Austenitic stainless steel

During World War II, the Schaeffler diagram was invented by Anton Schaeffler, who was then a budding metallurgist in the employ of two American welding electrode manufacturers, Harnischfeger Company and A.O. Smith Corporation. ==AISI 200 and 300 series==

Austenitic stainless steels are divided into 300-series and 200-series subgroups. In 300 series stainless steels, the austenitic structure is obtained primarily by adding nickel. The structure in 200 series stainless steels, however, is obtained by adding manganese and nitrogen, with a small amount of nickel content. This makes 200 series steels a cost-effective nickel-chromium austenitic type stainless steel. 300 series stainless steels are the larger subgroup. The most common austenitic stainless steel and most common of all stainless steel is Type 304, also known as 18/8 or A2. Type 304 is extensively used in such items as cookware, cutlery, and kitchen equipment. Type 316, also known as A4, is the next most common austenitic stainless steel. Some 300 series, such as Type 316, also contain some molybdenum to promote resistance to acids and increase resistance to localized attack (e.g. pitting and crevice corrosion). The higher nitrogen addition in 200 series gives them higher mechanical strength than 300 series. Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid. Alloy 20 has excellent mechanical properties and the presence of niobium in the alloy minimizes the precipitation of carbides during welding. == Heat resisting austenitic stainless steels ==

Heat resisting grades can be used at elevated temperatures, usually above . They must resist corrosion (usually oxidation) and retain mechanical properties, mostly strength (yield stress) and creep resistance. Corrosion resistance is mostly provided by chromium, with additions of silicon and aluminium. Nickel does not resist well in sulphur containing environments. This is usually taken care of by adding more Si and Al which form very stable oxides. Rare earth elements such as cerium increase the stability of the oxide film. Type309 and 310 are used in high temperature applications greater than . Note: ferritic stainless steels do not retain strength at elevated temperatures and are not used when strength is required. Austenitic stainless steel can be tested by nondestructive testing using the dye penetrant inspection method but not the magnetic particle inspection method. Eddy-current testing may also be used. == Precipitation Hardening grade EN1.4980 ==

Grade EN1.4980 (also known as A286) is not considered strictly as a heat resisting steel in standards, but this is popular grade for its combination of strength and corrosion resistance. It is used for service temperatures up to in applications such as: • Aerospace (standardized in AMS5731, AMS5732, AMS5737 and AMS5525 standards), • Industrial gas turbines, • Automotive (turbo parts), etc. == Thermomechanical Properties of Austenitic Stainless Steel ==

The face centered cubic (FCC) microstructure of austenitic stainless steel is enabled by the inclusion of nickel (cite). Under normal atmospheric conditions, the crystal structure of iron is body centered cubic (BCC), also known as alpha-iron or ferrite. Above approximately 910 °C (1183 K), the crystal structure changes to FCC gamma-iron, or austenite. By contrast, nickel maintains a FCC crystal structure regardless of temperature. Carbon atoms readily produce carbides from alloyed chromium, forming as precipitates within the metal. the formation of carbides can leech chromium from the Cr2O3 coating of the metal. When worked at cold temperatures or subjected to mechanical stresses beyond the yield point, austenitic steels will work harden, as dislocations formed within the crystal structure compound upon one another. Additionally, misalignment of crystalline structures results in the formation of body-centered tetragonal (BCT) martensite, a significantly harder crystal structure of steel. Above temperatures of approximately 480 °C (753 K), deformation becomes plastic as power-law creep occurs, with glide becoming predominant. The transition to climb-plus-glide power law creep is however not distinct, due to the temperature at which austenite forms; at this point, dynamic recrystallization of the metal begins, and creep of the material will accelerate as the steel rapidly becomes more malleable. As a result, these steels are usually not recommended for use above 700 °C for structural purposes, ==See also==