What is stainless steel?
Stainless steel is the generic name for a number of different steels used primarily because of their corrosion resistance. All stainless steels share a minimum percentage of 10.5% chromium. Chromium is always the deciding factor, although other elements, particularly nickel and molybdenum, are added to improve corrosion resistance. The success of the material is based on the fact that it has one unique advantage. The chromium in the stainless steel has a great affinity for oxygen, and will form a film of chromium oxide on the surface of the steel at a molecular level. The film itself is extremely thin, about 130 Angstroms and one Angstrom is one millionth of a centimetre. This layer is described as passive (does not react or influence other materials), tenacious (clings to the layer of steel and is not transferred elsewhere) and self-renewing (if damaged, more chromium from the steel will be exposed to the air and form more chromium oxide). This means that over a period of years a stainless steel knife can literally be worn away by daily use and will still remain stainless.
In many cases stainless steel is the most cost efficient solution. Although producing a component or fabrication in stainless steel will often be more expensive that using ordinary steel due to the higher cost of stainless steel and expertise required during processing it is the better life cycle costs of stainless steel that make it such an attractive material. Through the use of a stainless steel a significantly longer service life and less maintenance costs for equipment can be achieved. Furthermore, stainless steel is fully recyclable and has a high scrap value on de-commissioning.
Families of stainless steels and other corrosion resistant alloys
There are several families of stainless steel: Ferritic, Martensitic, Austenitic and Duplex. These names are derived from the crystal structure of the steels, which determines their metallurgical behaviour. Other corrosion resistant alloys are nickel based alloys and titanium and its alloys.
Ferritic stainless steels
Ferritic stainless steels are plain chromium stainless steels with a chromium content varying between 10.5 and 18% and a low carbon content. They are magnetic and not hardenable by heat treatment. Ferritic alloys have good ductility and formability but a relatively poor high temperature strength compared to that of austenitic grades.
Martensitic stainless steels
Martensitic stainless steels were the first stainless steels that were commercially developed (as cutlery) and have a relatively high carbon content (0.1 - 1.2%) compared to other stainless steels. They are plain chromium steels containing between 12 and 18% chromium. They are magnetic and hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery, aerospace and general engineering.
A relatively new group of martensitic stainless steels are the supermartensitic stainless steels. The supermartensitic grades combine high strength and low-temperature toughness with acceptable corrosion resistance in many applications. (please also see the section on supermartensitics in the "More Alloys" Fact File.)
Austenitic stainless steels
Austenitic stainless steels are non-magnetic. When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. This enhances their corrosion resistance and modifies the structure from ferritic to austenitic. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production (type 304 is the most commonly specified grade by far). They are not hardenable by heat treatment.
Super austenitic grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium, molybdenum and nitrogen to these grades.
Duplex stainless steels
These are stainless steels containing relatively high chromium levels (between 18 and 28%) and moderate amounts of nickel (between 4.5 and 8%). The high corrosion resistance and the excellent mechanical properties combination of duplex stainless steels can be explained by their chemical composition and balanced ('duplex') microstructure of approximately equivalent volume fractions of ferrite and austenite.
Super duplex grades have enhanced pitting and crevice corrosion resistance compared with the ordinary austenitic or duplex types. This is due the further additions of chromium, molybdenum and nitrogen to these grades.
Precipitation hardening stainless steels
Precipitation hardening stainless steels, like the martensitic types, can be strengthened (i.e. hardened) by heat treatment. The mechanism is metallurgically different to the process in the martensitic types. This means that either martensitic or austenitic precipitation hardening structures can be produced.
Nickel based alloys
Nickel based alloys are a family of alloys in which the principle constituent is nickel although it might be less than 50% of the total composition. Nickel based alloys are used for a wide range of applications requiring aqueous corrosion, high temperature strength of both. Also resistance to mechanical damage by many mechanisms such as fatigue or creep damage is high.
Titanium is the fourth most abundant structural metal in the earth's crust and is the ninth industrial metal. Titanium is used for its excellent corrosion and erosion resistance, high heat transfer efficiency, and superior strength-to-weight ratio. Once judged to be expensive, titanium, in life-cycle costing, is now more often seen to be economical. The key to cost-effective use is to utilise its unique properties and characteristics in the design rather than to substitute titanium for another metal.