ABSTRACT
The microstructure of iron-based alloys is very complicated and diverse, being influenced by composition, homogeneity, processing, and section size. Microstructures of castings look different from those of wrought products, even of the same composition and if given the same heat treatment. In wrought steels, the microstructures can look quite different when viewed from planes parallel or perpendicular to the deformation axis. In general, it is easiest to identify heat-treated structures after transformation and before tempering. For example, 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. Further, while ferrous metallographers tend to use nital almost exclusively for etching, nital is not always the best reagent to use to properly reveal all microstructures. It is unfortunate that some companies prohibit use of picral because picric acid can be made to detonate under certain conditions. Picral is an excellent etchant for revealing certain microstructural constituents in steel and accidents have been less common than for nital. Vilella’s reagent, which also contains picric acid, is also exceptionally valuable for certain compositions and microstructures. Sodium metabisulfite (K2S2O5) in water (10% SMB) is a very good generalpurpose reagent for steels (and safer to use than nital or picral), with a combination of the capabilities of both nital and picral.
