The straight grades of austenitic stainless steel pipe 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 thephysical requirements of straight grade, there is no minimum carbon requirement.
The “L” stainless steel grades are used to provide extra corrosion resistance after welding. The letter “L” after a stainless steel pipe 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 temperature 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” stainless steel grades are used. You may ask why all stainless steel are not produced as “L” stainless steel grades.
There are a couple of reasons:
Frequently the mills are buying their raw material in “L” stainless steel 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 certified304/304L; 316/316L, etc.
“H” Stainless Steel Grades
The “H” stainless steel 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” stainless steel 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” stainless steel grades are used where annealing after welding is impractical, such as in the field where stainless steel pipe and fittings are being welded.
The most common of austenitic stainless steel 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.
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%.
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.
Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for extra corrosion resistance.
Requires molybdenum content of 4.00% min.
Requires molybdenum content of 4.00% min. and nitrogen of .15% min.
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 stainless steel grades are primarily used in the aircraft industry.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)