A literature review of the stainless steel 21-6-9 and its potential for sandwich nozzles

Abstract

Volvo Aero Corporation has developed a laser welding technique for sandwich nozzles which will have advantages in high repeatability, only commercially available material used and flexibility over other alternative space rocket nozzle manufacturing methods. This master thesis is about the nitrogen strengthened high manganese stainless steel 21-6-9 (a.k.a. Nitronic 40) and its suitability as the main material for a sandwich nozzle and/or its parts (e.g. manifold and stiffeners). A literature review over the alloys physical, mechanical and process properties was performed and an experimental investigation of weld properties regarding laser welded tensile test, x-ray diffraction measurement on the retain delta-ferrite content and nitrogen diffusion during TIG welding was also performed. The conclusions from this master thesis are that the alloy 21-6-9 seems to fulfil most of the properties for a sandwich nozzle, but the retained delta-ferrite in welds and especially the ability of the metastable austenite to undergo strain and stress- induced martensite transformation at low temperatures could be described as potential “show stoppers” and must be further investigated. Feasibility tests are proposed for an evaluation of the possible delta- ferrite and martensite problems: • Cryogenic fracture toughness measurements on 21-6-9 welds. • LCF measurements with hydrogen environment or at cryogenic temperature for welded and unwelded samples. • Examine if martensite transformation can occur in sandwich nozzles made of 21-6-9. Other proposed and important tests: • Evaluate the risk of solidification cracking under high degree of restraint. • Evaluate a heat-treatment procedures for minimum grain growth when stress-relieving and for possible delta-ferrite reduction.Validerat; 20101217 (root