This photomicrograph shows the
microstructure of a nitrided Nitralloy at 500X with a Nital etch.Nitriding consists of exposing a quenched martensitic steel to an ammonia atmosphere at 500C to 650C, the high end of the tempering range of ordinary martensite. The Nitralloy steel contains 1.0% aluminum, and this element reacts with nitrogen to form a thin case containing extremely fine aluminum nitride precipitates. Nitrogen diffuses into the steel and combines with the aluminum in the interior. Aluminum can only diffuse very short distances in steel at these temperatures, and that forces the nitrides to create an extremely fine dispersion of aluminum nitride particles. On the other hand, nitrogen dissolves interstitially, so it is far more mobile and penetrates sufficiently far into the steel to create an effective case. The ammonia atmosphere is a source for atomic nitrogen as it dissociates. Atomic nitrogen has an extremely high chemical activity compared to diatomic molecular nitrogen, which is nearly inert with respect to the steel.
The martensite tempers while all this nitrogen absorption and nitride precipitation is taking place. The present structure is normal ... there is a faint network along the pror austenite grain boundaries near the outer surface ... and there are interior precipitates which are unresolved here.
Next, Specimen 9
is a nitrided thread gauge.