Austenitic grades
are those alloys which are commonly in use for stainless steel applications.
The austenitic grades are not magnetic. The most common austenitic alloys
are iron-chromium-nickel steel and are widely known as the 300 series. The
austenitic stainless steel tube, because of their high chromium and nickel
content, are the most corrosion resistant of the stainless steel 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 pipe contain a maximum of 0.08% carbon. There is
a misconception that straight grades contain a minimum of 0.035% 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 tubing type indicates low carbon (as
in 304L). The carbon is kept to 0.035% or under to avoid carbide
precipitation. Carbon in steel when heated to temperature in what is
called the critical range 800 °F to 1600 °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 steel 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
The higher the carbon
content the greater the yield 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 0.04% carbon and a maximum of 0.10% 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.
Type 304 |
The most common of
austenitic grades, containing approximately 18% chromium and 8% nickel. It is
used for chemical processing equipment, for food, dairy, and beverage industries,
for heat exchangers, and for the milder chemicals. |
Type 316 |
Contains 16% to 18%
chromium and 11% to 14% nickel. It also has molybdenum added to the nickel
and chrome of the 304. The molybdenum is used to control pit type attack.
Type 316 is used in chemical processing, the pulp and paper industry, for
food and beverage processing and dispensing and in the more corrosive
environments. The molybdenum must be a minimum of 2%. |
Type 317 |
Contains a higher
percentage of molybdenum than 316 for highly corrosive environments. It must
have a minimum of 3% “moly”. It is often used in stacks which contain
scrubbers. |
Type 317L |
Restricts maximum
carbon content to 0.030% max. and silicon to 0.75% max. for extra corrosion
resistance. |
Type 317LM |
Requires molybdenum
content of 4.00% min. |
Type 317LMN |
Requires molybdenum
content of 4.00% min. and nitrogen of .15% min. |
Type 321 |
These types have
been developed for corrosive resistance for repeated intermittent
exposure to temperature above 800 degrees F. Type 321 is made by the addition
of titanium and Type 347 is made by the addition of tantalum/columbium. These
grades are primarily used in the aircraft industry. |
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