Fig. 1. Aged 312 stainless steel weld metal where the sigma phase was colored by etching with Murakami’s reagent at 80°C. The magnification bar is 20 µm long.

The specimens were mounted and prepared metallographically. Each was etched with the reagents described below. They were re-prepared between etchants. The etchants used were as follows.

1. Vilella’s reagent (1 g picric acid, 4 mL HCl, 96 mL ethanol), used by swabbing for 10-20 s. This outlined the delta and the sigma and revealed the carbides in the austenite grain boundaries of the base metal and in the weld. Sigma phase was more sharply outlined than the delta ferrite. No austenite grain or twin boundaries were revealed.

2. Murakami’s reagent (10 g KOH or NaOH, 10 g potassium ferricyanide, 100 mL water), used at room temperature or 80-100ºF. At room temperature, carbides can be revealed at the austenite grain boundaries and within the weld. When used at or near the boiling point, both delta and sigma are revealed. Delta is not as clearly revealed as sigma. With KOH, sigma was colored orange-brown in 30 s, but after another 30 s nearly all of the color was gone. With NaOH, sigma was colored orange-brown after 60 s. The delta ferrite was colored better using NaOH than with KOH (very weak).

3. 20% NaOH in water at 3 V dc, 10 s, used to color both delta ferrite (tan/blue) and sigma (orange-brown). Carbides were visible but not strongly revealed.

4. 10% ammonium persulfate in water at 6 V dc, 10 s, was used to color the carbides. Contrast was slightly better than with Vilella’s, but both were excellent for revealing the carbides. This etch outlined both the delta ferrite and the sigma phase. Again, the sharpness of the phase boundaries was excellent for sigma and non-uniform for the delta ferrite.

5. Concentrated ammonium hydroxide at 1.5 V dc and at 6 V dc for 60 s, was used in an effort to reveal the structure. At 1.5 V dc, the carbides were revealed faintly after 60 s. At 6 V dc, the carbides were visible after 60 s but not with the contrast produced by Vilella’s or ammonium persulfate. Sigma was not colored. A stainless steel cathode was used for all electrolytic etching.

Fig. 2. Electrolytic etching with aqueous 10% ammonium persulfate does an excellent job revealing carbides, as in this example of sensitized 316 stainless steel base metal with M23C6 carbides in the grain boundaries in the heat-affected zone.

The study showed that a variety of reagents can be used to reveal constituents in stainless steels. Vilella’s reagent, and other similar general-purpose reagents, will reveal the carbides but will only outline delta ferrite and sigma. They are not useful for identification purposes. Electrolytic 20% NaOH was excellent for revealing and discriminating between delta ferrite and sigma as it colored delta ferrite tan and blue and sigma orange. Murakami’s works well, but it gave some erratic results. At room temperature, it only reveals the carbide but not as clearly as Vilella’s or ammonium persulfate. At temperatures near the boiling point, the standard formulation colored the delta ferrite non-uniformly but colored sigma uniformly. But there was no color difference. Ammonium persulfate is excellent for detecting sensitization because it reveals the carbides very clearly. It outlined both delta ferrite and sigma. Again, the delta ferrite was not sharply outlined but the sigma was. Ammonium hydroxide is not recommended for use as it did nothing better than any other etch, and it is very annoying to use.

Figure 1 shows 312 SS that was aged at 816°C for 160 h converting all of the ferrite to sigma. The specimen was etched with Murakami’s reagent at 80°C, coloring the sigma vividly. Figure 2 shows sensitization – carbides precipitated in the grain boundaries of the welded 316 stainless steel in the heat-affected zone – revealed very well using aqueous 10% ammonium persulfate electrolytically.