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 austenitic materials.
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 Vessel Code 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 design consideration.
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 grades.
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 composition range.
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 2,000o 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 of the ASME Boiler and Pressure Vessel Code for current requirements.