I have an exhaust on my car from Tube engineering,I'm trying to establish what grade of Stainless it is made of. Does anyone know if it is 300 or 400 series please?

Its probably 316 or if its a little cheaper, 304. I don't think anyone makes exhausts out of anything other than ausenitic alloy as 400 is an utter swine to weld.

Get a magnet on it; if its 300 series (316, 304 etc) the magnet wont stick. If it sticks, its ferritic stainless steel (400 series). It could also be 200 series stainless which is a low nickel, manganese stabilised austenitic "stainless" grade which will rust before your eyes if it so much as goes near water.

I generally thought that exhaust manifolds and the like would be easier to manufacture from ferritic stainless but I may be wrong.

I doubt it will be a duplex alloy but it is possible. full chemical analysis is the best way to find out for sure as people will often tell you you are getting one grade of alloy and sell you another (sometimes without knowing it themselves).

why do you ask?

[Edited on 7/5/11 by steven]

The reason I ask is: I'm fed up with taking a life time cleaning off the Blue/Brown before polishing it up. Someone suggested I use a pickling paste, so I have contacted a supplier and he wishes to known the grade before supplying me.

if you have visible oxides forming on a stainless exhaust I'm guessing it isnt 316 or 304 and you've got a low chromium "stainless" grade. Pickling paste is basically an acid flux to burn off the oxides. it will eat into any passive chromium oxide layer (this is what makes the steel stainless) too so use with caution or you could end up getting pitting corrosion which once it sets in, can be a real issue.

Try the magnet - it will tell you if its ferritic stainless (it will stick). If its non-magnetic, you'd have to do some chemical analysis to find the grade (200/300 series) and specific type but this will mean getting a sample off to a lab.

Just to be clear - the blue/brown you refer to, is this rust/corrosion or "blueing" from heat?

[Edited on 7/5/11 by steven]

Blueing from the heat.

the blueing is desirable Cr oxide layer, if you keep removing this, you keep exposing the metal to oxygen and you will, step by step, eat away your exhaust. That said, it wont happen overnight.

Personally, I'd leave the blueing well alone, but if you want to go down the route of acid etching and your supplier wants to know if it is 300/400 (austenitic/ferritic), a magnet will tell you.

Edit: just found this which looks to be a no acidic alternative:

http://www.youtube.com/watch?gl=GB&hl=en-GB&v=l_nSgyWR8dw

[Edited on 7/5/11 by steven]

I would just leave the oxide film on it, pickling paste is also not very nice stuff to use as it usually contains hydrofluoric acid which is very nasty stuff I avoid it at work when welding stainless and just wire brush while still hot to remove the scale.
Paul