“What you need to know”
(Probably more than you ever wanted to know!)
About Stainless Steel
Stainless steel is essentially a low carbon steel which contains Chromium at 10% or more by weight. It is this addition of Chromium that gives the steel its unique stainless, corrosion resisting properties.
The Chromium content of the steel allows the formation of a rough, adherent, invisible, corrosion-resisting Chromium Oxide film on the steel surface. If damaged mechanically or chemically, this film is self-healing, providing that Oxygen, even in very small amounts, is present. The corrosion resistance and other useful properties of the steel are enhanced by increased Chromium content and the addition of other elements such as Molybdenum, Nickel and Nitrogen.
There are more than 60 grades of stainless steel. However, the entire group can be divided into five classes. Each is identified by the alloying elements which affect their microstructure and for which each is named.
Why choose stainless steel over other materials?
The many unique values provided by stainless steel make it a powerful candidate in materials selection. Engineers, specifiers and designers often underestimate or overlook these values because of what is viewed as the higher initial cost of stainless steel. However, over the total life of a project, stainless steel is often the best value option. Some of the many benefits of stainless steel can be found below:
- Corrosion resistance
Lower alloyed grades resist corrosion in atmospheric and pure water environments, while high-alloyed grades can resist corrosion in most acids, alkaline solutions, and chlorine bearing environments, properties which are utilized in process plants.
- Strength-to-weight advantage
The work-hardening property of austenitic grades, that results in a significant strengthening of the material from cold-working alone, and the high strength duplex grades, allow reduced material thickness over conventional grades and therefore, cost savings.
- Long-term value
When the total life cycle costs are considered, stainless steel is often the least expensive material option.
- Impact resistance
The austenitic microstructure of the 300 series provides high toughness, from elevated temperatures to far below freezing, making these steels particularly suited to cryogenic applications.
- Fire and heat resistance
Special high Chromium and Nickel-alloyed grades resist scaling and retain strength at high temperatures.
- Stainless Steel is a “Green” material
To ensure a high quality of life, the materials that we use as consumers and manufacturers should meet not only technical performance standards, but have a long service life, be usable in a great number of applications, and be environmentally friendly. Once their service is complete, they should be 100% recyclable, thereby completing the life cycle to be used once again. Stainless steel is such a material.
The longevity of stainless is the result of the alloying composition and, therefore, it has a natural corrosion resistance. Nothing is applied to the surface that could add additional material to the environment. It does not need additional systems to protect the base metal, the metal itself will last.
Stainless steel needs less maintenance and its hygienic qualities means that we do not have to use harsh cleaners to get a clean surface. There is little or nothing to dump into the drain that could have an environmental impact.
Stainless steel products complete their service life. There is less concern about disposal since this material is 100% recyclable. In fact, over 50% of new stainless steel comes from old melted stainless steel scrap, thereby completing the full life cycle.
The easy cleaning ability of stainless steel makes it the first choice for strict hygiene conditions, such as hospitals, kitchens, abattoirs and other food processing plants.
- Aesthetic appearance
The bright, easily maintained surface of stainless steel provides a modern and attractive appearance.
- Ease of fabrication
Modern steel-making techniques mean that stainless can be cut, welded, formed, machined and fabricated as readily as traditional steels.
The above is courtesy of Suncor Stainless.
The following is a discussion of the various types of stainless steel. For other terms and their definitions you will encounter when dealing with stainless steel click here.
18-8: 300 series stainless steel having approximately (not exactly) 18% chromium and 8% nickel. The term "18-8" is used interchangeably to characterize fittings made of 302, 302HQ, 303, 304, 305, 384, XM7, and other variables of these grades with close chemical compositions. There is little overall difference in corrosion resistance among the "18-8" types, but slight differences in chemical composition do make certain grades more resistant than others do against particular chemicals or atmospheres. "18-8" has superior corrosion resistance to 400 series stainless, is generally nonmagnetic, and is hardenable only by cold working.
304: The basic alloy. Type 304 (18-8) is an austenitic steel possessing a minimum of 18% chromium and 8% nickel, combined with a maximum of 0.08% carbon. It is a nonmagnetic steel which cannot be hardened by heat treatment, but instead. must be cold worked to obtain higher tensile strengths.
The 18% minimum chromium content provides corrosion and oxidation resistance. The alloy's metallurgical characteristics are established primarily by the nickel content (8% mm.), which also extends resistance to corrosion caused by reducing chemicals. Carbon, a necessity of mixed benefit, is held at a level (0.08% max.) that is satisfactory for most service applications.
The stainless alloy resists most oxidizing acids and can withstand all ordinary rusting. HOWEVER, IT WILL TARNISH. It is immune to foodstuffs, sterilizing solutions, most of the organic chemicals and dyestuffs, and a wide variety of inorganic chemicals. Type 304, or one of its modifications, is the material specified more than 50% of the time whenever a stainless steel is used.
Because of its ability to withstand the corrosive action of various acids found in fruits, meats, milk, and vegetables, Type 304 is used for sinks, tabletops, coffee urns, stoves, refrigerators, milk and cream dispensers, and steam tables. It is also used in numerous other utensils such as cooking appliances, pots, pans, and flatware.
Type 304 is especially suited for all types of dairy equipment - milking machines, containers, homogenizers, sterilizers, and storage and hauling tanks, including piping, valves, milk trucks and railroad cars. This 18-8 alloy is equally serviceable in the brewing industry where it is used in pipelines, yeast pans, fermentation vats, storage and railway cars, etc. The citrus and fruit juice industry also uses Type 304 for all their handling, crushing, preparation, storage and hauling equipment.
In those food processing applications such as in mills, bakeries, and slaughter and packing houses, all metal equipment exposed to animal and vegetable oils, fats, and acids is manufactured from Type 304.
Type 304 is also used for the dye tanks, pipelines buckets, dippers, etc. that come in contact with the lormic, acetic, and other organic acids used in the dyeing industry.
In the marine environment, because of it slightly higher strength and wear resistance than type 316 it is also used for nuts, bolts, screws, and other fasteners. It is also used for springs, cogs, and other components where both wear and corrosion resistance is needed.
|Type Analysis of Stainless Type 304
316: For severe environments. Of course, there are many industrial processes that require a higher level of resistance to corrosion than Type 304 can offer. For these applications, Type 316 is the answer.
Type 316 is also austenitic, non-magnetic, and thermally nonhardenable stainless steel like Type 304. The carbon content is held to 0.08% maximum, while the nickel content is increased slightly. What distinguishes Type 316 from Type 304 is the addition of molybdenum up to a maximum of 3%.
Molybdenum increases the corrosion resistance of this chromium-nickel alloy to withstand attack by many industrial chemicals and solvents, and, in particular, inhibits pitting caused by chlorides. As such, molybdenum is one of the single most useful alloying additives in the fight against corrosion.
By virtue of the molybdenum addition, Type 316 can withstand corrosive attack by sodium and calcium brines, hypochlorite solutions, phosphoric acid; and the sulfite liquors and sulfurous acids used in the paper pulp industry. This alloy, therefore, is specified for industrial equipment that handles the corrosive process chemicals used to produce inks, rayons, photographic chemicals, paper, textiles, bleaches, and rubber. Type 316 is also used extensively for surgical implants within the hostile environment of the body.
Type 316 is the main stainless used in the marine environment, with the exception of fasteners and other items where strength and wear resistance are needed, then Type 304 (18-8) is typically used.
|Type Analysis of Stainless Type 316: