About Stainless Steel
Many types of stainless steels have been developed to resist different corrosion environments and working conditions ensuring safety, ductility and hygiene. Stainless steel is even used in systems to clean up the exhaust gases from cars and power stations.
Stainless steel is also recyclable: when scrapped, it can be re-melted to make something new.
Stainless steels are chromium containing steel alloys. The minimum chromium content of the standardised stainless steels is 10.5%. Chromium makes the steel ‘stainless’ this means improved corrosion resistance.
The better corrosion resistance is due to a chromium oxide film that is formed on the steel surface. This extremely thin layer, under the right conditions, is also self-repairing.
Other typical alloying elements are molybdenum, nickel and nitrogen. Nickel is mostly alloyed to improve the formability and ductility of stainless steel. Alloying these elements brings out different crystal structures to enable different properties in machining, forming, welding etc.
The four major types of stainless steel tube are
Austenitic or 300 series, stainless steels make up over 70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and a nickel content of at least of 7%, which makes the steel structure fully austenitic and gives it ductility, a large scale of service temperature, non-magnetic properties and good weldability.
The low carbon versions of the Austenitic Stainless Steel, for example 316L or 304L, are used to avoid corrosion problem caused by welding. The “L” means that the carbon content of the Stainless Steel is below 0.03%, this will reduce the sensitization effect, precipitation of Chromium Carbides at grain boundaries, due to the high temperature produced by welding operation.
The range of applications of austenitic stainless steel includes housewares, containers, industrial piping and vessels, architectural facades and constructional structures.
Ferritic stainless steels generally have better engineering properties than austenitic grades, but have reduced corrosion resistance, due to the lower Chromium and nickel content.
They are also usually less expensive. They contain between 10.5% and 27% chromium and very little nickel, if any, but some types can contain lead. Most compositions include molybdenum; some, aluminium or titanium.
The most common of these steels are 12% and 17% chromium containing steels, with 12% used mostly in structural applications and 17% in housewares, boilers, washing machines and indoor architecture.
Austenitic-Ferritic (Duplex) stainless steels have a mixed microstructure of austenite and ferrite, the aim being to produce a 50/50 mix, although in commercial alloys, the mix may be 40/60 respectively. Duplex steels have improved strength over austenitic stainless steels and also improved resistance to localised corrosion, particularly pitting, crevice corrosion and stress corrosion cracking.
They are characterised by high chromium (19–28%) and molybdenum (up to 5%) and lower nickel contents than austenitic stainless steels.
Duplex stainless steels properties are achieved with overall lower alloy content than similar performing super austenitic grades making their selection and use cost effective for many applications. The duplex structure delivers both strength and ductility. Duplex steels are mostly used in petrochemical, paper, pulp and shipbuilding industries.
Martensitic stainless steels are not as corrosion-resistant as the other two classes but are extremely strong and tough, as well as highly machineable, and can be hardened by heat treatment.
Martensitic stainless steel contains chromium (12-14%), molybdenum (0.2-1%), nickel (0-<2%), and carbon (about 0.1-1%) -giving it more hardness but making the material a bit more brittle. It is quenched and magnetic. This steel is mostly used in turbine blades and in knives.