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IIW-2326
Austenitic and austenitic-ferritic CrMnNi-stainless steels are suitable materials in the transport and automotive industry due to their high corrosion resistance and high strength that allows weight and cost savings. This study focuses on the laser weldability of a commercial lean duplex and an austenitic high manganese stainless steel. The impact of different laser sources, i.e. a 5 kW CO2- and a 4 kW Nd:YAG-laser, and of the main process parameters on the resulting weld quality will be investigated. One important aspect will concern the appearance of weld defects such as pores and hot cracks. The factors causing such internal imperfections will be analysed in order to find effective methods for preventing them. Weld microstructure and the associated corrosion and mechanical properties will be assessed with different techniques and adequate process parameters for high quality welds will be determined. The advantages and limitations of the applied welding processes will be evaluated for future applications.
Due to significant developments over the last decades, laser beam welding has become a well-established industrial process offering high processing speeds and causing low component distortions. But an important issue currently preventing its intense use, especially in the energy or plant construction sector where high alloy steels are applied, concerns hot crack formation. Although considerable advances in understanding hot cracking mechanisms have been made, most of the known influencing factors are metallurgical in character. The thermo-mechanical effects are barely considered or quantified. Up to the present, there exist numerous hot cracking tests that were however conceived for welding methods other than laser beam welding. Considering the special features of the laser welding process, such as high cooling rates and the narrow process zone, results obtained with other welding techniques and test procedures cannot be transferred to laser beam welding. In this study, the laser beam weldability of various stainless steels was examined in terms of their susceptibility to hot cracking by means of the controlled tensile weldability test, which was proven to be suitable for use in conjunction with CO2 laser welding. This test allows the application of tensile strain at a variable fixed cross-head speed transverse to the welding direction. Full and partial penetration bead-on-plate welds were produced. In a first attempt to determine the impact of the applied external strain on the local transient strains and strain rates near the weld pool, an optical system was used to measure the backside surface of partial penetration welds. The results showed the influence of the strain and the strain rates on hot crack formation. Furthermore, a classification of the studied austenitic, duplex and ferritic stainless steels according to the established test criteria (critical strain and cross-head speed) was conducted.
Thick-walled pipes made of steel are the basis of a large number of components in mechanical and installation engineering, e.g. for hydraulic components, in power station technology or in the petroleum industry. Particularly when strict positional and shape tolerances are demanded, the classical manufacturing process is chip-producing machining from the solid material. However, this is time-intensive and material-intensive. The laser/GMA hybrid processes with modern high-power lasers are a suitable alternative. With these processes, it is currently possible to weld components with a wall thickness up to approx. 15 mm with little distortion in one pass. However, in initial experiments, solidification cracks with longitudinal and vertical orientations in relation to the welding direction, so-called central line cracks, were established in welds on pipes. Within the framework of a research project, it was possible to determine the cause of the cracking and to find measures in order to avoid it. The technical boundary conditions could be complied with in this respect. The article gives an overview of the research project and the essential results.
Dickwandige Rohre aus Stahl sind die Basis einer Vielzahl von Komponenten im Maschinen- und Anlagenbau, zum Beispiel bei Hydraulikkomponenten, in der Kraftwerkstechnik oder der Petroindustrie. Insbesondere, wenn hohe Lage- und Formtoleranzen gefordert werden, ist das klassische Herstellungsverfahren ein Zerspanen aus dem Vollmaterial. Dies ist jedoch zeit- und materialintensiv. Als Alternative bieten sich die Laserstrahl-MSGHybridverfahren mit modernen Hochleistungslasern an. Mit diesen Verfahren ist es derzeit möglich, Bauteile bis zu etwa 15 mm Wanddicke verzugsarm und einlagig zu verschweißen. In ersten Experimenten wurden bei Schweißungen an Rohren jedoch in Bezug auf die Schweißrichtung längs und vertikal orientierte Erstarrungsrisse, so genannte Mittelrippendefekte, festgestellt. Im Rahmen eines Forschungsprojekts konnte die Ursache der Rissentstehung ermittelt und Maßnahmen zur Vermeidung gefunden werden. Dabei ließen sich die technischen Randbedingungen einhalten. Der Artikel gibt einen Überblick über das Forschungsprojekt und die wesentlichen Ergebnisse. ---------------------------------------------------------------------------------------------------------------------------------------
Thick-walled pipes made of steel are the basis of a large number of components in mechanical and installation engineering, e.g. for hydraulic components, in power station technology or in the petroleum industry. Particularly when strict positional and shape tolerances are demanded, the classical manufacturing process is chip-producing machining from the solid material. However, this is time-intensive and material- intensive. The laser/GMA hybrid processes with modern high-power lasers are a suitable alternative. With these processes, it is currently possible to weld components with a wall thickness up to approx. 15 mm with little distortion in one pass. However, in initial experiments, solidification cracks with longitudinal and vertical orientations in relation to the welding direction, so-called central rib defects, were established in welds on pipes. Within the framework of a research project, it was possible to determine the cause of the cracking and to find measures in order to avoid it. The technical boundary conditions could be complied with in this respect. The article gives an overview of the research project and the essential results.
Influence of the main laser welding parameters on process stability and resulting weld quality were investigated. Effects of weld edge preparation on the weld appearance and quality were considered. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Metallurgical characterisation of the samples regarding the resulting phase composition and hardness was conducted. Tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties. The results provide an insight into the advantages and limitations of the laser welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.
Manganese alloyed stainless steels represent a cost-effective alternative to conventional CrNi- stainless steels due to strong fluctuations of the market prices for nickel seen during the last years. In CrMnNi steels, nickel is partially replaced by lower-cost manganese and small amounts of nitrogen for stabilization of the austenitic phase. This also brings benefits regarding the mechanical properties, as it results in an increased material strength. Laser beam welding of such materials was investigated for direct comparison with standard CrNi steels. Main emphasis was laid on finding adequate process parameters to achieve a stable welding process and obtain a good weld quality. Two different laser sources, a 4.4 kW Nd:YAG and a 5 kW CO2 laser, were used to weld 1.5 mm stainless steel sheets in continuous wave mode. A high-Mn austenitic (1.4376) and a lean duplex (1.4162) steel, as well as the standard austenitic (1.4301) and duplex (1.4362) grades were selected as test materials. Both butt and lap joint configurations were studied. Experiments were carried out systematically, varying the welding speed, laser power and focal point position in order to determine adequate process windows. The influence of the shielding gas type and flow rate on the process stability and the weld quality were investigated. The effects of weld edge preparation on the weld appearance and quality levels attained were also examined. The obtained welded joints were subjected to radiographic tests for detection of internal imperfections. Also a metallurgical characterization of the samples regarding the resulting phase composition or balance and hardness depending on the welding process parameters was conducted. Furthermore, tensile and potentiodynamic tests were performed to evaluate the mechanical and corrosion properties, respectively. The results provide an insight into the advantages and limitations of the laser beam welding process for joining high-manganese alloyed stainless steels. Conditions for the production of defect-free and corrosion-resistant welds having good mechanical properties could be determined.