Metallographic Studies of Iron Carbide and Tungsten Carbide

Abstract

IN most metallographic investigations of steels or of cemented carbides the fine details of the carbides are not studied. We have just completed an etching study of iron carbides made in three different ways and of some tungsten carbide (1/32-in. equilateral triangles) single crystals. The iron carbide studies, made with Carapella's reagent (5 gm. ferric chloride, 2 ml. concentrated hydrochloric acid and 99 ml. methyl alcohol) etching for 20–60 min., revealed cracking as the principal deformation mechanism and twinning as the secondary agency. The cracking behaviour has been described previously, but the twinning appears to be a new observation1. In two cases only, out of hundreds of samples studied, the cracks found were aligned along the twinning planes in the carbides. These observations were made upon a carburized plain carbon steel, upon a carburized ‘Westinghouse Puron’ iron (spectrographically pure) and upon a pure, massive, polycrystalline, iron-manganese carbide kindly supplied by Dr. L. S. Darken of U.S. Steel Research from his recent ternary system studies. In the two carburized samples there was no possibility of mechanical work having caused cracking of the carbide. Fused caustic, 50 per cent nitric acid and 50 per cent hydrochloric acid were also successful etching reagents.