Iron-base alloys containing between 11%
and 30% chromium form a tenacious and highly protective chrome oxide
layer that gives these alloys excellent corrosion-resistant
properties. There are a great number of alloys that are generally
referred to as stainless steels, and they fall into three general
classifications.
Austenitic stainless steels
contain usually 8% to 12% nickel, which stabilizes the austenitic
phase. These are the most popular of the stainless steels. With 18%
to 20% chromium, they have the best corrosion resistance and are very
tough and can undergo severe forming operations.
These alloys are susceptible to
embrittlement when heated in the range of 593 to 816°C. At these
temperatures, carbides precipitate at the austenite grain boundaries,
causing a local depletion of the chromium content in the adjacent
region, so this region loses its corrosion resistance.
Use of “extra low carbon” grades
and grades containing stabilizing additions of strong carbide-forming
elements such as niobium minimizes this problem. These alloys are
also susceptible to stress corrosion in the presence of chloride
environments.
Ferritic stainless steels are
basically straight Fe-Cr alloys. Chromium in excess of 14% stabilizes
the low-temperature ferrite phase all the way to the melting point.
Since these alloys do not undergo a phase change, they cannot be
hardened by heat treatment. They are the least expensive of the
stainless alloys.
Martensitic stainless steels
contain around 12% Cr. They are austenitic at elevated temperatures
but ferritic at low; hence they can be hardened by heat treatment.
To obtain a significant response to
heat treatment, they have higher carbon contents than the other
stainless alloys. Martensitic alloys are used for tools, machine
parts, cutting instruments, and other applications requiring high
strength. The austenitic alloys are nonmagnetic, but the ferritic and
martensitic grades are ferromagnetic.
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