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Stainless steel: Understanding the difference between 302, 304 and 316 grades

Mike Plaster
Wednesday 25th May, 2016

Stainless steel is an exceptionally versatile and hardwearing material, and one which in modern life is impossible to avoid. As well as being an incredibly strong substance, stainless steel has the capacity to be moulded into a vast array of forms. Whether it’s the cutlery you eat with, the essential components of machines you use every day, or the litter bin you use, you most probably encounter stainless steel on an hourly basis. The fact that stainless steel continues to look good and perform well year after year is one of the reasons the material is the preferred choice for quality street furniture. As well as being durable, it must also represent an aesthetically pleasing addition to our public spaces.

What’s stainless steel made of?

At its most basic, stainless steel is a metal alloy formed of iron and chromium. The percentage of chromium will vary according to the desired quality of the end product, and other elements may be introduced to the alloy in order to give it specific attributes and capabilities. For example, more nickel will increase ductility and make the finished product tougher, especially in acidic environments. A greater quantity of chromium, one of stainless steel’s core ingredients will improve the corrosion-resisting qualities of the end product. Conversely, a larger mix of nitrogen increases mechanical strength, titanium would improve resistance to granular corrosion, and aluminium helps the end product to resist oxidisation.

What are its benefits? While other metals will rust and corrode, stainless steel will retain its finish, and its strength. However, despite all these other metals being added, it is the chromium in all grades of stainless steel that give it its power. Chromium enables a molecular layer of chromium oxide to form on the surface of the steel. Only around a millionth of a centimetre thick, this film nevertheless fiercely protects the metal beneath. It is passive, meaning it does not react with other materials, while its high level of tenacity is a measure of how it clings determinedly to the surface of the object. It is also a self-renewing film, which means that if it is forcibly removed from the stainless steel, it will quickly appear again – with more chromium oxide taking the place of that which has been removed.

The grades:

Grade 304

Most commonly, if stainless steel is being used, it will typically be a grade 304 or related product. Grade 304 stainless steel is comprised of around 18% chromium and 8% nickel. As well as offering good resistance to corrosion and ease when welding, this formula has excellent ductility and can be used in an extremely wide range of applications. In the home, you’ll find grade 304 used in sinks, pots, pans and sanitary surfaces. The substance is also used extensively in architectural cladding, handrails, transport containers, street furniture, surgical instruments and chemical plant equipment.

Grade 302

Grade 302 stainless steel is an alloy that, in addition to containing similar quantities of chromium and nickel to grade 304 (18% and 8% respectively), also contains higher carbon levels. Often sold in strip and wire forms, grade 302 stainless steel is primarily used by the manufacturers of conical compression springs. One of the product’s main attributes is that it is resistant to solvents, acids and chemicals. Grade 302 stainless steel is frequently found in spring form within machines used in kitchens, food processing businesses, medical settings and dairies.

Grade 316

After grade 304 stainless steel, grade 316 is perhaps the most commonly used. The substance contains molybdenum, which lends the product extra resistance to corrosion and oxidisation. This makes it particularly attractive to those looking to use the product in chloride or marine environments. Equally, the substance is ideal for the manufacturing of quality street furniture as it offers real durability along with an attractive finish and an ease of form. Grade 316 is very tough but equally, it has exceptional formability. These twin attributes make it ideal for the production of springs for the marine, oil and gas industries, which simultaneously need both movement and power. Another notable property of grade 316 stainless steel is that it will demonstrate high-level tensile strength, even in very high temperatures, and will not deform permanently under major mechanical stress.

All three grades of stainless steel present their own unique strengths and your choice of material should be determined by the practical requirements of your project, as well as the aesthetic you’re hoping to achieve.

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