Stainless steel is ‘ordinary’ (carbon) steel to which various other things, particularly chromium, have been added to make it ‘stainless’.  The chromium reacts with air to form a light, invisible oxide covering which protects from further corrosion.  If the covering is damaged, further corrosion takes place thus maintaining the film.

In the same way that carbon steel has various strengths, so stainless steel does as well, and combining this with the amount of ‘stainlessness’ can result in a wide variety (over 200) of types.  The position is further complicated by the fact that steel for different purposes has different standards and different ways of specifying.  The following is a guide to the various standards and the situation generally with regard to the use of stainless steel.

The types fall into 3 main usage groups:

1 Flat plate/sheet/strip
This is the easiest to define and is what is used to make angles, plates etc

2 Bar and rod
Precasters rarely use this, but others use it to make sockets and proprietary fixings

3 Fasteners
This is the term used for bolts, set screws, studding and nuts etc

Groups 1 & 2 are treated in the same way, group 3 is totally different.

Flat plate and sockets etc:

The common grades used for fittings are listed below, together with the various ways of specifying them, which may be found in specifications and literature -

'Old' UK USA Germany Europe
303S31 303 1.4305 X 10 CrNiS 18 9
304S16 304 1.4301 X 5 CrNi 18 10
304S11/304S31 304L 1.4306 X 2 CrNi 18 10
316S33/316S31 316 1.4401 X 5 CrNiMo 17 12 2
316S11 316L 1.4404 X 2 CrNiMo 17 13 2

The common application of these is as follows

303/1.4305 - generally used for sockets etc
304/1.4301 - suitable for rural, urban, light industrial.  Not suitable for heavy industrial or coastal
316/1.4401 - suitable for industrial and coastal

the L grades signify low carbon and need only be used when welding plates thicker than 16mm

For most angles and plates therefore ‘standard’ grade is 1.4301 (304)

and  ‘higher’ grade is 1.4401 (316)

For sockets, the ‘standard’ grade is 1.4305 (303) and the ‘higher grade is 1.4401 (316)