Experimental processing of FeCr sigma-phase powders
M.B. Cortie, J. Kerr, S. Nana
MATERIALS FORUM
Abstract
Ultra-ferritic Fe-35 to 40%Cr stainless steel may be completely converted to sigma phase by thermo-mechanical processing at temperatures in the range 600 to 900 C. The resultant intermetallic compound is brittle and readily milled to powder by conventional techniques. In this form it can be processed and shaped like a ceramic powder. The green component can then be converted back into metallic form by an appropriate heat treatment that raises the temperature above that at which sigma phase is stable, causing the material to revert to the ferritic structure. Under appropriate conditions, sintering and consolidation of the compact can be arranged to occur at this time. The sintering treatment must be followed by a fast cool to room temperature in order to prevent reversion to sigma phase. Prototype components have been produced by powder metallurgy and thermal spraying, and the concept has been extended to the production of simple shapes by laser-based rapid-prototyping.
Key words:intermetallic; South Africa; stainless steels; sigma phase; thermal spraying
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