Effects of Austenite and Martensite. The
austenitic and martensitic phases are discussed
together for ferritic stainless steels because they
are interrelated; one can occur as the result of
the other.
High-purity iron-chromium alloys are ferritic
at all temperatures up to the melting point if they
contain more than approximately 12% Cr. However,
the gamma loop in iron-chromium alloys
can be greatly expanded by the addition of carbon
and nitrogen. For example, it was found that
the ferrite-austenite boundary was extended to
29% Cr in alloys that contained 0.05% C and
0.25% N (