Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium. All stainless steels have a high resistance to corrosion due to the naturally occurring chromium-rich oxide film formed 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 corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.
The following information has been extracted from the ASSDA Reference Manual, published by ASSDA (Australian Stainless Steel Development Association) and should be used as a guide only.
Benefits of Stainless Steel
- Corrosion resistance - All stainless steels have a high resistance to corrosion. Low alloyed grades resist corrosion in atmospheric conditions; highly alloyed grades can resist corrosion in most acids, alkaline solutions, and chloride bearing environments, even at elevated temperatures and pressures.
- High and low temperature resistance - Some grades will resist scaling and maintain high strength at very high temperatures, while others show exceptional toughness at cryogenic temperatures.
- Ease of fabrication - The majority of stainless steels can be cut, welded, formed, machined and fabricated readily.
- Strength - The cold work hardening properties of many stainless steels can be used in design to reduce material thicknesses and reduce weight and costs. Other stainless steels may be heat treated to make very high strength components.
- Aesthetic appeal - Stainless steel is available in many surface finishes. It is easily and simply maintained resulting in a high quality, pleasing appearance.
- Hygienic properties - The cleanability of stainless steel makes it the first choice in hospitals, kitchens, food and pharmaceutical processing facilities.
- Life cycle characteristics - Stainless steel is a durable, low maintenance material and is often the least expensive choice in a life cycle cost comparison.
Types of Stainless Steel
In addition to chromium, nickel, molybdenum, titanium, niobium and other elements may also be added to stainless steels in varying quantities to produce a range of stainless steel grades, each with different properties. Austenitic and ferritic grades account for approximately 95% of stainless steel applications.
Austenitic
When nickel is added to stainless steel in sufficient amounts the crystal structure changes to "austenite". The basic composition of austenitic stainless steels is 18% chromium and 8% nickel. Austenitic grades are the most commonly used stainless steels accounting for more than 70% of production. Most common grades are 301, 304 & 316.
Basic properties: excellent corrosion resistance; excellent weldability; excellent formability and ductility; excellent hygiene characteristics; good high and excellent low temperature properties
Common grades & uses:
- Grade 301 – general purpose stainless steel with good corrosion resistance for most applications. Used where the high work hardening exponent is desirable. Can be supplied cold worked to give high strength. Used for structural applications such as rail carriages and wagons. Available as sheet and coil.
- Grade 304 – general purpose stainless steel with good corrosion resistance for most applications. Used for architecture, food processing, domestic sinks and tubs and deep drawing applications. Available as sheet, coil, plate and bars.
- Grade 316 – used where higher corrosion resistance is required, ie marine equipment. Can be welded up to 3.0mm thick without subsequent heat treatment. Available as sheet, coil, plate, tube and pipe.
Ferritic
These are plain chromium stainless steels with chromium content varying between 12 and 18%, but with low carbon content. Most common grades are 409, 430 & 444.
Basic properties: moderate to good corrosion resistance increasing with chromium content; not hardenable by heat treatment and always used in the annealed condition; magnetic; weldability is poor in thick gauges; formability not as good as the austenitics
Common grades & uses:
- Grade 409 – heat resistant steel, easily formed and welded. Mainly used for automotive exhausts or welded applications where superior performance to galvanised steel is required. Available as sheet and coil.
- Grade 430 – interior architectural components, stove and automotive trims, dishwasher and clothes dryer liners. Fusion welds tend to be brittle. Available as sheet, coil, plate and bar.
- Grade 444 – heat exchanger and hot water tanks, and in chloride containing waters. Not prone to chloride stress corrosion – superior resistance to pitting, crevice and intergranular corrosion. Possesses excellent deep drawing properties. Available as sheet and coil.