The use of heat tinting for metallographic study of complex phases in duplex stainless steel

Stainless steel alloys can be difficult to etch to reveal the intricate features of the microstructure due to their resistance to chemical attack. This can be particularly true of duplex stainless steels as its major phases (ferrite and austenite) can etch differently due to their chemical differences and crystal structures. This becomes increasingly complicated as secondary (sometimes detrimental) phases, for example sigma phase, chi, nitrides and carbides, form in the material during exposure to high temperatures. This paper demonstrates a thermal etching technique (heat tint) to reveal the microstructures and discusses potential issues and advantages. The resulting colours are related to the oxide formed on the surface of the phases in the material giving interference colours. The paper discusses the advantages of the technique in observing the complex structures as they exhibit clear differences in colour which can be used to quantitatively measure the phase fraction. Using this technique, it may be possible to remove some of the ambiguity seen in prediction of the formation of these phases.