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.
Low Carbon 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.
High Carbon 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.
Ferritic Grades Ferritic grades have been developed to provide a group of stainless steel to resist corrosion and oxidation, while being highly resistant to stress corrosion cracking. These steels are magnetic but cannot be hardened or strengthened by heat treatment. They can be cold worked and softened by annealing. As a group, they are more corrosive resistant than the martensitic grades, but generally inferior to the austenitic grades. Like martensitic grades, these are straight chromium steels with no nickel. They are used for decorative trim, sinks, and automotive applications, particularly exhaust systems.
Duplex Grades Duplex grades are the newest of the stainless steels. This material is a combination of austenitic and ferritic material. This material has higher strength and superior resistance to stress corrosion cracking. An example of this material is type 2205. It is available on order from the mills.
Precipitation Hardening Grades Precipitation hardening grades, as a class, offer the designer a unique combination of fabricability, strength, ease of heat treatment, and corrosion resistance not found in any other class of material. These grades include 17Cr-4Ni (17-4PH) and 15Cr-5Ni (15-5PH). The austenitic precipitation-hardenable alloys have, to a large extent, been replaced by the more sophisticated and higher strength superalloys. The martensitic precipitation-hardenable stainless steels are really the work horse of the family. While designed primarily as a material to be used for bar, rods, wire, forgings, etc., martensitic precipitation-hardenable alloys are beginning to find more use in the flat rolled form. While the semi-austenitic precipitation-hardenable stainless steels were primarily designed as a sheet and strip product, they have found many applications in other product forms. Developed primarily as aerospace materials, many of these steels are gaining commercial acceptance as truly cost-effective materials in many applications.
Super Alloy Grades Superalloys are used when 316 or 317 are inadequate to withstand attack. They contain very large amounts of nickel and/or chrome and molybdenum. They are usually much more expensive than the usual 300 series alloys and can be more difficult to find. These alloys include Alloy 20 and Hastelloy.