Like many scientific discoveries the origins of Stainless Steel lies in a serendipitous accident. In 1913 Sheffield, England, Harry Brearley was investigating the development of new steel alloys for use in gun barrels. He noticed that some of his samples didn’t rust and were difficult to etch. These alloys contained around 13% chromium.

The first application of these steels was in cutlery for which Sheffield subsequently became world famous. Simultaneous work in France led to the development of the first austenitic stainless steels.

Worldwide demand for Stainless Steel is increasing at a rate of about 5% per annum. Annual consumption is now well over 20 million tonnes and is raising in areas such as the construction industry and household appliances. New uses are being continuously found for the attractive appearance, corrosion resistance, low maintenance and strength of Stainless Steel.

The advantageous properties of Stainless Steel can be seen when compared to standard plain carbon mild steel. Although Stainless Steel have a broad range of properties, in general, when compared with mild steel, Stainless Steel have:

All stainless steel are iron-based alloys that contain a minimum of around 10.5% Chromium. The Chromium in the alloy forms a self-healing protective clear oxide layer. This oxide layer gives stainless steel their corrosion resistance. The corrosion of different grades of stainless steel will differ with various environments. Even trace amounts of some elements can markedly alter the corrosion resistance. Chlorides in particular can have an adverse effect on the corrosion resistance of stainless steel. Grades high in Chromium, Molybdenum and Nickel are the most resistant to corrosion.

Although the corrosion resistance of stainless comes from the presence of Chromium, other elements are added to enhance other properties. These elements alter the microstructure of the steel.

Stainless steel are grouped into families based on their metallurgical microstructure. The microstructure may be composed of the stable phases austenite or ferrite, a “duplex” mix of these two, martensite or a hardened structure containing precipitated micro-constituents.

Austenitic stainless steel contains a minimum of 16% chromium and 6% nickel. They range from basic grades like 304 through to super austenitic such as 904L and 6% Molybdenum grades.

By adding elements such as Molybdenum, Titanium or Copper, the properties of the steel can be modified. These modifications can make the steel suited to high temperature applications or increase corrosion resistance. Most steels become brittle at low temperatures but the Nickel in austenitic stainless makes it suited to low temperature or cryogenic applications.

Austenitic stainless steel is generally non-magnetic. They are not able to be hardened by heat treatment. Austenitic stainless steel rapidly work-harden with cold working. although they work harden.

The principal alloying elements are sometimes reflected in the name of the steel. As a common name of 304 stainless steel is 18/8, for 18% chromium and 8% nickel.

Applications for austenitic stainless steel include:

They are known for their moderate corrosion resistance and poor fabrication properties. Fabrication properties can be improved by alloy modifications and are satisfactory in grades such as 434 and 444. Ferritic stainless steel cannot be hardened by heat treatment and are always used in the annealed condition.

Ferritic stainless steel is magnetic. They are also not susceptible to stress corrosion cracking. Welding ability is acceptable in thin sections but decreases as section thicknesses increase.

High carbon and lower chromium content are the distinguishing features of martensitic stainless steel when compared with ferritic stainless.

Martensitic stainless steel includes 410 and 416. Hardened martensitic steels cannot be successfully cold formed. They are magnetic, have moderate corrosion resistance and poor welding ability.

Duplex stainless steel have high chromium and low nickel contents. This gives duplex stainless steel microstructures that include both austenitic and ferritic phases. They include alloys like 2304 and 2205. These alloys are so named due to their respective compositions - 23% chromium, 4% nickel and 22% chromium, 5% nickel.

By having both austenite and ferrite in the microstructure, duplex stainless steel feature properties of both classes. Although a compromise between the two ‘pure’ types, duplex grades can offer some unique property solutions. Duplex grades are resistant to stress corrosion cracking, but not to the same level as ferritic grades. The toughness of duplex grades is superior to that of the ferritic grades – but inferior to that of the austenitic grades.

Most importantly, the corrosion resistance of duplex steels is equal, or superior to 304 and 316 stainless steel. This is particularly so for chloride attack.

Duplex grades are readily welded. They also have high tensile strengths.