What is stainless steel-Types of stainless steel
Stainless steel stains less easily than other iron-based metals, but it’s not literally “stainless”. Just like standard steel, stainless can get marked up by fingerprints and grease, develop discoloration, and eventually rust. The difference is resilience. Stainless steel can withstand much more time and abuse before showing signs of wear.
All steels have the same basic iron and carbon composition, but stainless steel also contains a healthy dose of chromium—the alloy that gives stainless steel its famous corrosion resistance.
And this is where things get complicated. There are multiple grades under the stainless steel umbrella, each with slightly different alloy composition, and therefore slightly different physical characteristics.
Stainless steel must contain at least 10.5 percent chromium. Depending on the grade, it may contain much higher chromium levels, and additional alloying ingredients like molybdenum, nickel, titanium, aluminum, copper, nitrogen, phosphorous and selenium.
What is stainless steel ?
Stainless steel is iron base alloy that has a great resistance to corrosion. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust (or) stainless than other types of steel It is observed that a thin, transparent and very tough film forms on the surface of stainless steel which is inert or passive and does not react with many corrosive materials. In a temperature range of 235°c to 980°c, it exhibits strength, toughness and corrosion resistance superior to other metals. It is thus ideally suited for handling and storage of liquid helium, hydrogen, nitrogen and oxygen that exist at cryogenic temperature. The property of corrosion resistance is obtained by adding chromium only (or) by adding chromium and nickel together. Stainless steel is manufactured in electric furnaces.
Why stainless steel is corrosive resistance
Stainless steels are very slightly oxidisable. Slight oxidation forms very thin film of oxide and this oxide film acts as a protective coating and in this way further corrosion is stopped. This protective film of oxide is so thin that the colour and beauty of the basic materials is not affected.
Types of Stainless steel
Stainless steels can be classified into the following groups:
Austenitic Stainless steel:
Austenitic steels have austenite as their primary phase. These are alloys containing atleast 24% chromium and nickel combined and the percentage of each of them alone is not less than 8%. These steels may contain other elements for particular purposes. Standard austenitic steels are vulnerable to stress corrosion cracking. Higher nickel austenitic steels have increased resistance to stress corrosion cracking. These are non-hardenable by heat treatment and non-magnetic but usually exhibit some magnetic response depending on the composition and the work hardening of the steel.
The most common of austenitic grades, containing approximately 18% chromium and 8% nickel. It is used for chemical processing equipment, for food, dairy, and beverage industries, for heat exchangers, and for the milder chemicals.
Contains 16% to 18% chromium and 11% to 14% nickel. It also has molybdenum added to the nickel and chrome of the 304. The molybdenum is used to control pit type attack. Type 316 is used in chemical processing, the pulp and paper industry, for food and beverage processing and dispensing and in the more corrosive environments. The molybdenum must be a minimum of 2%.
Contains a higher percentage of molybdenum than 316 for highly corrosive environments. It must have a minimum of 3% “moly”. It is often used in stacks which contain scrubbers.
Martensitic Stainless steel
These are alloys of iron, carbon and chromium. The difference between the percentage of chromium and 17 times the percentage of carbon is less than 12.5%. These are also called chromium steel, and are harden able and magnetic. They are used where high strength and moderate corrosion resistance is required.They have generally low weld ability and form ability. They are magnetic.
Basic martensitic grade, containing the lowest alloy content of the three basic stainless steels (304, 430, and 410). Low cost, general purpose, heat treatable stainless steel. Used widely where corrosion is not severe (air, water, some chemicals, and food acids. Typical applications include highly stressed parts needing the combination of strength and corrosion resistance such as fasteners.
Contains lower carbon than Type 410, offers improved weldability but lower hardenability. Type 410S is a general purpose corrosion and heat resisting chromium steel recommended for corrosion resisting applications.
Has nickel added (2%) for improved corrosion resistance. Typical applications include springs and cutlery.
Ferritic Stainless steel
These are alloys of iron, carbon, steel. These steels are based on Chromium with small amounts of Carbon usually less than 0.10%. These steels have a similar microstructure to carbon and low alloy steels. They cannot be hardened by heat treatment.Ferritic steels are also chosen for their resistance to stress corrosion cracking. They are not as formable as austenitic stainless steels. They are magnetic.
The basic ferritic grade, with a little less corrosion resistance than Type 304. This type combines high resistance to such corrosives as nitric acid, sulfur gases, and many organic and food acids.
Has lower chromium and added aluminum to prevent hardening when cooled from high temperatures. Typical applications include heat exchangers.
Contains the lowest chromium content of all stainless steels and is also the least expensive. Originally designed for muffler stock and also used for exterior parts in non-critical corrosive environments.
Primarily used in chemical plants and piping applications, the duplex stainless steels are developing rapidly today and have a microstructure of approximately equal amounts of ferrite and austenite. Duplex stainless steels typically contain approximately 22-25% chromium and 5% nickel with molybdenum and nitrogen. Although duplex and some austenitics do have similar alloying elements, duplexes have higher yield strength and greater stress corrosion cracking resistance to chloride than austenitic stainless steels.
Precipitation-hardening stainless steels are chromium-nickel stainlesses, which contain alloying additions such as aluminum, copper or titanium that allow them to be hardened by a solution and aging heat treatment. They can be either austenitic or martensitic in the aged condition. Precipitation- hardening stainless steels are grouped into three types: martensitic, semiaustenitic and austenitic. The martensitic (such as Type 630) and semiaustenitic (such as Type 631) can provide higher strength than the austenitic (such as Type 660, also known as A286).
Applications and uses
- Chemical industry – Large storage tanks and containers for liquids and solids
- Food industry – food processing equipment like milking machines, storage tanks, hauling tanks, piping, valves, milk trucks etc. Also used in breweries, wine-making and the fruit juice industry.
- Domestic uses – 304 stainless steel can resist the corrosive action of acids found in fruits, milk, meat and vegetables, and is fairly resistant to abrasion. Therefore, it is widely used in sinks, tabletops, stoves, refrigerators, cutlery, utensils and various other appliances.
- Architectural uses – paneling, molding, railings, décor.
- Industrial applications – tools, bolts, nuts, screws etc.
- Automotive and aerospace industries – fuel tanks, various structural components.
- Hospitals – surgical equipment.
- Commercial – sanitary fittings, pipes.
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