Stainless is a term that was applied to cover a broad range of steel types and grades developed for oxidation or corrosion resistant applications. Stainless steels are fundamentally iron alloys containing a minimum of 10.5% chromium, although other alloying elements (molybdenum, copper, titanium, nickel) may also be used in specific proportions to improve their structure and properties such as strength, formability and cryogenic toughness.
The main condition for stainless steels is that they should be corrosion resistant for a specified application or environment. Selection of an ideal ‘grade’ of stainless steel must meet the corrosion resistance needs along with the physical or mechanical properties required to realize the total service performance requirements. The stainless function of the steel results from the chromium content to undergo passivation, forming an inert film of chromium oxide on the surface. This inert layer prevents further corrosion by obstructing oxygen diffusion to the steel surface to stop corrosion from spreading into the bulk of the metal [1]. Masteel UK is an expert in stainless steel and can offer guidance regarding the best grade for a specific application.
Which Stainless Steel?
Stainless steels are not only categorized by the alloy metal content but also by their crystalline structure. The 300 series stainless steels, covered specifically in this article, have an austenitic crystalline structure, which is face-centered cubic with four atoms in the unit cell for higher density. In fact, these Austenite steels make up more than 70% of total stainless-steel production and are the most popular material across a variety of industries including: pharmaceutical, food production, and building.
They possess a maximum of 0.15% carbon (low carbon is vital to the properties of stainless steel), a minimum of 16% chromium, and adequate nickel and/or manganese to retain an austenitic structure at temperatures ranging from the cryogenic region to the melting point of the alloy. As a reference, a representative composition of 18% chromium and 10% nickel, normally known as 18/10 stainless, is often used to manufacture cutlery.
300 Series
The 304 (A2) is the most extensively used austenitic stainless-steel and this is also known as 18/8 to describe its composition of 18% chromium and 8% nickel. The 304 stainless steel has good oxidation resistance in intermittent service up to 870 °C and in continuous service up to 925 °C.
The second most frequently-used austenitic stainless-steel is the 316 grade (A4), which is also known as marine grade stainless, and used mainly for its increased resistance to corrosion. Type 316 is fundamentally an austenitic chromium- nickel stainless steel that contains an extra 2-3% molybdenum. The molybdenum boosts general corrosion resistance, enhances resistance to pitting from chloride ion solutions (for instance seawater and de-icing salts), and offers increased strength at elevated temperatures.
What is the Difference Between 304 and 316 Stainless Steel?
The straightforward answer is that 304 has 18% chromium and 8% nickel while 316 has 16% chromium, 10% nickel and 2% molybdenum. Both of these 300 grade steels are known for their excellent welding and forming properties, which give them applications across many industries. Masteel is a key supplier of 304 and 316 grades of stainless steel and can provide both cut profiles and advice on fabrication and properties.
Where Is It Used?
Grade 304 is a particular favorite in the food processing industries mainly in milk processing, beer brewing and wine making, pharmaceutical manufacture, and petrochemicals. In this area, benches, troughs, pressure vessels, sinks, chemical containers, heat exchangers, and storage tanks are all produced from 304 stainless. The steel is highly resistant to common acids and is extremely easy to fabricate into the items required, although some staining or patina may occur over a period of time.
This grade of stainless is also used in architectural applications for railings, paneling, and trim as it will offer a long service life and retain a good appearance. The 304 stainless steel does have a weakness, i.e., it is likely to corrode from chloride solutions, or from saline environments in coastal areas. Chloride ions may cause areas of corrosion, called ‘pitting’ which can pass under the protective chromium barriers to attack internal structures. Solutions with just 25 ppm of sodium chloride can have a corrosive effect.
For more tough applications within the marine environment, where there is contact from more powerful acids and chemicals, grade 316 is recommended. This has nearly the same physical and mechanical properties as the 304 stainless steel. The 316 stainless steel is common in a number of industrial applications involving processing chemicals, as well as high-saline areas such as coastal regions and outdoor areas where de-icing salts are common. Due to its non-reactive qualities, 316 is also used in the manufacture of medical surgical instruments. In addition, it is the preferred material in the pharmaceutical sector, where reaction vessels have to be extremely clean.
Conclusion
Stainless steel is considered to be one of the most significant inventions of the 20th Century and pervades so many areas of human life. Although arguments remain over its particular origin, Harry Brearley experimenting with chromium steel alloys at the Brown Firth Laboratories in 1913 in Sheffield is recognized for its discovery. Masteel UK can presently shoulder the mantle of responsibility left behind by Brearley and provide 304 and 316 stainless-steel grades to fulfill the most challenging of customer requirements.
References and Further Reading
- The Discovery of Stainless Steel, http://www.bssa.org.uk
- Jianhai Qiu. “Stainless Steels and Alloys: Why They Resist Corrosion and How They Fail”. http://www.corrosionclinic.com/corrosion_resources/stainless_steels_why_how_p1.htm
This information has been sourced, reviewed and adapted from materials provided by Masteel UK Ltd.
For more information on this source, please visit Masteel UK Ltd.