What Does 18/8 Stainless Steel Mean?
18/8 stainless steel is the most widely used and flexible austenitic form of stainless steel. The numbers 18/8 represent the composition of this steel; it is 18% chromium and 8% nickel, making it very resistant to corrosion and oxidation.18/8 stainless steel is also highly durable and easy to fabricate. It's also simple to clean and available in various appearances and finishes. 18/8 stainless steel may also be known as austenite steel or 304 grade steel.
Stainless steel is a low-carbon steel that contains chromium at 10% or sometimes more by weight. The chromium gives the steel its stainless, corrosion-resistant properties. Stainless steel's chromium content also allows the formation of a rough, corrosion-resistant chromium oxide film on the steel surface. If it is damaged in any way, this film is regenerates—as long as oxygen, even in very small amounts, is present.
While 18/8 steel's ability to resist corrosion—as well as its other useful properties—are enhanced by its increased chromium content. However, the addition of other elements, like molybdenum, nickel and nitrogen, can also play a part in this. There are over 60 grades of stainless steel, but the entire group can be divided into just about five classes with each being identified by the alloying elements that affect their microstructure (for which they are named) and how they react with these elements.
Corrosionpedia Explains 18/8 Stainless Steel
18/8 stainless steel is an outstanding metal for applications like:
- Auto trim and molding.
- Kitchen equipment.
- Wheel covers.
- Truck bodies.
- Exhaust manifolds.
- Storage tanks.
- Pressure piping and vessels.
The percentage of nickel and chromium present in this grade of stainless steel provides this metal with good corrosion resistance properties, particularly to moderately caustic and acidic solutions. It also performs similarly in most non-severe conditions, except in welding. In cases where stainless steel is to be welded, stainless steel 304L grade is preferred, as it offers more resistance to intergranular corrosion.
In atmospheric and pure water environments, lower alloyed grades are the better option because they tend to better resist corrosion, while high-alloyed grades resist corrosion in most acids, chlorine and alkaline solution-heavy environments—properties often used in process plants.
In comparison to the 400 series, 18/8 grade stainless steel has better resistance to corrosion. It can be hardened by cold working and is not magnetic. 18/8 stainless steel has superior corrosion resistance but shows signs of corrosion when exposed to chlorides which is why it would not be a good choice for marine applications.
The label “18-8” designates products that made from 300-series stainless. It refers to the 18% chromium and 8% nickel alloy mixture—the two different alloys in the steel. All 300 series stainless steel are made up of this 18/8 mixture, although there may be differences in its chemical composition when comparing the different grades of the 300 series. Certain grades tend to be more resistant than others against different types of corrosion.
Austenitic stainless steels can have yield strength of about 200 megapascals (MPas) while manufactured and they can be subsequently strengthened by cold working, which can increase the yield strength up to ten times. They can retain their ductility at cryogenic temperatures and their strength at high temperatures, which is not possible for ferritic grades of stainless steels. Their corrosion resistance is very high—right from daily use to withstanding boiling seawater. However, though superior 18/8 stainless steels have a low resistance to cyclic oxidation when compared to ferritic stainless steels and they are also prone to stress corrosion cracking. 18/8 stainless steel's endurance limit is also lower (~30% of their tensile strength) than ferritic steel's (~50 - 60% of their tensile strength). This increases 18/8 stainless steel's susceptibility to fatigue failures when compared to ferritic grades.