Advice welding smal parts of SS

[voipio]

I don't think oxides of iron, chrome, nickel, or whatever, will outgass their oxygen under high vacuum, else we would be able to de-rust all our old tools in a vacuum chamber. The gap between the circular blanking piece and the wall of the tube will tend to act as a small air reservoir, and empty more slowly as the vacuum is drawn down.

 

[Misterg]

They look like they might be 'sugaring' due to insufficient purging (crystaline chromium compounds?).

I thought that looked like a vacuum flange on the end if the part...

For high vacuum, I agree that the inside needs to be 'clean' - any porosity or structure that can adsorb water or other contaminants will out-gas and degrade the vacuum.

 

[Robert1998]

Thank you

 

[Robert1998]

Yes is a KF16 or QF16 flange. About the black things, the problem is not just purging but also overheating right?

 

[Misterg]

About the black things, the problem is not just purging but also overheating right?

I'm not the right person to give a definitive answer, but as far as I understand, it's down to purging / gas coverage (and maybe cleanliness). Hopefully somebody more knowledgeable will answer.

I would think that for high vacuum, you will need to have full weld penetration so that there is a smooth surface on the inside.

 

[Brad93]

Ok, I know I have overheated the weldings. And I got these nodules in the other side. What are they?



The pictures have benn taken after using pickling fluid to clean SS steel. They all were black and after the solution some become grey and others remain black.

Are they iron oxide? I mean, they have grow, so, I think they cant be just steel, so maybe they contain oxigen. That is important because this part is meant to operate under high vacuum, so, it it contains oxigen it will gassing that oxigen ruin the vacuum

Sugaring

Proper name is carbide precipitation.

Your purge wasn't good enough.

 

[Brad93]

This is how small your welds need to be. This is no purge, just aluminium chill bar resting behind the joint.

#5 standard ceramic not a gas lens.

Heat management is the key

 

[Hitch]

Did you tack it together under purge?

It's such a small piece, the purge would benefit from being further away, ie a piece of spare pipe fixed on the end.

If there's only one open end, pipe in, an a vent out next to it works on a small piece.

Certainly not the easiest thing for a novice to weld up.

 

[chrismac]

Sugaring

Proper name is carbide precipitation.

Your purge wasn't good enough.

Thats a new one for me, only ever heard it called oxidization, every days a school day.

 

[Brad93]

Thats a new one for me, only ever heard it called oxidization, every days a school day.

Whilst the stainless is oxidised, the actual black sugary looking formations are, as said above, crystals of chromium carbide

Did you tack it together under purge?

It's such a small piece, the purge would benefit from being further away, ie a piece of spare pipe fixed on the end.

If there's only one open end, pipe in, an a vent out next to it works on a small piece.

Certainly not the easiest thing for a novice to weld up.

I think you're right @Hitch. Small isolated spots could well be tacks

 

[Robert1998]

Thank you all

 

[Robert1998]

One question come to my mind. So I need more penetration, so I should increase the amperage a bit and purging inside to avoid oxidation/carbide precipitation right?

Maybe I can increase the penetration using pulses Tig right?

 

[chrismac]

A nice V prep on both parts, purge and weld slowly until you get the hang of closed butts because thats effectively what you are doing. probably better using no wire on the first run, but the issue is you'll need so little wire for the 2nd it will be hard to make it tidy.
Personally I would not bother using pulsed tig with the experience you have.
Good luck

 

[Brad93]

A nice V prep on both parts, purge and weld slowly until you get the hang of closed butts because thats effectively what you are doing. probably better using no wire on the first run, but the issue is you'll need so little wire for the 2nd it will be hard to make it tidy.
Personally I would not bother using pulsed tig with the experience you have.
Good luck

given the thickness Id just put a slight bevel on each part, knock the sharp edge off and hit it hot and fast autogenous

 

[Robert1998]

Thank for all your advice. This forum and its people are great

 

[Robert1998]

Ok here it is the second try

I made this set up to have gas purging while I weld. Is not the mos eficient way, but worked nice



This is the result, for my inexperienced eye doesn´t look horryble.





Buuuuut, while welding one part, the purging gas pipe slipped and I got a nice bulb of sugarization. So I drill that point and I made a new weld.

What do you think? Do I pass the exam?

 

[chrismac]

Much better, why dont you wire brush it once it has cooled a little,

Be aware your gas hoses need to be nice and tight on the fittings as not to draw air in and contaminate your purge/weld gas,

You get a thumbs up from me, with practise you will get even better.

 

[Robert1998]

Thank you. Yes. I'm going to brush it but I wanted to present the part as it get after welding.

 

[Hitch]

3m clean and strip wheels are great for cleaning pipe welds. They blend lumps and edges slightly as well as remove colour.

 

[Brad93]

You don't want your purge to only come on once your start welding! There will be mixture between oxygen and argon.

You need to purge the air for a while before welding to be 100% sure it's only argon inside the tube.

If this was being done professionally you'd use a purge monitor on the first one to work out how long you need to purge with argon before you achieve less than 25-50ppm of oxygen.