laser-GMA-hybrid welding. Results obtained from comparative investigations between tubular cored and solid wire electrodes relating to process stability are summarised.