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Austenitic grades are those alloys which are commonly in
use for stainless applications. The austenitic grades
are not magnetic. The most common austenitic alloys are
iron-chromium-nickel steels and are widely known as the
300 series. The austenitic stainless steels, because
of their high chromium and nickel content, are the most
corrosion resistant of the stainless group providing
unusually fine mechanical properties. They cannot be
hardened by heat treatment, but can be hardened significantly
by cold-working.
Straight Grades
The straight grades of austenitic stainless
steel contain a maximum of .08% carbon. There is a misconception
that straight grades contain a minimum of .03% carbon,
but the spec does not require this. As long as the material
meets the physical requirements of straight grade, there
is no minimum carbon requirement.
"L" Grades
The "L" grades are used to provide extra
corrosion resistance after welding. The letter "L" after
a stainless steel type indicates low carbon (as in 304L).
The carbon is kept to .03% or under to avoid carbide precipitation.
Carbon in steel when heated to temperatures in what is
called the critical range (800 degrees F to 1600 degrees
F) precipitates out, combines with the chromium and gathers
on the grain boundaries. This deprives the steel of the
chromium in solution and promotes corrosion adjacent to
the grain boundaries. By controlling the amount of carbon,
this is minimized. For weldability, the "L" grades are
used. You may ask why all stainless steels are not produced
as "L" grades. There are a couple of reasons:
- "L" grades are more expensive
- Carbon, at high temperatures imparts
great physical strength
Frequently the mills are buying their
raw material in "L" grades, but specifying the physical
properties of the straight grade to retain straight grade
strength. A case of having your cake and heating it too.
This results in the material being dual certified 304/304L;
316/316L, etc.
“H” Grades
The “H” grades contain
a minimum of .04% carbon and a maximum of .10% carbon
and are designated by the letter “H” after
the alloy. People ask for “H” grades primarily
when the material will be used at extreme temperatures
as the higher carbon helps the material retain strength
at extreme temperatures.
You may
hear the phrase “solution
annealing”. This means only
that the carbides which may have
precipitated (or moved) to the
grain boundaries are put back into
solution (dispersed) into the matrix
of the metal by the annealing process. “L” grades
are used where annealing after
welding is impractical, such as
in the field where pipe and fittings
are being welded.
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