What is stainless steel

Stainless steels are alloys containing iron, relatively low carbon, a minimum 10.5% chromium and up to 30% nickel. However they are typically 18% chromium and 8% nickel. For increased corrosion resistance or for manufacturing requirements in specifi c applications, chromium may be increased and other elements such as manganese, aluminum, titanium and/or molybdenum may be added as required. Stainless steels are sometimes called corrosion-resisting steels since the terms stainless may suggest non-staining. It does not mean non-staining in all environments, but less staining and more resistant to corrosive attack when compared with steels containing less than 10.5% chromium. What Makes Stainless Steel Resist Corrosion? Chromium [the key element in stainless steels] in excess of about 10.5% forms a tenacious, refractory oxide protective fi lm on any exposed layer, i.e. a corrosion barrier. This relatively impervious fi lm is considered to be selfhealing or self-restoring. It will, if broken, repair itself spontaneously upon re-exposure to an oxidizing agent such as air or nitric acid. Resistance to corrosive attack under specifi c conditions is enhanced by progressively increasing chromium over 10.5% and/or by the addition of other elements such as nickel, molybdenum and copper. Types of Stainless Steel Stainless steels are grouped according to the metallurgical structure which forms when they are cooled from high temperatures. Four basic types of stainless steel recognised are: • Duplex • Austenitic • Ferritic • Martensitic Duplex Stainless Steel Duplex has a structure of approximately equal amounts of ferrite and austenite. Chromium content varies from 18%-28% and 4.5%-8% nickel, with most grades also containing Molybdenum. features: • High resistance to stress corrosion cracking • Increased level of passivity • Good welding and formability • Higher tensile and yield strengths than austenitic and ferritic grades.