Back Purging Stainless

[tab]

I'm thinking about starting to do a bit of stainless tig welding, doing a few searches and having a read on here i see that back purging comes up quite a bit to stop the weld getting contaminated from behind.

My question is, how essential is it to actually do it??

I'm only asking as while i'm practicing i dont want to be wasting gas that i dont need to be wasting. I'm thinking along the lines of when i do some t fillet runs will i need to protect the back if the weld doesnt fully penetrate, i've done lots of mild steel where the weld doesnt come right through the back and never had any weld strength problems so wonder if stainless was the same?

Thanks in advance for any advice/answers

 

[hotrodder]

Fillets will be fine unless we're talking really thin stuff and that can be sorted by clamping a heat sink (chunk of copper or ally) to the back. Partial pen butts in thicker material will also be OK. It's when the back of the weld gets hot enough to scale (or melt in the case of full pen butts obviously) that you need to purge... chromium is very reactive (it's chromium oxide that makes stainless, err, stainless) and the result can be a right mess known as a 'coked' or sugared weld if the purge is skipped. While that isn't a big issue for practice (and even some jobs) the weldpool behaves differently compared to what it should be like and finished welds will be less 'stainless' than they should be, typically be dirtier/greyer and very likely to have lack of fusion defects

 

[langy]

I wouldn't worry about it while practising

 

[tab]

Brilliant, cheers folks thats hit the nail on the head with everything i was wondering about.

 

[Burf86]

I have worked in a lot of places, from sheet metal to pipework and have never seen anyone back purge, anything. It is not essential. It's just the right thing to do if you want it perfecto

 

[hotrodder]

It's not about perfection, it's about corrosion resistance. Even the 'acceptable' heat tint colours from welding lower it...
https://boc.com.au/boc_sp/downloads/gas_brochures/BOC_216601_Purging_While_Welding_Brochure_v7.pdf
http://www.bssa.org.uk/topics.php?article=2

A sugared/coked weld from not purging means you've 'burnt' out a bunch of the chromium. From a mechanical POV the weld almost certainly contains oxide inclusions and possibly has LOF (lack of fusion) defects where the coked mess is. How much of an effect it has on the amount of chromium left to keep the steel 'stainless' i don't know but the weld will be less corrosion resistant than it should be. Whether/how important any of that is depends on the application

 

[Burf86]

Well, that just goes to show that everyone I have worked for (and me) doesn't have a clue....

 

[biffa]

Also for food grade stuff you can have that horrible sugar on the back of a weld whether it be pipe or a vessel because it is a dirty trap, not stainless but to weld titanium you need a purge and a trailing shield as any more than straw colour is unsuitable as it ruines the properties of titanium

 

[Burf86]

Does purging/not purging affect the strength, or it is just the corrosion resistance? I didn't manage to find info relating to this question in the BOC link.

 

[biffa]

If purging is done correctly you will have weld reinforcement both sides of the weld, but if it was not purged you would only have weld reinforcement one side so if purging is done correctly it does improve strength