A number of etchants have been reported in the literature to selectively outline, outline and color, or attack specific types of carbides in steels. These etchants have been developed during the first half of the 20th century but have not been studied systematically since the development of modern analytical techniques. To evaluate these etchants, eight specimens (seven different compositions) with M3C, M23C6, M7C3, M6C, M2C and MC carbides were studied, first by electron-backscattered diffraction (EBSD) to verify the carbides present. The matrix of each specimen was darkened to measure the total carbide content. Then, the various etchants were tried and the results were compared to past publication results. Quantitative measurements were made after each etchant was used. This revealed some minor differences with the prior literature and showed that, while useful for qualitative evaluations, they are not useful for quantitative measurements.
Metallographers have used certain selective etchants for years to color specific carbides as an aid to phase identification. However, only a few of these etchants have had wide usage (e.g., alkaline sodium picrate to color cementite since 1906 due to the work by Kourbatoff). The behaviors of the others are less well known. Even for alkaline sodium picrate, however, few metallographers know that it also outlines and colors M6C carbides.
The nominal compositions of the alloys used in this study are given in Table 1. For the white cast iron (WCI) specimen, Table 1 gives the actual composition. The 440C martensitic stainless steel, the M42 and the T15 high-speed steel specimens are all annealed powder-metallurgy alloys with fine carbide sizes. The other tool steels were made by ingot technology. The D3 specimen was austenitized at 1066ºC, which is higher than the recommended temperature (1010ºC) to dissolve all of the small carbides, then quenched to form martensite. The W1 and the M4 specimens were spheroidize annealed. Both spheroidize annealed and quenched and tempered specimens of M42 high-speed steel were evaluated. The white cast iron specimen was, of course, in the as-cast condition. The specimens were prepared and analyzed using EBSD on a focused ion-beam unit by Dr. J. R. Michael. The results are given in Table 2.
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