There are a number of different grades of stainless steel, and I want to look at the properties of two, namely A2 and A4 and discuss where they can be used.
Firstly though, lets define Stainless Steel: Also known as inox steel from the French “inoxydable” is a steel alloy with a minimum of 10.5 % chromium. The chromium helps the alloy to resist staining and corrosion. The main point to make here is that it helps resist corrosion, it does not prevent it. Perhaps we could say “A highly corrosion-resistant grade of steel”
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:
Non-metal additions are also made, the main ones being:
The British Stainless Steel Association discuss the ‘discovery’ of stainless steel here: http://bit.ly/ex4yNI
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.
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 – but more:
A2 > Type 304 can also be called 18/8 because it ‘approximately’ contains 18% Chromium and 8 % nickel
So, A2 (304, 18/8) is an 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, stoves, refrigerators , pots, pans dairy equipment, brewing industry, fruit industry, food processing plants, dye tanks, pipelines, and more
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” from bar stock.
These classes have different mechanical properties. For example A2-70 has a tensile strength of 700 Nmm-2 and 450 Nmm-2 proof stress. The BSAA has some good comparison details here: http://bit.ly/eXXLZP, and there are more tables of properties here: http://bit.ly/70aDdB
In summary, stainless steel 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 it is best to go for A4
You can see a specification sheet here: DOCUMENTS & BROCHURES : When you get their click Log In As Guest
http://home.cogeco.ca/~mquill/stainless.html : A discussion of stainless steel for rigging
http://www.cip.ukcentre.com/rust2.htm: Chloride attack on Stainless Steel
http://www.mcnallyinstitute.com/04-html/4-1.html: Corrosion problems with Stainless Steel