Stainless steels are iron base alloys that include chromium, carbon and other elements, mainly nickel, molybdenum, manganese, silicon and titanium. Chromium, present in no less than 10%, provides higher corrosion resistance than a simple iron alloy. This feature is due to the passivation of the alloys in an oxidizing environment.
They are very used on a wide range of industrial applications due to their excellent mechanical properties and also their corrosion resistance. There are five different families; four of them are classified by their particular crystal structure formed in the alloy: Austenitic, ferritic, martensitic and duplex (austenitic and ferritic); while the fifth correspond to precipitation-hardened alloys, based more on the type of heat treatment used than the crystal structure.
Heat treatments were performed in stainless steels to produce changes on their physical and mechanical properties, their residual stress level and restore their maximum corrosion resistance. Very often with the same treatment achieves a satisfactory corrosion resistance and excellent mechanical properties.
Austenitic and superaustenitic are more resistance to corrosion than ferritic and martensitic, because chromium carbides decompose and Chromium and Carbon remain in solid solution by rapid cooling from high temperature. However, if it is cooled slowly, as in welding, between 870 and 600ºC, chromium carbides precipitated in grain boundary leaving Chromium poor the area beside the edge, which makes the phenomenon called “intergranular corrosion”. This can be fixed downing to the lowest the Carbon content (0.03%), or adding Niobium or Titanium; these elements have a higher tendency to form carbides than Carbon, allowing it remain in solid solution on the iron and thus maintain its ability to resist corrosion.
They are stainless steel with high nickel content (4 to 37%) to stabilize the austenite. They could also content molybdenum, copper, silicon, aluminum, titanium and niobium, elements that are used to obtain certain characteristics.
Main properties of Austenitic and Superaustenitic stainless due to FCC structure, which provides high ductility, formability, toughness and excellent impact resistance. They can be hardened by cold working, but not by heat treatment, cause nickel stabilizes the austenite at room temperature.
Oxidation resistance is far superior to other types of stainless steels due to above reasons, which favors welding procedures that can be done perfectly; they are very used to manufacture tubes for the chemical and petrochemical industry, where corrosion is a crucial service condition.
They tend to be non-magnetic and sometimes, when they have been cold-worked they can be. Cold forming is a way to improve their mechanical properties, specifically yield strength that is relatively low compared to other materials. Cold-working and the section reduction increase the yield strength and the tensile strength limits, while decrease the steel elongation.