Development and performance introduction of austenitic stainless steel

Austenitic stainless steels are non-magnetic stainless steels that contain high levels of chromium and nickel and low levels of carbon. Known for their formability and corrosion resistance, austenitic stainless steels are the most widely used grades of stainless steel.

Ferritic stainless steels have a body-centered cubic (BCC) grain structure, but austenitic stainless steels are defined by their face-centered cubic (FCC) crystal structure, with one atom at each corner of the cube. This grain structure is formed when a sufficient amount of nickel is added to the alloy in standard 18% chromium alloys (8%-10%).
Austenitic stainless steels, in addition to being non-magnetic, cannot be heat treated. However, they can be cold worked to improve hardness, strength and stress resistance. A solution anneal heated to 1045°C followed by quenching or rapid cooling will restore the original state of the alloy, including removing alloy separation and re-establishing ductility after cold working.
Nickel-based austenitic stainless steel is called 300 series stainless steel. The most common of these is grade 304, which typically contains 18% chromium and 8% nickel.
8% is the minimum amount of nickel that can be added to a stainless steel containing 18% chromium in order to completely transform all ferritic structure into austenitic stainless steel. For 316 stainless steel, molybdenum can also be added to a level of about 2% to improve corrosion resistance.
While nickel is the alloying element most commonly used to produce austenitic stainless steels, nitrogen offers another possibility. Stainless steels with low nickel and high nitrogen content are classified as 200 series stainless steels. However, because it is a gas, only a limited amount of nitrogen can be added before harmful effects occur, including forming nitrides and weakening the gas porosity of the alloy.
The addition of manganese (also an austenite former) mixed with nitrogen makes it possible to add higher quantities of gas. As a result, these two elements, along with copper (which also has austenite-forming properties), are often used to replace nickel in 200-series stainless steels.
In the 1940s and 1950s, when nickel was in short supply and prices were high, the 200 series was also known as chromium-manganese (CrMn) stainless steel. Now considered a cost-effective alternative to 300-series stainless steels, which can offer the added benefit of increased yield strength.
Straight line grades of austenitic stainless steels have a maximum carbon content of 0.08%. Low carbon grades or "L" grades contain a maximum carbon content of 0.03% to avoid carbide precipitation.
Austenitic stainless steels are nonmagnetic in the annealed condition, although they may become slightly magnetic when cold worked. They have good formability and weldability, as well as excellent toughness, especially at low or low temperatures. Austenitic grades also have low yield stress and relatively high tensile strength.
Although austenitic stainless steels are more expensive than ferritic stainless steels, they are generally more durable and corrosion resistant.