Inside TIG joints on stainless

[Lawnmowerboy]

Hi all, Can't seem to get the hang of doing inside corners with the TIG, material is 1.2mm 316 stainless, gas is 8lpm, about 40a

I can't seem to get the edges to melt in i just blow holes in the piece being joined and it goes all furry on the back. There isn't any gaps and the bits i've been using are laser cut so perfectly square straight edges.

 

[langy]

I would use a 1mm tungsten & a gas lense, 5-7 gas and 1mm rod

 

[Shox Dr]

First off, laser cut SS, is a bitch to weld. Clean the edges

 

[langy]

First off, laser cut SS, is a bitch to weld. Clean the edges

Your right, I didn't spot the laser cut bit, must be something to do with the heat I guess but they do weld better when sanded

 

[backpurge]

First off, laser cut SS, is a bitch to weld. Clean the edges

It can be down to what gas they are using on the laser, you quite often get a layer of oxide on the cut surface which as you say is a git to weld and needs to be cleaned off

 

[chevy2]

Turn the amps down a bit and concentrate the heat slightly more to the piece without the edge.

 

[langy]

It can be down to what gas they are using on the laser, you quite often get a layer of oxide on the cut surface which as you say is a git to weld and needs to be cleaned off

That's interesting as I've often wondered why

 

[penfold]

That's interesting as I've often wondered why

I imagine it's just the thicker than normal oxide layer; cleanliness is next to godliness, especially when TIG is involved.

 

[langy]

I imagine it's just the thicker than normal oxide layer; cleanliness is next to godliness, especially when TIG is involved.

Most definitly !!!

 

[hotrodder]

Pretty much all been said but as always arc length, arc length, arc length

The back of the joint going furry is known as coking or sugaring and is the result of oxidation - mild steel just scales lightly but chromium is far more reactive which is why stainless welds are purged (argon supplied to the back of the weld to exclude 02). Even partial pen welds (like fillets) on thin stainless are very prone to this as there's very little margin for error.

 

[Lawnmowerboy]

They use nitrogen on the laser, Will backpurging help?

 

[zx9]

They use nitrogen on the laser, Will backpurging help?

So if you use nitrogen on the laser, are the laser cut edges effectively nitrided? Could help explain why the laser cut edges are more difficult to TIG.

 

[hotrodder]

With regards to blowing holes, no. Stopping the back of the joint coking up if you get things too hot, yes. For a fillet, especially when practicing or even when practical on thin stuff then clamping a heat sink to the back can be just as effective- beyond heat sinking it'll also help keep the atmosphere at bay. It's also kinda cheating as, with practice, it's possible without

 

[Shox Dr]

So if you use nitrogen on the laser, are the laser cut edges effectively nitrided? Could help explain why the laser cut edges are more difficult to TIG.

Yup, on steel that is. Not sure what it does to Stainless

 

[backpurge]

If you use nitrogen as opposed to oxygen you should get an oxide free cut edge, if the cut faces are discoloured they are getting contaminated from somewhere. Is the nitrogen from a generator or a tank/cylinder bundle etc

 

[Lawnmowerboy]

Yeah, Big air prod. 1200kg cylinder thing!

 

[backpurge]

Yeah, Big air prod. 1200kg cylinder thing!

Edges should be oxide free then