Stainless Steel Grades, Uses and Properties

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At Stanton Marine the vast majority of our stainless steel fasteners (nuts, bolts and washers) are A4 grade, otherwise known as Marine Grade.  For a brief explanation in relation to stainless steel and its grades please see below:

Stainless steel differs from carbon steel by the amount of chromium present. Unprotected carbon steel rusts readily when exposed to air and moisture. This iron oxide film (the rust) accelerates corrosion by forming more iron oxide. Stainless steels contain sufficient chromium to form a passive film of chromium oxide, which prevents further surface corrosion and blocks corrosion from spreading into the metal’s internal structure.  Passivation only occurs if the mixture of chromium is high enough.

Other alloying elements are added to enhance the structure and properties such as formability, strength and cryogenic toughness.   These include metals such as:
Nickel, Molybdenum, Titanium, Copper
Non-metal additions are also made, the main ones being:
Carbon, Nitrogen

Grades of stainless steel A2, A4 in relation to fasteners.  To all intents and purposes, when we see type A2 stainless steel, we can also call it Type 304. Likewise A4 grade can be called type 316.

Stainless steel, when used for fasteners (nuts, bolts, screws etc) have British standards. BSENISO 3506 replaces BS6105. Part 1 covers bolts, screw and studs, part 2 nuts and part 4 tapping screws.

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

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 and is the material used for our fasteners.

The A4-70 class has a typical tensile strength of 700 Nmm-2 and 450 Nmm-2 proof stress.

Some of our A4 fixings .....
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Some of our A4 fixings .....
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