Identification of Carbides in Tool Steel by Selective Etching

Abstract

This paper describes simple metallographic technique for selective etching of individual types of carbides (MC, M2C and M6C and M7C3) in tool steel. Electrolytic etching in chromic acid was used in order to reveal the MC carbides. Chemical etching in permanganate solution revealed the M2C and M6C carbides, while the electrolytic etching in the latter solution enabled to observe M7C3, M2C and M6C carbides. These techniques were demonstrated on an experimental niobium-containing tool steel prepared by powder metallurgy. The results confirm that the MC carbides are highly thermally stable, while the M2C carbides decompose during austenitizing at the temperature of 1050 °C and higher. The M7C3 carbides dissolve in the austenite significantly. This exact and simple observation of the carbides behaviour enables to describe the role of particular carbides on heat treatment behaviour and also to save the carbide-forming elements, where the important ones (tungsten, vanadium) are listed as critical raw materials and the others (chromium and molybdenum) are also strategic.