Metallographic Etching

Image analysis work is facilitated if the etchant chosen improves the contrast between the feature of interest and everything else. Thousands of etchants have been developed over the years, but only a small number of these are selective in nature. Although the selection of the best etchant, and its proper use, is a very critical phase of the image analysis process, only a few publications have addressed this problem.

A classic example of the different behavior of etchants is given in Figure 20.2 where low-carbon sheet steel has been etched with the standard nital and picral etchants, and also a color tint etch.

Etching with 2% nital reveals the ferrite grain boundaries and cementite. Note that many of the ferrite grain boundaries are missing or quite faint; a problem that degrades the accuracy of grain size ratings. Etching with 4% picral reveals the cementite aggregates (one could not call this pearlite as it is too nonlamellar in appearance and some of the cementite exists as simple grain boundary films) but no ferrite grain boundaries. If one is interested in the amount and nature of the cementite (which can influence formability), then the picral etch is far superior to the nital etch as picral revealed only the cementite.

Tint etching with Beraha’s solution (Klemm’s I could also be used) colored the grains according to their crystallographic orientation. With the development of color image analyzers, this image can now be used quite effectively to provide accurate grain size measurements since all of the grains are colored.

Figure 20.3 shows a somewhat more complex example of selective etching. The micrograph shows the ferrite-cementite-iron phosphide ternary eutectic in gray iron.

Etching sequentially with picral and nital revealed the eutectic, Figure20.3a, surrounded by pearlite. Etching with boiling alkaline sodium picrate, Figure 20.3b, colored the cementite phase only, including in the surrounding pearlite (a higher magnification is required to see the very finely spaced cementite that is more lightly colored). Etching with boiling Murakami’s reagent, Figure 20.3c, colors the iron phosphide darkly and will lightly color the cementite after prolonged etching. The ferrite could be colored preferentially if Klemm’s I was used.

Selective etching has been commonly applied to stainless steels for detection, identification and measurement of delta ferrite, ferrite in dual phase grades, and sigma phase.

Figure 20.4 shows examples of the use of a number of popular etchants to reveal the microstructure of a dual phase stainless steel in the hot rolled and annealed condition.

Figure 20.4a shows a well delineated structure when the specimen was immersed in ethanolic 15% HCl for 30 minutes. All of the phase boundaries are clearly revealed, but there is no discrimination between ferrite and austenite. Twin boundaries in the austenite are not revealed. Glyceregia, a etchant for stainless steels, was not suitable for this grade, Figure 20.4b, as it appears to be rather orientation sensitive.

Many electrolytic etchants have been used for etching stainless steels, but only a few have selective characteristics. Of the four shown in Figures 20.4c to f, only aqueous 60% nitric acid produced any gray level discrimination between the phases, and that was weak.

All revealed the phase boundaries nicely, however. Two electrolytic reagents, shown in Figures 20.4g and h, are commonly used to color ferrite in dual phase grades and delta ferrite in martensitic grades.

Of these, aqueous 20% sodium hydroxide, Figure 20.4g, usually gives more uniform coloring of the ferrite. Murakami’s and Groesbeck’s reagents have also been used for this purpose. Tint etchants have been developed by Beraha that color the ferrite phase nicely, as demonstrated in Figure 20.4i.

Selective etching techniques are not limited to iron based alloys, although these have more thoroughly developed than for any other alloy system. Selective etching of beta phase in alpha-beta copper alloys has been a popular subject.

Selective etching has a long historical record for identification of intermetallic phases in aluminum alloys. This method was used for many years before the development of energy-dispersive spectroscopy. Today, it is still useful for image analysis work.

Figure 20.5 illustrates the structure of a simple WC-Co sintered carbide, cutting tool. In the as-polished condition, Figure 20.5a, the cobalt binder can be seen faintly against the more grayish tungsten carbide grains. A few particles of graphite are visible.

In Figure 20.5b, light relief polishing has brought out the outlines of the cobalt binder phase, but this image is not particularly useful for image analysis.

Etching in a solution of hydrochloric acid saturated with ferric chloride, Figure 20.5c, attacks the cobalt and pro- vides good uniform contrast for measurement of the cobalt binder phase.

Following this with Murakami’s reagent at room temperature reveals the edges of the tungsten carbide grains, useful for evaluation of the WC grain size, Figure 20.5d.