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Design of low nickel stainless steels


Research objectives and content
The aim of this project is to analyse and study a specific compositional range of alloying elements in order to develop austenicferritic duplex stainless steels of lower nickel contents than are found in existing 22% Cr duplex alloys in such a way that the proposed l formulations will exhibit an optimised balance between properties (mechanical, corrosion resistance and weldability), fabricability and l production (process and raw materials) cost. Said alternative duplex alloys will primarily feature lower nickel relative to the standard duplex grades in hand, and higher contents of l nitrogen, manganese and copper. Accordingly, the overall composition will be modified so that, while at least matching the common l mechanical and corrosion resistance properties, there is a solidification (porosity and castability) problem, or hot workability loss, and the | trend to brittle phase precipitation is reduced. Heat treatments will be studied in detail for designing the phase balance and phase l compositions towards optimum corrosion resistance and also for a better understanding of the kinetics of precipitate formation influencing corrosion resistance and other properties. Special emphasis will be paid to the welding area. The project is intended to disclose the actual effects of alloying elements on the austenite-ferrite microstructure and properties of these alternative duplex alloys, when they are subjected to different thermal and/or mechanical treatments, for the issue has received little study, sometimes leading to contradictory conclusions, and is clearly deserving of more attention because of its significance for new alloy designing. The main concern in the research program proposed is the determination of partitioning coefficients for the selected range of alloying elements The key tool at the microscopic level will be microanalysis. Two different instruments will be used a Scanning Electron Microscope equipped with a X-ray microprobe and an lon Beam Microscope. The combination of both instruments will provide evidences for the determination of grain composition at the different stages of the process underwent at the industrial scale for duplex alloys. Furthermore, taking advantage of the magnetic behaviour of ferrite and austenite phases a Scanning Probe Microscope equipped with magnetic probe will be used. This way a clear correlation between composition, magnetic behaviour and microhardness will be assessed.
Training content (objective, benefit and expected impact)
The requested training will suppose a complement of the current abilities acquired by the applicant during the past years. In this research project nitride chemistry plays a fundamental role as far as metal-nitrogen interactions will determine in part partitioning coefficient between phases. The knowledge acquired up to now will be profited widening the application field of nitride chemistry. Furthemmore, metallurgical processes and characterisation techniques relevant for this project as well as for materials science in general: vacuum melting, microanalysis (using both ion and electron microprobes) or scanning probe microscopy will be managed. Said this, the research training will open opportunities either in public research systems or industrial ones by close contact with regional industries devoted to the production of stainless steels. As previously stated, the field of these non-conventional alloys is currently at an experimental level. The specific research proposed is aimed to study and analyse using microprobe techniques partitioning coefficients between phases within the selected composition range as a way to design new low cost alloys. The developed materials will have to become an alternative to duplex in applications in which these are of customary use. Therefore, the low nickel duplex stainless steels will be potentially used in the off-shore plants, tanks, heat exchange tubes, marine cables, food industry, pulp and paper industry, pharmaceuticals, environmental protecting system.
Links with industry / industrial relevance (22)
This proposal is aimed to be developed as a part of a whole project in the framework of the European Community for Steel and Carbon (ECSC). It has been classified in the field F3 and given the reference number P401 5, if funded it will start in September 1 998. Said this, it is clear the industrial links of the proposal which is related to the one co-presented by Acerinox S.A. (Spain), Avesta Sheffield AB (Sweden), Creusot-Loire Industrie (France), Instituto de Soldadura e Qualidade (Portugal) and Universidad de Sevilla (Spain). In this framework, the ferrite-austenite equilibrium will be carried out by the the thermodynamic computer program Thermo-Calc, correlation between the chemical composition and the pitting corrosion will be established, the mechanical properties of the new alloys assessed, workability, corrosion resistance, yield stress, tensile strength, total elongation, etc. monitored. As a result of these measurements, experimental evidence enough for enough for performing experimental heats at the industrial level will be accumulated. performing experimental heats at the industrial level will be accumulated.


Universidad de Sevilla
S/n,avenida Americo Vespuccio
41092 Sevilla