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Identification of Phases in Stainless Steels by Etching

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

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.

Color Metallography

octahedride-grain_1-wsColor has historically seen limited use in metallography, mainly due to the cost of film and prints and the difficulty and cost of reproducing images in publications. However, with the growth of digital imaging, capturing color images is much simpler and cheaper. Also, printing images in color is inexpensive for in-house reports, and can be distributed cheaply on CDs, although reproduction in journals is still expensive. Color does have many advantages over black and white. First, the human eye is sensitive to only about forty shades of gray from white to black, but is sensitive to a vast number of colors. Tint etchants reveal features in the microstructure that often cannot be revealed using standard black and white etchants. Color etchants are sensitive to crystallographic orientation and can reveal if the grains have a random or a preferred crystallographic texture. They are also very sensitive to variations in composition and residual deformation. Further, they are usually selective to certain phases and this is valuable in quantitative microscopy. By George Vander Voort

(Courtesy of the Microscopy Society of America – www.microscopy.org) – Original Published by Microscopy Today, November 2005 – Volume 13, Number 6 – www.microscopy-today.com

 

Metallography of Surface Treatments & Coatings

1center-wsA wide variety of surface treatments and coatings are applied to metals to enhance their performance, for example, to improve fatigue resistance, increase wear resistance, corrosion or oxidation resistance.

Some of these treatments involve diffusion of one or more elements into the metal or alloy followed by post heat treatments. These processes included the familiar processes of carburizing, nitriding, and carbonitriding but also included less familiar processes such as ion nitriding, chromizing and boronizing. There are also a variety of coatings that are deposited by electroless or electrolytic means, or by physical or chemical vapor deposition, or by thermal or plasma spray.  The technological significance of these processes is enormous. By George Vander Voort

Microstructure of Ferrous Alloys

The microstructure of iron-base alloys is very complicated and diverse, being influenced by chemical composition, material homogeneity, processing and section size. This article offers a brief explanation of the terminology describing the constituents in ferrous alloys, and offers a basic review of steel microstructures.

Microstructures of castings look different from those of wrought products, even if they have the same chemical composition and are given the same heat treatment. In general, it is easiest to identify heat-treated structures after transformation and before tempering. For example, if a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature used for the product increases toward the lower critical temperature. Further, ferrous metallographers tend to use nital almost exclusively for etching, but nital is not always the best reagent to use to properly reveal all microstructures.

Metallography of Superalloys

While specimen preparation of superalloys for metallographic examination is relatively straightforward, the metallographer must take into consideration some of the inherent characteristics of these complex alloys, such as high toughness, presence of large amounts of strengthening phase and high corrosion resistance, to ensure getting samples that clearly reveal their complex microstructures.

Superalloys are complex alloys of Fe-Ni, Ni-, and Co-base compositions. Their microstructure can be quite complex due to the potential for a variety of phases that can form in heat treatment or service exposure conditions. This article discusses the use of new metallographic materials to prepare these alloys and the different etchants required to reveal the structure of these alloys properly as a function of alloy composition, heat treatment and microstructural phases. This discussion is limited to iron-nickel and nickel-base alloys, but most of the comments are also applicable to Co-base alloys.