Because of their sub-microscopically fine
distribution the nitrides in the diffusion
zone of low carbon steels generally can not
be seen after etching of the metallographic
sample. However, after heating the sample
to tempering temperature (200-400° C for
15-30 min), the nitrogen in solid solution
precipitates in the form of γ’ nitride needles.
These nitride needles can be etched so that
the diffusion zone becomes visible and its
thickness can be measured (Fig. 4).
On alloy steels the diffusion zone will be
etched dark with Nital, but the nitrides can
not be resolved with an optical microscope
(Fig. 5).