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

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

The fracture mechanisms in duplex stainless steels at sub-zero temperatures

The aim of the thesis was to study the susceptibility for brittle failures and the fracture process of duplex stainless steels at sub-zero temperatures (°C). In the first part of the thesis plates of hot-rolled duplex stainless steel with various thicknesses were used to study the influence of delamination (also known as splits) on the fracture toughness. The methods used were impact and fracture toughness testing. Light optical microscopy and scanning electron microscopy were used to investigate the microstructure and fracture surfaces. It was concluded that the delaminations caused a loss of constraint along the crack front which resulted in a stable fracture process despite the presence of cleavage cracks. These delaminations occurred when cleavage cracks are constrained by the elongated austenite lamellae. The pop-in phenomenon which is frequently observed in duplex stainless steels during fracture toughness testing was shown to occur due to these delaminations. The susceptibility for pop-in behaviour during testing increased with decreasing plate thickness. The toughness anisotropy was also explained by the delamination phenomenon.In the second part of the thesis duplex stainless steel weld metals from lean duplex and super duplex were investigated. For the lean duplex weldments with different nickel contents, tensile, impact and fracture toughness testing were conducted from room temperature to sub-zero temperatures. The result showed that increased nickel content decreased the susceptibility for critical cleavage initiation at sub-zero temperatures. The super duplex stainless steel weldment was post weld heat treated. The fracture sequence at low temperature was critical cleavage fracture initiation after minor crack-tip blunting and ductile fracture. Energy-dispersive X-ray spectroscopy investigation of the weld metals showed that substitutional element partitioning is small in the weld metal. However, for the post weld heat treated weldments element partitioning occurred which resulted in decreased nickel content in the ferrite.QC 20131108</p