Metallographic etching reagents. III: For alloy steels

Abstract

There has been lacking in the existing literature a rationale which would guide one in making an intelligent selection of a suitable etching reagent for identifying, by means of metallographic etching reagents, the constituents present in various alloy steels, ferro-alloys, and special alloys. This investi-gation was undertaken to find such a rationale for alloy steels and ferro-alloys containing chromium, tungsten, and vanadium. Experimental data on the etching behavior of typical constituents found present in these steels and ferro-alloys and in high-speed steels toward various alkaline solutions under oxidizing and nonoxidizing conditions were obtained, using the simple immersion and electrolytic etching methods. The principles governing this behavior were determined from these data and results of tests made with certain solutions. Briefly stated, they are based on (1) two distinct effects obtained with alkaline etching solutions in the immersion method, (a) a chemical attack of the constituent by the alkali, and (6) the oxida-tion of the constituent by nascent oxygen formed in the solution ; and (2) similar effects by electrolytic etching with solutions of weakly dissociated acids, both alkaline and metallic salts of these acids, and alkalis, although the nascent oxygen is derived from different sources, (a) a secondary reaction between the acid formed in the first stages of the electrolysis and water, and (6) by discharge of hydroxyl ions on the anode (specimen). A differentiation of the various constituents is thus made possible. A quite complete, though brief, review of the literature on equilibrium dia-grams of the alloy systems and on constituents found in alloy steels and ferro-alloys is included.