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Archives for June 2011

Identification of Phases in Stainless Steels by Etching

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page-2-1_wsNumerous etchants have been used to selectively reveal matrix phases and second-phase constituents in ferritic, martensitic, ferritic-martensitic, austenitic, ferritic-austenitic (duplex) and precipitation hardenable stainless steels. Procedures for identification of second-phases, such as carbides, sigma and chi, and delta ferrite in austenitic or precipitation hardenable stainless steels using selective etchants are described.

The microstructural constitution of stainless steel is quite complex and exposure to high operating temperatures adds to the complexity as a variety of phases can be observed. In addition to the matrix phases of ferrite, austenite and martensite, and duplex austenite-ferrite and (less commonly) ferrite-martensite, there are numerous possible minor constituents.  In the carbide family, M23C6 (face-centered cubic) and M7C3 (hexagonal) carbides are the most common, but M6C (face-centered cubic) and MC (face-centered cubic) carbides, are also observed in certain alloys. Certain nitrides may be observed and the intermetallic phases, sigma, σ (tetragonal), chi, χ (body-centered cubic) and Laves, η (hexagonal).

A Layman’s View of Plasma Spray Coating Metallography

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The paper describes the approach taken in the author’s first metallographical study of plasma sprayed coating. After consideration of the preparation problems known to be associated with porous materials, the coating materials and the substrate material, and coatings in general, literature on metallographic studies of plasma sprayed coatings was identified, obtained and reviewed.

A coating section of each type was carefully removed from the substrate, refrigerated in liquid nitrogen, broken and examined with the SEM to reveal the nature and degree of porosity and bonding defects. Specimens were prepared using two different approaches and two variations of one of the approaches. Polishing was conducted using DP-Plan cloths and two different diamond sizes followed by two stages of diamond polishing with conventional cloths and finally using colloidal silica on a vibratory polisher. Vibratory polishing time must be critically controlled for two of the three coatings. Conventional etchants were tried to enhance microstructural detail but the Pepperhoff interference layer method was found to be more useful.