stainless grades and disimillar??

[asmeix6g]

Hi all, right got a question here thats been bugging me for a while regarding different grades of stainless... i hear all the time, "316 is pure stainless" etc how true is this? stainless is an alloy so surely it cannot be pure? 304 308 316 are different compositions right? so why could one be purer than another?
Also disimillar rods... this old guy at work who everyone thinks is a welding god said 309 is pure stainless so when you weld disimillar its 'purity' is still really high in the weld? how do dissy rods actually work, whats the science behind it?
Also whilst im popping questions out willy nilly, at the moment were doing some big gas seperating vessel for sellafield nuclear powerstation, the WPS is single V, then air arced back and welded.... how come we can use air arc? i would of thought it introduces too much carbon to the party?? :S
so confused right now...

 

[TechnicAl]

Ive never heard of pure stainless

309 is over alloyed (23%Cr 12% Ni 65%Fe) so that when it is diluted by mild steel (100%Fe) you end up with 304 (19%Cr 9%Ni) or thereabouts
If you do the same to 308L you end up with 12%Cr 5% Ni which gives a microstructure which is very crack susceptible.

316L is slighltly higher alloyed than 308L so it has better corrosion resistance in some circumstances.

Air Arc tends to oxidise / nitrides the surface, ideally the surface should be ground to remove any imurities added by the gouging. Remember air is predominently nitrogen. Adding carbon isnt as easy as most people imagine.

makes sense?

 

[KemppiFrog]

Not much of a reply, but 316 is more corrosion resistant in marine work, while 304 is a bit stronger. So all fittings (and rigging) in the boat world are normally 316 an L suffix means it is suitable for welding. As for 'pure', they are alloys and designed for different uses.

 

[Hitch]

It's not even possible to have pure stainless surely..... its all stainless whether it be 304....316....321......varying quantities of different things make it up....as you say.

 

[TechnicAl]

Well it starts with 12% Cr which is the minimum alloying addition to impart stain-less properties. Therefore they all contain Chromium.

I wouldnt call any of them pure stainless but maybe 12%Cr Steel (410 grade) could be called Basic Stainless.

 

[asmeix6g]

hey thanks technicAL yes, it does make sense. its been bugging me for months! especially the 'pure stainless' thing! like you say its an alloy so how can it be pure?
so if 309 is 23%Cr 12% Ni 65%Fe what is 316?
also i assume 309 and 312 are for different grades of stainless to carbon steel? im assuming they are both over alloyed but to different degrees so its better suited for one grade than the other.
what grades would you use 309 on and what grades would you use 312 on?
Thanks so much for this info much respect

 

[KemppiFrog]

316 has a bit of molibdenem (too late to spell...)
KF

 

[TechnicAl]

316 is 19Cr 12Ni 3 Mo and the rest is Iron (there are impurities)

312 is 30Cr 10Ni and in many instances does the same as 309 but it has double the tensile strength so its good for welding high strength steels that are difficult to weld (crack easily). With 312 youre less likely to get hydrogen cracking than if you used something else of similar strength.

309 is most for SS to MS but it can also be used to clad MS. There is a 309Mo that is used to clad mild steel so that the surface is basically 316L (309Mo diluted by MS).

Its one of those where its easy once you know

 

[medroberts]

Hi TechnicAl,
Could you tell me if 29/9 rods are the same as 312-16 rods only another name ?
Med Roberts

 

[TechnicAl]

Yes

29/9 or 30/10 or 312....all the same