Differing grades of stainless steel bolts, including A2 and A4

Differing grades of stainless steel bolts, including A2 and A4

Stainless steel is available in a variety of differing grades, its discovery is generally accredited to Harry Brearley in 1912, however, there were a number of other researchers and investigators who played significant roles in the development of stainless steel bolts.

We will look more closely at two particular grades of stainless steel, A2 and A4, discussing their properties and their uses.

Let's begin with defining what stainless steel actually is…. Stainless steel is also sometimes known as inox steel (which comes from the French word ‘inoxydable' which means simply ‘stainless') which is a steel alloy containing a minimum of 10.5% chromium. The chromium addition to the alloy helps prevent staining and corrosion to steel, without totally preventing it.

Stainless steel is distinguished from normal carbon steel by the amount of chromium contained within it. Carbon steel (when unprotected) rusts easily when in contact with water/moisture and air – iron oxide is formed (or commonly referred to as rust) which then accelerates the corrosion process by forming more and more ‘rust' which then penetrates into the structure of the steel and compromising its load capacity. Stainless steel contains sufficient chromium to form a barriers against the iron oxide called chromium oxide. The chromium oxide forms a film over the steel and prevents further corrosion to the surface and restricts it, preventing the progress into the internal structure of the metal.

Other elements are added to alloy to enhance the structure and properties; such as formability, strength and cryogenic toughness. These include metals such as
Stainless steel, when used for fasteners (nuts, bolts, screws etc) have British standards. BS EN ISO 3506 replaces BS6105. Part 1 covers bolts, screw and studs, part 2 nuts and part 4 tapping screws.

Typically, type A2 stainless steel can also be called or referred to as Type 304. Likewise, A4 grade can be called type 316.

A2 > Type 304 can also be called 18/8 because it contains approximately 18% Chromium and 8 % nickel

So, A2 (304, 18/8) is austenitic steel and is non-magnetic. The chromium provides a corrosion and oxidation resistance, however, it can tarnish. It is immune to foodstuffs, sterilizing solutions, most organic chemicals and dyestuffs, also a wide variety of inorganic chemicals. As such it is used extensively for sinks, tabletops, cooking tops, refrigerators, cooking pots and pans, dairy equipment, brewing equipment, fruit industry, food processing plants, pipelines, and more due to its hard wearing, stain and pitting resistance – it also helps in applications where bacteria could be an issue (I refer to catering as an obvious example) as it possesses easy to clean properties which aids the end user.

However, for marine conditions you need more resistance to corrosion. Adding molybdenum (2-3%) to the mix provides this extra cover – and gives us the A4 grade

A4 grade then is also austenitic, non magnetic and suitable for all the same applications as A2 BUT, has the added advantage of being suitable for marine solutions. Often called Marine Grade stainless steel. The molybdenum increases the corrosion resistance to withstand attack from many industrial chemicals and solvents and of course, chlorides. Used in the production of inks, photographic chemicals, surgical implants, and the marine environment. In the UK 75% of the population lives within 50 miles of the coastline – meaning that should you be looking to lay some new decking (for example), then stainless steel bolts and screws should be preferable due to their anti-corrosion properties.

Both A2 and A4 grades come in three property classes: 50 (soft) , 70 (cold-worked) & 80 (high-strength) – the most common of which is the 70 class which is cold drawn" from bar stock.

In summary, stainless steel bolts fasteners should be used when you need to minimise the risk from corrosion. A2 is a very good grade for this, but when you need that little bit more, for example in a marine environment, then experience tells us that A4 is the way to go.
Chemical industry, Medical field, Aeros pace or Military, Petroleum chemical industry, Car, Bicycle, Motor vehicle
manufacturing, High corrosive environment, etc.

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