Etching of Duplex Stainless Steels

Modern duplex stainless steels have a phase balance around 50/50 austenite/ ferrite and many specifications require the ferrite content to be determined.  A common requirement is to etch electrolytically in 40% NaOH or KOH, because this stains the ferrite light brown and makes ferrite determination easier.  However, this etch does not clearly show grain boundaries or third phases, such as sigma, chi and nitrides.  These are best shown by first etching in 10% oxalic acide followed by the hydroxide stain etch (see below).  If the microstructure is viewed by polarised light or bright field illumination, the sigma/chi phases appear orange.  Nitrides appear as black dots in the ferrite or decorate the ferrite sub-grain boundaries (see below).



UNS S32760 showing sigma and chi phases           UNS S32760 showing nitrides in the ferrite

Posted on: 21st April 2017

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Image (top left) by Agnieszka