Stainless is a term that was adopted to cover a wide range of steel types and grades developed for corrosion or oxidation resistant applications. Stainless steels are essentially iron alloys containing a minimum of 10.5% chromium although other alloying elements (Nickel, Molybdenum, Titanium, Copper) may also be used in specific proportions to enhance their structure and properties such as formability, strength and cryogenic toughness.

The main condition for stainless steels is that they should be corrosion resistant for a stated application or environment. Selection of a suitable ‘grade’ of stainless steel must meet the corrosion resistance requirements along with the mechanical or physical properties needed to realize the overall service performance requirements. The stainless function of the steel comes from the chromium content to undergo passivation, forming an inert film of chromium oxide on the surface. This inert layer prevents further corrosion by hindering 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 advise on the best grade for your application.

Which stainless steel?

Stainless steels are not only classified by the alloy metal content but also by their crystalline structure. The 300 series stainless steels, which are being considered here 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 over 70% of total stainless-steel production and are the most popular material across a range of industries including: food production, pharmaceutical and building. They contain a maximum of 0.15% carbon (low carbon is important to the properties of stainless steel), a minimum of 16% chromium, and sufficient 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 typical composition of 18% chromium and 10% nickel, commonly known as 18/10 stainless, is often used to manufacture cutlery.

300 Series

The 304 (A2) is the most widely used austenitic stainless-steel and this is also known as 18/8 to describe its composition of 18% chromium and 8% nickel. 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 common austenitic stainless-steel is the 316 grade (A4), which is also called marine grade stainless, used primarily for its increased resistance to corrosion. Type 316 is essentially an austenitic chromium- nickel stainless steel that contains an additional 2-3% molybdenum. The molybdenum increases general corrosion resistance, improves resistance to pitting from chloride ion solutions (for example seawater and de-icing salts), and provides increased strength at elevated temperatures.

What is the difference between 304 and 316 stainless steel?

The uncomplicated answer is that 304 contains 18% chromium and 8% nickel while 316 contains 16% chromium, 10% nickel and 2% molybdenum. Both of these 300 grade steels are known for their exceptional welding and forming properties, which give them applications across a number of industries. Masteel is a major supplier of 304 and 316 grades of stainless steel and can provide both cut profiles and advice on properties and fabrication.

Where is it used?

Grade 304 is a particular favourite in the food processing industries particularly in beer brewing, milk processing & wine making, pharmaceutical manufacture and petrochemicals. In this area benches, sinks, troughs, pressure vessels, heat exchangers, chemical containers and storage tanks are all produced from 304 stainless. The steel is very resistant to common acids and is very easy to fabricate into the items required, although some patina or staining may occur over a period of time.

This grade of stainless is also used in architectural applications for panelling, railings and trim as it will give a long service life and retain a good appearance. 304 stainless steel does have a weakness, it is liable to corrosion from chloride solutions, or from saline environments in coastal areas. Chloride ions may cause areas of corrosion, called ‘pitting’ which can pass beneath the protective chromium barriers to attack internal structures. Solutions with only 25 ppm of sodium chloride can have a corrosive effect.

For more robust applications within the marine environment of with contact from more powerful acids and chemicals grade 316 is recommended. This has virtually the same physical and mechanical properties as 304 stainless steel. 316 stainless steel is common in many industrial applications involving processing chemicals, as well as high-saline environments such as coastal regions and outdoor areas where de-icing salts are common. Because of its non-reactive qualities, 316 is also used in the manufacture of medical surgical instruments and in the pharmaceutical industry, where reaction vessels have to be clean beyond reproach, 316 is the material of choice.

Finally

Stainless steel is one of the great inventions of the 20th Century and pervades so many areas of our lives. Although arguments remain over its exact origin, Harry Brearley experimenting with chromium steel alloys at the Brown Firth Laboratories in 1913 in Sheffield is credited with its discovery. Masteel UK can now shoulder the mantle of responsibility left behind by Brearley and provide 304 and 316 stainless-steel grades to satisfy the most demanding of customer requirements.

References

  1. The Discovery of Stainless Steel, http://www.bssa.org.uk/about_stainless_steel.php?id=31
  2. 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