This article gives a brief introduction to the following steel types:

• Austenitic
• Duplex
• Ferritic
• Martensitic

Austenitic

These are alloys containing primarily chromium and nickel. Austenitic steels have austenite as their primary phase (face centered cubic crystal). The most common austenitic alloys are the 300 series. The austenitic stainless steels, because of their high chromium and nickel content, are the most corrosion resistant of the stainless group.

Austenitic steels are not hardenable by heat treatment but can be hardened significantly by cold-working. . The austenitic grades are not magnetic. 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.

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. Often mills buy their raw material in "L" grades, but specify the physical properties of the straight grade to retain straight grade strength. This results in the material being dual certified 304/304L; 316/316L, etc.

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.

Duplex

Duplex Stainless Steels have a structure that contains both ferrite and austenite. Duplex alloys have higher strength and better stress corrosion cracking resistance than most austenitic alloys and greater toughness than ferritic alloys, especially at low temperatures. The corrosion resistance of duplex alloys depends primarily on their composition, especially the amount of chromium, molybdenum, and nitrogen they contain. Duplex alloys are often divided into three sub-classes: Lean Duplex (AL 2003™ alloy), Standard Duplex (AL 2205™ alloy), and Superduplex (AL 255™ Alloy and UNS S32760).

Ferritic

Ferritic Stainless Steels, which are part of the 400 series of stainless alloys, have chromium as their major alloying element and are typically low in carbon content. Like martensitic grades, these are straight chromium steels with no nickel. Ferritic steels have ferrite (body centered cubic crystal) as their main phase. Ferritic steel is less ductile than austenitic steel. These steels are magnetic but cannot be hardened or strengthened by heat treatment. These grades can be hardened by cold rolling, but cannot be hardened as much as the austenitic alloys.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.

Martensitic

The martensitic grades are straight chromium steels containing no nickel. The major alloying addition in martensitic stainless steels is chromium in the range of 11 to 17%. The carbon levels can vary from 0.10 to 0.65% in these alloys. They may be tempered and hardened. The high carbon enables the material to be hardened by heating to a high temperature, followed by rapid cooling (quenching). Martensite gives steel great hardness, but it also reduces its toughness and makes it brittle, so few steels are fully hardened. These steels are magnetic.