The microstructure of iron-based alloys is very complicated, being influenced by composition, homogeneity, processing and section size. Microstructures in coarse-grained steels are much easier to observe than in fine-grained steels. Of course, steels are normally made with a fine grain size for best mechanical properties.
In general, it is easiest to identify heat-treated structures after transformation and before tempering. But, in most applications, hardened steels must be tempered and they are usually examined in this condition. If a mixed microstructure of bainite and martensite is formed during quenching, these constituents will become more difficult to identify reliably as the tempering temperature given the product increases towards the lower critical temperature.