Ferritic stainless steels were first invented around 100 years ago and they can be regarded as the simplest stainless alloys - an addition of chromium to iron to improve corrosion resistance. Some ferritic grades have been in large scale commercial production for many years, but the variety of grades available today has only been possible because of new melting and refining technologies. A large number of ferritic grades now exist and a great deal of very active alloy development is continuing.
Ferritic grades are similar to carbon steels in mechanical properties and behaviour in fabrication. They are substantially ferritic at all temperatures so do not transform to austenite, and there are not hardenable by heat treatment. Because they do not transform when heated during welding, grain growth can take place in the heat affected zone, and poor toughness can result. For this reason, ferritic grades are usually restricted to sheet metal thickness up to about 3.0 mm. An exception is the lean structural grade 1.4003, which can be welded at plate thickness.