David N. French, Sc.D.
President of David N. French, Inc., Metallurgists, Northborough, MA.
Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (4 printed pages)
Austenitic stainless steels are a class of alloys with a face-centered-cubic
lattice structure of austenite over the whole temperature range from room
temperature (and below) to the melting point. In ferritic steels there is a
transformation from the body-centered-cubic lattice structure of ferrite to the
face-centered-cubic lattice structure of austenite. The temperature of this
transformation depends upon the composition but is about 1340o
F for a plain-carbon steel similar to the SA178 or SA210 grades. When 18%
chromium and 8% nickel are added, the crystal structure of austenite remains
stable over all temperatures. The nickel-based alloys with 35-70% nickel and
20-30% chromium, while not strictly steels (a steel must have at least 50%
iron), do have the face-centered-cubic lattice arrangement and are also called
Our discussion will be limited to austenitic stainless steels. This class of
alloys has excellent corrosion resistance and excellent high-temperature
tensile and creep strength. They have been used in superheaters and reheaters
for 35 years or so and have provided excellent performance.
For high-temperature boiler applications, three general grades, 304, 321, and
347, are the most widely used. Within these classifications are other grades,
designated by a following capital letter, L or H. The differences are only in
the carbon content. Table I lists these differences.
For use at temperatures above 1000oF, the ASME Boiler & Pressure
requires a minimum of 0.04% carbon for adequate creep strength. For superheater
and reheater applications, the H grade is preferred as this assures the proper
carbon content for use at temperatures where creep strength is the important
There are two other grades, 304N and 304LN. The "N" indicates a nitrogen
content of 0.10-0.16% ( for improved strength) and the "L" again signifies a
maximum carbon content of 0.035%.
The 304,321, and 347 grades are all in the classification of 18% chromium, 8%
nickel with some slight variations in the range of these alloying elements.
Table II lists the chromium and nickel composition requirements for the three
There are different ASME specifications, depending upon the form which the
material is used. Tubes are covered in SA213, pipes are covered in SA376,
plates are covered in SA240, and each product form has a slightly different
Other differences among these three grades are the additions of titanium in
321, and columbium and tantalum in grade 347. For 321, the titanium is 0.60%
maximum; and for 347, the columbium plus tantalum shall not exceed 1.0%. There
are other requirements on the minimum amount of these alloying elements, based
upon the carbon content. There are also some other minor differences in the
nickel range, depending upon the product form. However, except for these,
relatively speaking, minor differences, they all fall within the broad
classification of the 18-8 austenitic stainless steels.
The material specification requires all of these materials to be provided in the
solution-annealed condition. That is, the final heat treatment is performed at
a temperature of 1900-2000oF, depending upon the particular grade.
For the 321H grade, there is a further requirement: a grain size of ASTM No.7
or coarser is specified to insure adequate creep strength. A solution anneal at
F minimum is usually sufficient to meet this specification requirement.
After the high-temperature solution anneal, the microstructure will be equiaxed
austenite. The word "equiaxed" means that the dimensions of an individual
austenite grain will be essentially the same, regardless of orientation or
direction. The material is in the fully annealed condition and will be a
single-phased material with only some non-metallic inclusions inherent to steel
making, apparent within the microstructure.
Unlike the ferritic steels that have dramatic microstructural changes depending
upon the peak operational or failure temperature, there are no abrupt
microstructural changes in the austenitic stainless steels. What
microstructural changes do occur, occur over a range of temperatures. All of
these grades will sensitize, that is, form chromium carbides along the
austenite grain boundaries. The formation of these carbides reduces the
chromium content of the austenite grains at the boundary, and, therefore,
reduces the local corrosion resistance along the grain boundaries.
To prevent sensitization, additions of titanium to make the alloy 321 and
columbium and tantalum to form 347 were invented. If these alloys are given a
second heat treatment, called a stabilization anneal, at 1600-1650o
F after the solution anneal, titanium carbide or columbium-tantalum carbide
will form preferentially to chromium carbide. With all of the carbon removed as
innocuous carbides, no chromium carbide can form. There is no loss of chromium
at the grain boundaries, and no loss of corrosion resistance, and thus no
sensitization. However, since in boiler applications 321 and 347 are not given
a stabilization anneal, 321 and 347 will sensitize just the same as 304.
One other microstructural constituent will form at elevated temperatures, and
that is a chromium-iron intermetallic called "sigma phase."
Both the sensitization and the formation of sigma phase occur over long periods
at ill-defined temperatures. Both will occur at temperatures beginning at about
l,000oF and will form more rapidly at slightly higher temperatures.
Since the formation of chromium carbide and sigma phase are governed by the
ability of individual atoms to move or diffuse through the lattice, these
atomic movements will occur more rapidly at higher temperatures. As the
temperature is increased above 1200oF, however, chromium carbide
begins to redissolve in the austenite; thus the rate of carbide formation and
growth decreases. By about 1600oF, chromium carbide is completely
gone from the microstructure. Sigma phase is unstable and redissolves above a
temperature of about 1600o
F; the exact temperature depends on the composition.
One other change in the microstructure that will occur over long periods of time
is grain growth. Depending upon the time and temperature, grain growth can
begin at temperatures as low as 1150oF-1200o
F if the time is long enough.
Unfortunately, from an estimation of operating-temperature perspective, all of
these changes within the microstructure of austenitic stainless steel occur
over a range of temperatures and over a range of times. There are no discrete
temperatures that indicate with any degree of precision the peak failure or
operating temperature. Thus there are only estimates of operating temperature
and not an accurate "calling card" within the microstructure as there are in
the ferritic steels.
In summary, the 18 chromium-8 nickel austenitic stainless steels have been used
for several decades in high-temperature applications within a steam generator.
They have excellent high-temperature tensile and creep strengths and excellent
corrosion resistance. The microstructural changes during long-term operation
are more subtle than in the ferritic steels.
Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.