Influence of nitrogen-doped shielding gas for welding of medium manganese austenites for cryogenic applications

Abstract The relevance of technical gases is constantly increasing due to environmental and climate policy requirements. The storage and transport of liquefied natural gas (LNG) takes place under cryogenic conditions with substantial volume reduction due to significant storage efficiency. Consequently, there are specific requirements for the mechanical properties of the applied materials at cryogenic temperatures. The materials used so far have been cold-hard, high-nickel austenites, and martensitic steels of the X8Ni9 type. The austenitic materials offer good processing properties, but due to their comparatively low strength and high costs, these materials are not attractive. In contrast, the welding of the martensite has a significant negative impact on the processing quality and the automation of the process. In order to address the problems of automation, material costs, and mechanical properties, particularly high strength and cold toughness, the application of innovative austenitic steels with a medium as well as a high manganese content is suggested. For the qualification of medium manganese austenites as a substitute for maritime applications, the welding processing of an X2CrMnNiN17-7–5 (1.4371) is considered under the aspects of the standards and guidelines of the International Association of Classification Societies (IACS), the International Maritime Organization (IMO) and the classification society Det Norske Veritas (DNV). Preliminary investigations have revealed that the application of a conventional filler metal of type G 20 16 3 Mn N L in combination with nitrogen doping of the M12-ArC-2.5 shielding gas according to DIN EN ISO 14175 results in a significant enhancement of the mechanical properties. The addition of 4% nitrogen into the shielding gas caused a diffusion-induced increase in the nitrogen content of the weld metal and a simultaneous increase in strength. Additionally, there were no negative influences on the austenitic microstructure. In consideration of the qualification specifications for welded joints and filler metals of the standards and guidelines according to IACS, IMO, and DNV, the joint welds are examined, and the qualification of the welding process is envisaged. For this purpose, comparative welds of the mentioned material combination are carried out and compared with both the initial gas mixtures and the nitrogen-doped shielding gas.