Post 389 ⇒ by Gautam Shah →
Stainless steel is an alloy steel, with low carbon content but with 10 to 30% Chromium. Chromium provides resistance to corrosion. The presence of nickel adds to corrosion resistance in acid environments and fabrication characteristics. The presence of molybdenum improves corrosion resistance in sea water or chloride conditions. Other elements such as, titanium, aluminum, niobium, copper, nitrogen, sulfur, phosphorus, and selenium may be added to increase corrosion resistance in specific environments and impart special mechanical characteristics, such as, form-ability, weldability, machine-ability, strength, hardness, etc.
Chromium was first reduced from its ore in late 17th C, and by beginning of 20th C, it began to be used to form alloy with steel. The stain-less characteristic is achieved with chromium in greater than 10.5 %. The first stainless steels were straight iron-chromium alloys, which were used for production of cutlery.
This article created for Interior Designers, avoiding complex terms and too many technical details. A lay person or non expert like Interior designer is required to decide about using stainless steel, few basic quarries arise. Is the SS completely rust proof? Is it a food grade material? Can it be cast?, Can it be rolled and formed without problems like fracture or cracking? Is it magnet-proof? Can it be welded and How? Can it be machined?
Most stainless steels are first melted in electric-arc or basic oxygen furnaces and subsequently refined in another steel-making vessel, mainly to lower the carbon content. In the argon-oxygen decarburization process, a mixture of oxygen and argon gas is injected into the liquid steel. By varying the ratio of oxygen and argon, it is possible to remove carbon to controlled levels by oxidizing it to carbon monoxide without also oxidizing and losing expensive chromium.
There are THREE major groups of stainless steels: Ferritic, Austenitic, and Martensitic.
FERRITIC (AISI 400 series)
These steels contain less than 0.10% carbon and are magnetic. Ferritic steels cannot be heat hardened and so cannot be welded well. Ferritic steels lack toughness, ductility, poor in extra elongation and susceptible to cracking at high temperature. These are less expensive and so used for certain applications such as for cold work. The general purpose grade is 430.
AUSTENITIC (AISI 200 and 300 series)
These stainless steels form the widest and most popular range of products (nearly 70%). These can be formed, welded and withstand cryogenic to red-hot temperatures. These steels can be made very strong by cold work, but are not harden-able by heat treatments. These are nonmagnetic. They contain between about 16 and 25% chromium. Nickel-containing steels are non-ferromagnetic and used for geophysical surveying equipment, X-ray scanners, etc.
MARTENSITIC (AISI 400 series)
These type of steels share some qualities with ferritic stainless steels, but have higher carbon content, up to 1%. The carbon content allows tempering and hardening such as for knives and tools. These steels are useful in conditions where strength of the steel supersedes the need for corrosion resistance. These steels are easy to machine but produce a serrating chip to a slowdown of the productivity. Martensitic stainless steels are selected for better tensile strength, creep, and fatigue strength properties, with reasonable corrosion and heat resistance.
Duplex steels are a combination of ferritic and austenitic steels. With addition of elements such as Aluminium, Copper and Niobium, Precipitation hardening steels become very strong.
Welding Martensitic Stainless Steels: Martensitic stainless steels are most difficult of the stainless steel alloys to weld. Higher carbon contents produce greater hardness and so chances of cracking. Localized stresses associated with Martensitic transformation, also cause cracking. Specific atmosphere (argon) welding reduces cracking.
Stainless steels are used for curtain walls, spandrels panels, mullions, windows, doors, panelling, cladding, partitions, grills, louvers, railings, fascias, ceilings’ panels, signage base, stairs, toilets, basins, bathtubs, shower stalls, automobile components, trimmings, building hardware, furniture, cabinets, etc. In industry these are used for food and pharmaceutical machinery, chemical plants, reaction vessels, storage tankers, liquid packing drums and tins. Flexibility and elasticity permit construction of snap assemblies. Stainless steels are used for surgical instruments, equipment, body implants and orthopedic support systems. Stainless steels are used for high temperatures, such as in aircraft jet engines or gas turbines.