Stainless Steel Strips

The basic composition of stainless steel is iron (Fe) and chromium (Cr). This is the simplest form of stainless steel, with this family known as the ferritic stainless steels because their crystal structure is called ferrite. (This is also the structure of mild steel.) The ferritic stainless steels are magnetic like ordinary steel. A commonly used grade is Type 430 (S43000) which is used for automotive trim and inside dishwashers and clothes dryers. They are often the least expensive stainless steels but can be more difficult to form and weld.

If you wish to make carbon steel strong and hard, such as for a drive shaft or wear plate, the mill might increase the carbon content, and then heat treat the steel by quenching and tempering it. The same can be done with stainless steel – if the carbon content of ferritic stainless steels is increased, it produces the family of martensitic stainless steels, used for items such as knives, razor blades and corrosion resistant bearings. Martensitic grades are strong and hard, but are brittle and difficult to form and weld. Type 420 (S42000) is a typical example. Like ferritic stainless steels, martensitic stainless steels are magnetic.

The majority of stainless steels contain nickel (Ni), which is added for a number of reasons but particularly to change the crystal structure from ferrite to austenite. Austenitic stainless steels are ductile, tough and, most importantly, easy to form and weld. These steels are not magnetic in the annealed condition. The most common example is Type 304 (S30400) or “18/8” – the most widely used stainless steel in the world. The lower carbon version, Type 304L (S30403) is always preferred in more corrosive environments where welding is involved. There are numerous applications for this grade, ranging from domestic kitchen sinks and building facades to commercial food processing equipment and chemical plant piping.

Molybdenum (Mo) is added to some stainless steels to increase their corrosion resistance, particularly in marine and acidic environments. It increases an alloy’s pitting and crevice corrosion resistance. These corrosion forms are caused by the common and highly aggressive chloride ion (Cl¯), which is present in salts, such as sea salt and table salt. When 2-3% molybdenum is added to Type 304 or 304L, it creates Type 316 (S31600) or 316L (S31603) stainless steel. They are sometimes referred to as the marine grades of stainless steel, since they are widely used for items such as boat fittings. They are also known as the acid resistant grades, since they have better corrosion resistance in some acids such as sulphuric acid. But their range of applications is wide, from building facades in aggressive atmospheres to piping onboard chemical tankers.

Halfway between the ferritic and austenitic stainless steels is a family called the duplex stainless steels, which are about 50% ferrite and 50% austenite. Because of this duplex structure, they are resistant to stress corrosion cracking which can affect the austenitic stainless steels in hot waters containing chlorides. The most common duplex stainless steel is 2205 (including both S31803 and S32205) and it is used in many applications such as hot water tanks.

Nitrogen (N) is added to some stainless steels, but is very important in duplex grades. It has several beneficial effects. Like nickel, nitrogen promotes austenite (especially important for welding) and, like molybdenum, it improves resistance to pitting and crevice corrosion. It also increases strength. Duplex stainless steels are inherently stronger, but a grade such as 2205, which contains about 0.15% nitrogen, has over twice the yield strength of Type 316L. Thus, 2205 is commonly used in tanks for seagoing chemical tankers where both strength and corrosion resistance are required, and for components such as rods and connectors for glass curtain walls in public buildings where the high strength means that the components can be small and so make the structure seem lighter and more transparent.

There is one more family – the precipitation hardening stainless steels. This is a specialized family which has very high strength achieved by adding elements such as copper, which form very fine particles during heat treatment. They generally have slightly higher corrosion resistance than the martensitic stainless steels but, at best, they have slightly less resistance than Type 304. They are commonly used in the aerospace and defence industries, but also find use in items such as pump shafts. 17-4PH (S17400) is a typical example.

In addition to the common grades mentioned above, there are many more specialized grades of stainless steel for applications which require greater corrosion resistance or higher strength. Three examples are Alloy 904L (N08904), which was originally developed for sulfuric acid service, the super-austenitic grade Alloy 254 (S31254), representing a group of 6% Mo stainless steels; and the grade Alloy 2507 (S32750), representing a group of super-duplex alloys. The last two are ‘seawater resistant’ – they will not suffer pitting or crevice corrosion when immersed in ambient temperature seawater. There are also grades developed for such special needs as improved machinability. Cast versions of most wrought grades are also available, usually slightly modified to improve castability.

Nickel-containing stainless steels and nickel alloys play an important role in providing corrosion resistant, and hence leak resistant, materials of construction for projects internationally. Some of these materials also play a critical role in handling gas production, particularly in liquefied form, thus helping to develop difficult-to-access gas reserves.