Vacaero

Manufacturers of heat treating and brazing vacuum furnaces and controls, complete hot zone and vacuum furnace retrofits, thermal spray coatings, plasma, HVOF and paint coating services.

VAC AERO Service Experience Trust
Canada FrançaisAccessibility |

Metallographic Specimen Preparation for Electron Backscattered Diffraction

by

Although some publications have claimed that mechanical specimen preparation is inadequate for producing damage-free specimens for EBSD, this is certainly not true. Our methods have concentrated upon producing the best possible surfaces using an automated grinder-polisher with standard consumable products in a reasonable amount of time and at low cost. Furthermore, these methods are highly reproducible as demonstrated by extensive tests on many metals and alloys from aluminum to zirconium.

Success depends first, and foremost, upon limiting cutting damage by using the proper blade and cutter. Next, commence grinding with the finest abrasive that will remove the cutting damage in a reasonable time and make all of the specimens in the holder co-planar. Polishing is done in counter rotation with a low holder rotational speed to keep the cloth as uniformly covered with abrasive and lubricant as possible. The grinding and polishing steps must keep the surface perfectly flat for best results. After the final polish, a general purpose etch can be applied, with the specimens in the holder, to evaluate the success of the preparation and determine what the structure is. Then, remove the etched surface by repeating the final step but with about half the required time. Preparation procedures are influenced by the crystal structure of the specimen. Face-centered cubic specimens will always exhibit more damage from each step than less ductile HCP and BCC metals and alloys.

Picric Acid – Hazards & Safe Usage

by

Picric acid (2,4,6-trinitrophenol, [(NO2)3C6H2OH]) is widely used in metallography labs for the common steel etchants known as picral, a 4% solution in ethanol, Vilella’s reagent, 1 g picric acid and 5 mL HCl and 100 mL ethanol,  and alkaline sodium picrate (2 g picric acid, 20 g NaOH, 100 mL water) for coloring M3C and M6C carbides, as well as several other formulations.

Picric acid was formulated by Peter Woulfe, a British chemist, in 1771, although Glauber is claimed to have written about it in 1742. The name comes from the Greek word pikros which means bitter, as picric acid has a bitter taste (it is toxic). Initially it was used to dye fabrics yellow. In the early 20th century, workers producing picric acid were sometimes called canaries, because their skin also became stained yellow.

Identification of Phases in Stainless Steels by Etching

by

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

by

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

by

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

by

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

by

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.