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Eingeladener Vortrag
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Duplex stainless steels (DSS) are frequently used, especially in applications requiring high strength combined with high corrosion resistance in aggressive media. Examples include power plant components and maritime structures. During welding of these steels, local variations in chemical composition can occur. This results in ferritization of the material and negatively affects the mechanical properties of the components. In this work, tungsten inert gas (TIG) welding experiments were performed with DSS. Chemical composition analysis was realized in situ by using Laser Induced Breakdown Spectroscopy (LIBS). The aim of the work is to quantitatively measure the chemical composition in the weld seam of various DSS and to identify possible influences of welding parameters on the microstructure of the material. The chemical concentrations of the main alloying elements Cr, Ni, Mn on the surface of the sample during the welding process and the cooling process were measured. Mn and Ni are austenite stabilizers and their content increases during welding by using certain high alloyed filler material. Spectra were recorded every 1.3 s at a spacing of approximately 2 mm. During the cooling process the location of the measurement was not changed. The LIBS method is proofed to be suitable for the quantitative representation of the chemical compositions during the welding process.
Nichtrostende Duplexstähle (DSS) werden häufig verwendet, insbesondere bei Anwendungen, die eine hohe Festigkeit in Verbindung mit einer hohen Korrosionsbeständigkeit in aggressiven Medien erfordern. Beispiele hierfür sind Kraftwerkskomponenten und maritime Anwendungen. Beim Schweißen dieser Stähle kann es zu lokalen Schwankungen in der chemischen Zusammensetzung kommen. Dies führt zu einer Ferritisierung des Werkstoffs und beeinträchtigt die mechanischen Eigenschaften der Bauteile. In dieser Arbeit wurden Versuche zum Wolfram-Inertgas-Schweißen (WIG) mit DSS durchgeführt. Die Analyse der chemischen Zusammensetzung wurde in situ mit Hilfe der laserinduzierten Plasmaspektroskopie (LIBS) durchgeführt. Ziel der Arbeit ist es, die chemische Zusammensetzung in der Schweißnaht verschiedener DSS quantitativ zu messen und mögliche Einflüsse der Schweißparameter auf die Mikrostruktur des Werkstoffs zu ermitteln. Gemessen wurden die chemischen Konzentrationen der Hauptlegierungselemente Cr, Ni, Mn auf der Oberfläche der Probe während des Schweißprozesses und des Abkühlungsprozesses. Mn und Ni sind Austenitstabilisatoren, deren Gehalt beim Schweißen durch die Verwendung bestimmter hochlegierter Schweißzusatzwerkstoffe zunimmt. Die Spektren wurden alle 1,3 s in einem Abstand von etwa 2 mm aufgezeichnet. Während des Abkühlungsprozesses wurde der Ort der Messung nicht verändert. Die LIBS-Methode eignet sich nachweislich zur quantitativen Darstellung der chemischen Zusammensetzungen während des Schweißvorgangs.
Duplex stainless steels (DSS) are used in all industries where corrosion problems play a major role. Examples include the chemical industry, the food industry and shipping industries. DSS have a balanced phase ratio of ferrite (α) and austenite (γ). Unlike single-phase stainless steels, DSS combine the advantages of these and can therefore fit many industry requirements, such as weight saving or high mechanical strength. When these steels are welded, alloying elements can burn off and condense as thin layers on cold surface regions. This loss of chemical elements can lead to changes in the microstructure. With the help of Laser-Induced Breakdown Spectroscopy (LIBS), chemical element distributions were visualized. The results were compared with those of conventional measurement methods, such as energy dispersive X-ray analysis (EDS) and X-ray fluorescence analysis (XRF), and the results from LIBS could be validated. LIBS is suitable as a fast, straightforward measurement method for producing line scans along the weld seam and provides spatially resolved information on accumulation phenomena of burned off alloying elements. LIBS is very well suited for the detection of sub-surface elements due to the exclusively superficial ablation of the material. In addition, the measurement method has been calibrated so that quantitative statements about element concentrations can also be made.
Duplex stainless steels (DSS) are frequently used, especially in applications requiring high strength combined with high corrosion resistance in aggressive media. Examples include power plant components and maritime structures. During welding of these steels, local variations in chemical composition can occur. This results in ferritization of the material and negatively affects the mechanical properties of the components. In this work, tungsten inert gas (TIG) welding experiments were performed with DSS. Chemical composition analysis was realized in situ by using Laser Induced Breakdown Spectroscopy (LIBS). We could quantitatively measure the chemical composition in the weld seam of various DSS and identify possible influences of welding parameters on the microstructure of the material. The chemical concentrations of the main alloying elements Cr, Ni, Mn on the surface of the sample during the welding process and the cooling process were measured. Mn and Ni are austenite stabilizers and their content increases during welding by using certain high alloyed filler material.
Erstellung von Tiefenprofilen auf Schweißnähten mittels Laserinduzierter plasmaspektroskopie (LIPS)
(2021)
Duplexstähle sind hochlegierte, nichtrostende Stähle. Sie bieten aufgrund des ausgewogenen Phasenverhältnisses von Austenit und Ferrit einige physikalische Vorteile gegenüber anderen nichtrostenden Stählen. Beim Schweißen können sich durch die Kondensation von Schweißrauch und verdampftem Material verschiedene chemische Elemente auf der Oberfläche der erstarrten Schweißnaht anreichern. Dann ist die Bildung einer schützenden Chromoxidschicht nicht mehr gewährleistet und es kann an diesen Stellen zu Lochfraß kommen. Frühere Arbeiten haben gezeigt, dass die Anreicherung von Mangan und Chrom an der Oberfläche der Wärmeeinflusszone des geschweißten Edelstahls 304L mit LIBS gemessen werden kann. Wir stellen die Ergebnisse der Optimierung der LIBS-Parameter für präzise Dickenmessungen solcher dünnen Schichten und für Tiefenprofilmessungen vor. Dazu haben wir galvanisch beschichtete Kupferproben mit bekannten Schichtdicken verwendet. Die Mangankonzentration im Schweißgut ist reduziert. Dies hat einen großen Einfluss auf die metallografische Struktur des Materials.
Many applications in industry require a material-to-material joining process of Duplex Stainless Steels (DSS). Therefore, it is essential to investigate the material’s properties during a welding process to control the weld quality. With the help of Laser-Induced Breakdown Spectroscopy (LIBS), the chemical composition during the Tungsten Inert Gas (TIG) welding process of DSS could be monitored in situ. The chemical composition could be quantitatively measured using pre-established calibration curves. Although the surface temperature and the welding plasma have a high influence on the spectral intensities, reliable composition measurements were possible. The concentration of alloying elements could be mapped during the TIG welding process.
Duplex steels are high-alloyed, stainless steels. They offer some physical advantages over other stainless steels due to the balanced phase ratio of austenite and ferrite. During welding processes, when welding fumes and vaporized material condensates, different chemical elements can accumulate on the surface of the solidified weld. Then, the formation of a protective chromium oxide layer is no longer guaranteed and pitting corrosion can occur at these places. Previous work has shown that the accumulation of manganese and chromium on the surface of the heat-affected zone of the welded high-grade steel 304L can be measured by LIBS.
We present the results of the optimization of LIBS parameters for precise thickness measurements of such thin films and for depth profile measurements Therefore, we used galvanically coated copper samples with known film thicknesses. The Concentration of manganese is reduced in the weld metal. This has a high impact on the metallografic structure of the material.
Welding processes of duplex stainless steels cause an unbalanced austenite (γ)/ferrite (δ) ratio due to high cooling rates and changes in chemical composition. That causes a degradation of mechanical properties and corrosion resistance. In situ monitoring of the weld pool is to be realized with the help of laser-induced breakdown spectroscopy (LIBS). A major advantage of this technique is the highly accurate time and spatially resolved measurement of the chemical composition during welding. Previous research has established that the LIBS method is suitable to detect chemical elements during welding and to show a distribution of selected elements. Chemical composition in the WM and HAZ can now be quantified using calibration curves generated by certified reference materials (CRM). Furthermore, a cooling rate can be plotted against the measured electron temperature.
Measurement of hydrogen concentration in steels by laser-induced breakdown spectroscopy (LIBS)
(2021)
Current efforts to achieve lightweight construction and the required reduction in CO2 emissions and an increase in energy and resource efficiency call for the increasing use of high-strength fine-grain structural steels. However, as the strength of higher-strength fine-grain structural steels increases, so do the associated joining challenges. Particular attention must be paid to hydrogen-assisted cold cracking. The influence of hydrogen reveals itself less in the strength but has a significant effect on the deformability [1, 2]. The degradation of the material properties can lead to zero ductility, where the values of the yield strengths coincide with the tensile strengths.
Laser-induced breakdown spectroscopy (LIBS) is a spectroscopic technique that can be used to determine elemental compositions without pre-treatment of the samples. Short, high-energy laser pulses ablate a small volume (< 0.1 mm3) of the examined material and ionize it to form a plasma. The decaying plasma emits element-specific light. This light is spectroscopically analysed and allows to detect qualitatively the present elements and to quantify them with help of a standardization routine. The simple experimental set-up and the fast, nearly non-destructive analysis procedure characterize the LIBS analysis. The LIBS method allows a time and spatially resolved in situ measurement of steel components in use.
Even low hydrogen concentrations (~ 1 wt.-ppm) in steel can be measured with the LIBS method and can be quantified with help of certified reference materials. The results are compared with results gained with the well-established carrier gas hot extraction method.
Schweißprozesse von nichtrostenden Duplexstählen verursachen aufgrund hoher Abkühlungsraten und Änderungen der chemischen Zusammensetzung ein unausgewogenes Austenit (γ)/Ferrit (δ)-Verhältnis. Das verursacht eine Degradation der mechanischen Eigenschaften und der Korrosionsbeständigkeit. Die In-situ-Überwachung des Schweißbades soll mit Hilfe der laserinduzierten Plasmaspektroskopie (LIPS) realisiert werden. Ein großer Vorteil dieser Technik ist die hochgenaue zeit- und ortsaufgelöste Messung der chemischen Zusammensetzung während des Schweißens. Bisherige Untersuchungen haben ergeben, dass die LIBS-Methode geeignet ist, chemische Elemente während des Schweißens zu detektieren und eine Verteilung ausgewählter Elemente aufzuzeigen. Chemische Zusammensetzungen können mit vorab erstellten Kalibrierkurven dargestellt weerden.