A corrosion - resistant type of alloy steel which contains a minimum of 12% Cr. This later is the element which confers upon the steel its property of resisting attack by the atmosphere or by a number of chemical regents. The effect is attributed to the ability of chromium to form a thin, but very tenacious, film of oxide at the surface of the alloy which resists attack by most oxidising agents. The resistance to corrosive attack is enhanced if nickel be added, and is further improved by small additions of molybdenum and copper.
Types of Stainless steel
There are Three main types of stainless steel – Ferritic, Martensitic and Austenitic. The ferritic steels are magnetic and have a low carbon content and contain chromium as the main element, typically at the 13% and 17 % levels. The martensitic steels are magnetic containing typically 12% chromium and a moderate carbon content ; they are hardenable by quenching and tempering like plain carbon steels and find their main application in cutlery manufacture, aerospace and general engineering. The austenitic steels are nonmagnetic and in addition to chromium , typically at the 18% level, they contain nickel which increases their corrosion resistance and are the most widely used group of stainless steel. Other grades include precipitation hardening, duplex and highly alloyed grades.
All stainless steels have a high resistance to corrosion due to their chromium content. This remarkable resistance to attack is due to the naturally occurring chromium –rich oxide film which is always present on the surface of the steel. Although extremely thin, this invisible inert film is tightly adherent to the metal and extremely protective in a wide range of corrosion media. The film is rapidly self repairing in the presence of oxygen and damage by abrasion, cutting or machining is instantaneously repaired. In general the corrosion resistance of these steels improves as the chromium content increases and in particular the addition of nickel to create the austenitic grades strengthens the oxide film and raises their performance in more aggressive conditions.
The additions of molybdenum to either the ferritic or austenitic steels improves their pitting corrosion resistance and produces the most resistant steels in either ferritic steels or austenitic categories.
The ferritic steels are used in the more mildly corrosive environments, being often used in trim work and somewhat less demanding applications.
In the home, stainless steel does not suffer from the unsightly rusting common to ordinary steels and is normally unaffected by the variety of tap waters, food, drink, soaps and detergents which are encountered. With the correct care and attention a clean, bright, attractive finish is maintained both ferritic and austenitic steels are used as is appropriate. As a result of their superior corrosion resistance and ease of cleaning, austenitic stainless steels are used widely in food processing, brewing, soft drink processing equipment, and in catering equipment, The austenitic steels are also highly resistant to the wide range of rural and industrial atmospheres encountered, resulting in extensive use in architectural, structural, and street furniture applications. Their resistance to attack by acids, alkalis and other chemicals, has led to wide use in the chemical and process plant industries. The selection of the correct grade and finish of steel for a given application is very important to obtain the corrosion resistance required the presence of aggressive impurities in corrosive media can necessitate the use of one of the higher alloyed stainless steel grades.
The ferritic stainless steels can be welded in thinner section, but it is recommended that advice be sought on welding practices. The austenitic stainless steels can be readily welded by the full variety of welding techniques. Incorrect welding practices, particularly on thicker sections, can impair corrosion resistance if the correct steel grade has not been chosen. This can be overcome y the use of stabilized steels or by the use of extra low carbon steels (ELC). Brazing and soldering can be carried out successfully with lighter section materials.
There is a wide range of stainless steels with other attractive properties to complement their corrosion resistance. The appropriate austenitic steels, in particular, can give excellent from-ability and impact strength. Their high temperature strength and scaling resistance upto 1100C are extremely useful for many applications, before much more highly alloyed materials have to be considered. Other grades arer resistant to slow elongation under stress at high temperatures, these are the creep resistant grades. Austenitic steels also perform well at low temperature, down to – 200 C and colder, where they do not embrittle and lose ductility or shock resistance.
When deformed the strength of the austenitic steels is raised significantly by work hardening, which can be useful design property. There are specially alloyed steels containing nitrogen, which also have built in higher strength, for such as pressure vessel applications. Again it is important that the correct grade of steel is selected to give the best combination of properties for the application Stainless steels are available in wide range of product forms including.
Plates, Sheets, Strips, Billet, Round Bar, Road, Wire, Pipe Tube, Fittings, Forgings, Rings, Castings, Sections-Hot Rolled, Extruded, Drawn, Cold Formed, Sintered Products. The flat product forms can be obtained in a variety of surface finishes, and advise should be sought on the suitability of the different finished for particular applications.