304 Stainless Steel: Strength, Structure and Properties

Steel is quite rightly described as 'the backbone of modern industry'. The material is an alloy composed primarily of iron and carbon: adding other elements such as chromium creates different types or 'families' of steel, one of which is the widely-used stainless steel.

304 stainless steel is made up of 66-75% iron, 18-20% chromium and 8-10.5% nickel, along with traces of other elements including carbon, manganese, phosphorus, sulphur and silicon. Its density is measured at about eight grams per cubic centimetre.

304 is the most commonly produced grade, accounting for over 50% of the stainless steel consumed by the global market. The material is austenitic, in common with around 70% of commercially-produced stainless steels. The term 'austenitic' refers to the material's molecular structure and results from the addition of nickel to the alloying mix.

Austenitic stainless steels like grade 304 display many useful qualities, such as:

• Excellent resistance to corrosion
• Zero magnetism when annealed
• The ability to be work-hardened
• High ductility and ease of forming
• Exceptional weldability with or without fillers
• Hygienic and easy to clean
• Good performance at extremes of temperature

Standard 304 stainless steel has a carbon content of around 0.08%. Other varieties in this family are 304L (lower carbon percentage of 0.03%) and 304H (higher carbon percentage of up to 0.1%). The lower carbon content of 304L is often used for large welding components as it is more ductile. 304H is primarily used for high-temperature applications as the higher carbon content helps to improve strength.

In comparison to mild steel, the yield strength of austenitic stainless steel is a significantly low proportion of its tensile strength (for more detailed information about yield and tensile strengths, please see the previous edition of the BS Stainless blog). Whereas the yield strength of mild steel is between 65% and 70% of its tensile strength, this figure drops to between 40% and 45% in the family of austenitic stainless steel.

This yield strength can be significantly improved by cold working the material. Stainless steel for making spring wire, for example, can have its yield strength raised to up to 8%% of its tensile strength by cold working.

You can find out more information about the different types and properties of various grades of stainless steel on the BS Stainless website, along with a complete catalogue of our extensive product portfolio.