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Stainless Steel

Stainless Steel plays a much larger role in our lives than one might think. The next time you sit down to eat, just imagine what it would be like to eat with a rusted fork and knife. It is hard to think of a material more important to the industrial world than stainless steel. While its commercial use is the most recognizable application of stainless steel, i.e., cutlery, its application in the industrial world has a much greater impact. Were it not for stainless steel we wouldn't have the corrosion resistant vessels and piping that we do to transport and hold some of the chemicals that we depend on in our daily lives. Oil production, for instance, would be virtually impossible. The popularity of stainless steel is mostly due to the fact that so many other metals cannot resist corrosion in atmospheric and pure water environments while stainless steel can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, elements which are often used in process plants.

What is Stainless Steel?
Steel is an alloy of iron and carbon. Iron and steel are the most widely used metals, mainly due to their strength, low cost, availability, and it wide ranging versatility. Iron and steel are both susceptible to corrosion and require the addition of other elements like chromium to improve their properties. There are three main types of steel: plain carbon or non-alloy steel, alloy steel and stainless steel.

In metallurgical terms, stainless steel is defined as an iron alloy with a minimum of 10.5% chromium content. Also referred to as chromium steel, its name originates from the fact that stainless steel "stains", or rusts, less easily than ordinary steel. Stainless steel has higher resistance to oxidation or rust and corrosion caused by other environments. The chromium forms a thin, invisible layer on the surface of the steel. If that layer is cut or in any way corroded, the chromium atoms automatically combine with oxygen, thus repairing the surface layer. This self-healing feature makes stainless steel a highly corrosion resistant, low maintenance material for all kinds of industrial applications. Chromium not only increases steel's resistance to wear, oxidisation and scaling, it improves its strength and its ability to be hardened.

Stainless Steel Advantages
Corrosion Resistant
Unlike most metals which cannot resist corrosion in atmospheric and pure water environments, stainless steel can withstand even the most caustic and corrosive elements in the process industry.
Fire and Heat Resistant
Special high chromium and nickel-alloyed grades of stainless can resist scaling and retain strength at extremely high temperatures.
Hygienic
The easy cleaning ability of stainless makes it a popular choice as a construction material when there are strict hygiene conditions as there are in the food processing and pharmaceutical industries.
Aesthetic Appearance
There is nothing quite as pleasing as the bright and easily maintained surface of stainless steel.
Ease of Fabrication
Modern steel processing methods can make stainless steel that can be altered in a variety of ways. It can be cut, welded, formed, machined, and fabricated as easily as traditional steels.
Long Term Value
When the total life expectancy of stainless steel is considered, its often the least expensive construction material when compared to other materials.

History of Stainless Steel
1913 - Harry Brearley invents stainless steel containing 0.24% carbon and 12.8% chromium.
1913 - first used for cutlery when Krupp Stahl adds nickel to the mix.
1914 - 400 series of martensitic and 300 series of austenitic stainless steels are developed.
1920 - a whole variety of chromium and nickel combinations are experimented with including grades 20/6, 17/7 and 15/11.
1924 - W.H. Hatfield invents 18/8 stainless steel (18% chromium, 8% nickel) which is also know as 304 Stainless Steel
1925 - Hatfield invents 18/8 with titanium which is known as 321 Stainless Steel.

Kinds of Stainless Steel
There are five families of stainless steel: Ferritic, Martensitic, Austenitic, Duplex and Precipitation-Hardening or PH. Stainless steel is available in the form of plate, sheet, strip, foil, bar, wire, semi-finished products, pipes, tubes, and tubing. Each family name is derived from the atomic structure of the steels, which determines how they behave.

1. Austenitic - is a nonmagnetic alloy (grade 304, 316L) that contains chromium (18%) and nickel. Austenitic stainless is the most widely used category of steel. A typical composition is 18% chromium and 8% nickel, commonly known as 18/8 stainless steel or 304.
2. Ferritic - is an alloy that contains chromium (10.5 - 18%) and iron with some carbon content. Ferrite stainless steel is magnetic, cannot be hardened and is the second most widely used steel. Generally used in automotive trim and exhaust systems, hot water tanks, and interior architectural trim. This alloy is highly corrosion resistant, but far less durable and ductile than austenitic steel.
3. Martensitic - is an alloy containing chromium (12 - 18%) and carbon, but no nickel. This alloy is a magnetic material that is not as corrosion resistant as the other two alloys, but are extremely strong and tough as well as highly machineable. Unlike the other two, this alloy can be hardened by heat treatment.
4. Duplex - is an alloy that contains chromium (18 - 28%), and nickel. As a combination of austenitic and ferretic steels, this alloy is stronger than both of the individual steels. Duplex steels are highly resistant to corrosion and cracking and are often used in heat exchangers and other process applications. Duplex steel is twice as strong as the austenitic stainless steel and is more corrosion resistant than martensitic stainless steel.

The composition of a stainless steel is indicated very loosely by a number series. For instance, the 200 series, is an austenitic stainless steel that contains chromium, nickel, and manganese. The 300 series is also an austenitic stainless steel that contains chromium and nickel. The 400 series is a ferritic and martenstic stainless steel composed of straight chromium, which means that it cannot be hardened by heat treatment. The newer Duplex stainless steel is indicated by 318. Some of the more familiar classes include the following:

304 stainless steel is an austenitic alloy that is easily formed and welded, and typically used for sheet metal products that are not subject to harsh environments.
304L stainless steel is one of the most frequently used austenitic alloys in the stainless steel family. It is used when parts are to be welded, especially if the weld might come in contact with chlorinated water.
316 stainless steel is a nonmagnetic material that contains molybdenum and a higher nickel content. 316 stainless steel, an austenitic alloy, is very corrosion resistant but is subject to attack if exposed to high levels of chlorine for a long period of time.
316L stainless steel is a highly corrosion resistant austenitic alloy that is second only to 304 stainless steel in importance. 316L stainless steel is commonly used in heavy gauge welded components, as it offers higher creep, stress-to-rupture and tensile strength at elevated temperatures.

Applications of Stainless Steel
The superior quality of stainless steel encourages its use as a material for cooking utensils, cutlery, fasteners, architectural hardware, equipment for use in chemical, petroleum, petrochemical, and textile plants, dairy and food manufacturing, health, medical and sanitation applications, to the power generation, pharmaceutical and transportation industries. In the food industry, components are manufactured from 304 and 316 stainless steels, utilising their properties of being able to resist attack from acid food substances and wash down chemicals, whilst not contaminating the food itself.

Cutlery and Kitchen Ware
The most well known application stainless steels is probably for cutlery and kitchenware. The finest cutlery uses specially produced 410 and 420 for the knives and grade 304 (18/8 stainless, 18% chromium 8% nickel) for the spoons and forks. The different grades used such as 410/420 can be hardened and tempered so that the knife blades will take a sharp edge, whereas the more ductile 18/8 stainless is easier to work and therefore more suitable for objects that have to undergo numerous shaping, buffing and grinding processes.

Chemical, processing and oil & gas industries
The chemical, oil and gas and processing industries are probably the most demanding users of stainless steel. Used in the construction of everything from tanks, pipes, pumps and valves, stainless steel can be found in most industrial processing equipment. Stainless steel excels at being able to handle and store materials as corrosive as dilute nitric acid. Special grades of stainless have also been developed to have greater corrosion resistance at a wider range of temperatures. Used in the construction of heat exchangers, desalination plants, sewage plants, and offshore oil rigs, stainless steel makes a variety of corrosive processes possible.

Power generation
Stainless steel is used by the power generation industry to combat corrosion, usually at extremely high temperatures. Grades composed of nickel are used for high temperature strength and oxidation resistance in fossil fueled power plants. Nickel based stainless steel is also used in flue gas desulfarization units. The nuclear power industry also uses stainless steel for both power generation and radiation containment. Ventilation shafts are made using stainless steel, which can be sealed in emergencies to seal off plants for years. Steam and gas turbines use stainless because of its corrosion resisting and heat resisting qualities.

Food production
Stainless steel is very popular in the production and storage of food. Stainless steel 304 and 316 are the most popular grades. While 304 is more popular than 316, the latter is reserved mostly for harsher environments. The popularity of stainless steel in food production doesn't have as much to do with the corrosive nature of the food itself, but in the fact that stainless steel is so hygienic and easy to clean. When contamination is an issue, stainless steel is a very important consideration. For example in some food production lines, 316 is recommended so that the production equipment can be submerged in strong anti-bacteriological cleaning agents. Another great advantage is that stainless steel imparts no taste to the food that it stores.

Architecture, building and construction
Because of its shiny surface, stainless steel is becoming more and more of a staple in architecture, building and construction industries. Modern buildings often use stainless steels for cladding, roofing and facades. Moreover, stainless steel is very low maintenance and its durable properties make it a great material of construction where vandalism is an issue, as it is in public transport, ticket machines and street furniture. When reinforced concrete first started to be used it was considered that the carbon steel used would not rust, as cement, obviously derived from limestone, is alkaline. However, constantly using grit salt on bridges can change the pH to acidic thereby rusting the steel which expands and cracks the concrete. Stainless steel reinforcing bar, although initially expensive, is proving to have very good life cycle costing characteristics.

Medical applications
Not is a great deal of medical equipment - such as surgical instruments, orthopedic beds, cabinets and examination machines - made using stainless because of its hygienic qualities, it is now possible to use it to replace broken bones in the body. Because it can withstand the action of body fluids, stainless steel can be used for everything from medical implants to artificial hips. Pharmaceutical companies use stainless for pill funnels and hoppers and for piping creams and solutions.

Automotive
The automotive industry continues to make use of stainless steel, not only for automotive parts such as exhaust systems and catalytic converters, but also for some of the structural components. With low maintenance costs, less environmental impact and greater concern with life cycle costs, stainless steel will remain a popular material of choice for this industry.

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