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root fill and clap
Why isn't a back purge necessary for carbon steel TIG pipe welding? And while we are there, what protects the internal root bead of a MIG pipe weld?
Technic Al
Because its not used for highly corrosive chemicals.

If you dont back purge stainless, the weld metal will oxidise and that will result in lower corrosion resistance.

The shielding gas will protect the mild steel weld
root fill and clap
Thanks for that, but what's stopping the (carbon steel) internal bead from bubbling away in porosity without atmospheric protection? Because by that rational, there wouldn't be a need for a gas shield on the external weld areas.
I was told the Tig answer by a very good and learned lecturer 25 years ago and have completely forgotten the precise answer and haven't been given a satisfactory reply to the question since.
Regarding stainless, the bead is compromised completely not just oxidised!
Anyone else have suggestions?
The gas from the torch will flow around the leading edge of the weld pool and give a degree of protection to the inside. It is not perfect but adequate. Compare carbon steel weld which has had a back-purge and one that has not and you will see the difference. As Technic Al says it is not needed. Only time it is needed is welding alloyed carbon steels. Back-purge was also used in the past to avoid suck-back. Welding stainless is a different kettle of fish. Even with common or garden grades tou have to keep the oxygen level below 1000ppm to avoid potential corrosion problems caused by a depletion of the passive layer just below the surface.
The filler wires for mig and tig welding contain de-oxidisers such as Aluminium, Titanium, Manganese and Silicon which helps prevent porosity along with the gas shield from the torch, Dav
I think the de-oxiders in the wire are there to cater for the steel being welded.
root fill and clap
I understand what you are saying regarding the flow around effect, but if you try TIG welding, say a flat plate with the argon turned down low enough to just protect the tungsten, the weld pool will just bubble away and the weld will have no integrity. Also, when welding a pipe and filling in the last 'hole' there will be a period of no flow around.
I think the answer lies somewhere at the chemical level, like with OA welding where the fuel gases and oxygen have a reducing effect and then in addition the secondary combustion gasses provide more protection.
I recently asked a senior welding inspector/supervisor for a prominent pipe contractor to a major oil company this very question and he had no idea. But instead of saying so, he made up an answer and just babbled! It degenerated into the two of us disagreeing about his farcical claim that CO2 was an inert gas, because 'thats what MIG stands for'!

Hi All!

Interesting subject!

For years, I knew you had to purge stainless and also chrome alloys above a certain percent chrome . But such a simple question has us at a loss to explain adequately!

Why does a carbon steel pipe butt not require a purge? Why is the internal root not full of porosity?

Why does a MIG S/S root not oxidise like a TIG root when no purge is present?


root fill and clap
Thanks Rodof!
In my inarticulate, roundabout way, that's what I was trying to ask!
A "scientific" reason why mild steels very forgiving to weld:
Iron oxide melts at a lower temperature than the plain steel it oxidised from.
That's why you can oxy-acetylene cut steel - the oxide flows away - you see that.
With welds, the iron oxide floats to the surface and is still flowing around as a liquid when the steel solidifies. So it "keeps out of the way" and doesn't cause trouble.

For stainless steels chromium oxide has a melting point way up at over 2435C (iron melts at 1538C), so the oxide takes control and creates that hideous non-flowing encrustation if you don't backpurge a stainless weld.
mtb pete
But aluminium oxide has a much higher melting temp than aluminium and (while it can't be cut with oxy fuel) can be welded without a purge. It's not just because of the reverse polarity moving oxides as the arc can't 'clean' the back of the weld and it's easier to put a nice root in without a root gap so torch shielding gas doesn't help

The deoxidisers in er type steel wires surely play a significant role- carbon steel doesn't weld well autogenously with TIG and using oxyfuel wire usually results in porosity with TIG. Doesn't really alter the amount of scaling on the root side though

From the way stainless oxidises without a purge it has a significantly higher affinity for oxygen than iron (at least at high temps). More so than it's position in a general reactivity series would sugggest? Higher than the elements usually used as deoxidisers?

The MIG stainless coking less/not coking... could it be as simple as the faster freezing weldpool? In the case of TIG autogenous tacking thin stainless the back doesn't coke as long as you're quick.
I think you've got the temp thing wrong try some welding. In my book stainless and steel alloys have a lower melting point than LCS but I could be wrong. I also think you're bang on with why we needn't purge. I reckon that it's something to do with millscale, I got distinctions in my city and guilds but that's as far as my tech' know how goes. Logic tells me that if you got a bit of 3mm plate and had a play with the tig, it wouldn't be long before you could sink a bead through, said plate would be distorted to hell and the bead would be covered in millscale.
That's my ten bobs worth.
Interesting Posts!

Several years ago, one of the Instructors at was then Mitsui Babcocks was given the task of making a 2" S/S pipe butt weld with no purge or paste and getting it passed the 'Bomb', After several attempts he finally did manage to pass the x-ray with acceptable results! It wasn't pretty, but it was acceptable !

Technic Al
Mild steel will be protected by the shielding gas until it has solidified after which it wont be affected much by the atmosphere.

Stainless continues to be affected by the atmosphere after the shielding gas has moved on. The Cr can still be oxidised when the stainless has solidified. The back side of the root is blocked off from getting more shielding gas.

When filling and capping stainless this isnt as much of an issue because there will be more gas available for shielding.

Note that you can 'coke' a ss root even when filling out, if too hot - point being the shielding gas when welding the root is not the only consideration - thats why you leave the purge on untill at least the hot-pass is completed.
3.2 Inspector
If you can "coke" the root of a SS weld - you can make a lot of money :)
Technic Al
Thats right

and with mild steel the hot pass will probably improve the preoperties of the root pass
Nanjing, the reason they are called de-oxidisers is because that is exactly what they do. They help prevent oxidisation of the weld.
To quote Mr W Lucas, DSc, PhD, CEng, FIM, FWeldI. "Rimming steels, in particular, can suffer from outgassing and porosity unless a filler wire containing deoxidants is used "
Technic Al
I dont think anyone still makes rimming steel. As far as I am aware its all concast these days.

All MS filler wires contain deoxidisers otherwise they would be full of holes at best and very difficult to use. Some contain more than others dependent on their range of application. Rusty plate would need a lot of deoxidising. Silicon is the most used but too much makes the weld brittle, too little and its full of holes.

The shielding gas used also affects the deoxidisation. You dont need much Si if you are intending to use Ar but if you go for CO2 you will need significantly more.
Hi All!

Technic Al, they may not produce rimming steel in great quantities any more but let me assure you, there is still plenty of the evil stuff out there!

Also, why is it, 1% chrome often proves more difficult to weld without coking than 2% chrome? Especially, when neither should require a purge according to general consensus?

3.2 Inspector
Are you serious about 1% Cr get oxidation more often than 2 1/4 Cr?

Technic Al

Really............I havent seen any rimming steel for many years. I assume you are seeing it on repairs to old structures ?

I dont know why 1 Cr should coke more than 2 Cr. I would expect them to be about the same. You live and learn
What is rimming steel?
nigel 42

suggest you do google
plenty of info
KK thanks.

For back purging carbon steel try argon + 20% CO2 (not an Argoshield type mix), have used it a lot on sprinkler system pipework and you don't get the thin layer of slag type coating which sometimes forms on the surface of the root bead which can clog up valves etc
root fill and clap
Thanks for all the replies.
OK so from this forum, in a nutshell, the reasons for carbon steel GTAW not requiring a back purge are-
- The oxides have a lower melting point and don't interfere with the weld pool.
- The de-oxidisers present in the filler wire.
- The flow around effect of the shielding gas.

I have asked quite a few knowledgable people this question now, and the one common thread is that there's a different reason from almost everyone!
So I'm still none the wiser!!!
The answer could be all those reasons you have listed, not one specific factor....?
Or a combination therof.
Hi All!

root fill and clap,

Some very good answers!

Here's my take on the subject,

Stainless Steels and most high alloy creep resistant steels (9Cr) have a fairly high percentage of Chromium in there make up. With S/S, it's usually classed as stain-less with over 12% Cr, with most S/S having 18% Cr.

Chromium is a very reactive metal and has a very powerful oxidant nature. When at normal temperature, Stainless Steels will quickly react with the Oxygen in the air to form a very thin protective 'Skin' over the surface of the metal, protecting it from attack (often called 'Passivation). Very similar to aluminium!

However, when this oxide layer is melted or removed as in welding, any oxygen present is free to attack or combine with the available Chromium and 'coking' or severe oxidisation will be present!

Generally, any steel with a Chromium content % of more than 5% will require an inert or other purging gas to reduce/eliminate Oxygen contact with the molten internal root weld pool to prevent this and also for any filler passes when the 'skin' is likely to be melted from heat i.e. hot pass and maybe first fill.

As your average Carbon/Mild steel does not have anywhere near the % Cr to cause any of these problems, the only effect is a mill-scale or 'rust' like appearance of the internal root with no purging gas. The reduction effects of the overspill of welding gas will protect the internal root profile long enough for a C/S weld to solidify and protect from atmospheric attack (less time at a temperature to be affected than S/S) A.K.A welding with no gas! Also, I would include the powerful de-oxidants in modern fillers that mtb pete mentioned, must play some part!

When MIG welding S/S, a purging gas is always the best way, but I have done jobs for nuclear work that didn't include a purge, I think it's just down to the speed of the root run and very small HAZ! Typically, closed butt etc, you still get the oxidisation but it's not as bad as TIG but not great for corrosive applications!


Excellent explanation R O G, thanks.
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