Stainless steel is the name of a high alloy steel used mainly for its anti-corrosion properties. The main characteristic of the stainless steel family is that they all contain a minimum of 10.5 percent of chromium. This enables the steel to withstand corrosive atmospheres or erosive chemical environments. When ordinary
Carbon Steel is exposed to air or water, it corrodes and forms a brown colored iron oxide-commonly called rust, on its surface. They will scale off and eventually the entire piece of steel will corrode and converted to rust. But when enough chromium (more than about 10%) is added to ordinary steel, the oxide on the surface formed will become a very thin, virtually invisible material and protective against wide ranges of corrosive medium. This is what we call stainless steel. There are over 60 different grades of stainless steel that is divided into 5 different categories. Stainless steel is more expensive than carbon and alloy steel and only accounts for a small number of steel used in the global market, but it is non the less a very valuable international market.
History of Stainless Steel
Sheffield, England born Harry Brearly discovered stainless steel in 1913. In an experiment, he found that steel`s corrosion resistance increased when the chromium content is raised to 12 percent or more. He is seen as the person who laid the foundation for the development of various stainless steel grades.
The development of stainless steel almost came to a complete standstill during the First World War. By the late 1920`s it was found that two grades of stainless steel namely martensitic stainless steel and austenitic stainless steel were the most versatile and useful. Martensitic stainless steel has a chromium content of between 13 and 18 percent while austenitic stainless steel contains 18 percent chromium and 8 percent nickel.
In today`s terms stainless steel is used as a generic term to describe corrosion resistant steel that has a minimum chromium capacity of 10.5 percent. The chromium creates a passive and self renewing chromium oxide film around the steel at atomic level and this prevents the iron from rusting.
Development of stainless steel still continues today and stainless steel is mainly divided into austenitic, ferritic and martensitic, duplex and precipitation hardening categories.
Corrosion of Stainless Steel
Stainless steel is susceptible to certain types of corrosion despite the fact that one of the main reasons for using stainless steel is its resistance to corrosion. The different kinds of corrosion affecting stainless steel are: General Corrosion, pitting corrosion, crevice corrosion, stress corrosion cracking, sulphide stress corrosion cracking, intergranular corrosion, galvanic corrosion and contact corrosion.
Corrosion in stainless steel occurs when the protective layer of chromium-oxide (Cr2O3) is broken down. This can spread as microscopic or visibly spots over the surface of the steel.
Categories of Stainless steel
Stainless steel is divided into 5 main categories. These are Austenitic, martensitic, ferritic, duplex and precipitation hardening.
200 Series Austenitic – Alloys: 201, 202, 203, 204 & 205 is a chromium-nickel-manganese alloy with high strength in the annealed. This series is non-magnetic, not heat treatable and has excellent formability for sever forming applications. It is used in structural applications and for the manufacture of washing machine tubs.
300 Series Austenitic – Alloys: 301, 302, 303, 304, 305, 308, 309, 310, 314, 316, 317, 321, 330, 347, 384 is chromium-nickel alloy and its strength is developed by cold working. The series is non-magnetic, not heat treatable and has good formability. By adding molybdenum, the corrosion resistance is increased. It is used in food equipment, chemical equipment and architectural applications.
400 Series Ferritic – Alloys: 405, 409, 429, 430, 434, 436, 442, 446 is straight chromium alloy, magnetic but it is not heat treatable. This series is mainly used for automotive trim and cooking utensils.
400 Series Martensitic – Alloys: 403, 410, 414, 416, 420, 422, 431, 440 is straight chromium alloy, magnetic and can be hardened by heat treatment. It is used for fasteners, pump shafts and turbine blades.
Precipitation Hardening – Alloys: 13-8, 15-5, 15-7, 17-4, 17-7 is chromium-nickel, martensitic or austenitic. Develop strength by precipitation hardening reaction due to heat treatment. It is mainly used for valves, gears and petro-chemical equipment.
Duplex – Alloys: 329, 2205, 2304, 2507, 3RE60 is chromium-nickel-molybdenum. This series is more resistant to stress corrosion cracking than austenitic and it is also tougher than fully ferritic alloys. It is mainly used in doorlines or pressure shafting.
Application of Stainless Steel
Stainless steel is a very versatile and useful material used in a wide variety of applications. It is used in the following fields:
Cutlery and kitchenware Home appliance Chemical processing and oil & gas industries Power generation Food production Architecture, building and construction Medical applications Automotive parts.
