Contaminated weld pool. beginer question

[zardoz]

I am doing TIG City and Guilds level 2 at evening class. I jumped straight to level 2 as there were no places left on the level 1 class, and I had done a few classes at TIG and can Oxy ace to a reasonable level, so i crossed my fingers and joined up. I am now getting the hang of it, and have passed the first of five test pieces, with a butt weld tested with a cupping test and found no splits in the weld. I started to look out for the reflection of the TIG arc in the pool of molten metal (as my tutor advised me to look out for) and was amazed as i actually started to be able to see its reflction in the liquid metal weld. The tutor tells me if i touch down with the tungsten, to stop and change or re grind it, which i now always try to do if i notice a touch down. Though sometimes even when i have been sure not to contaminate the tip by touching down i still seem to get contamination in the weld pool before i have finished my run. I can see this as I can no longer view the reflection of the tip in what appears to be a now dirty weld pool with "scum" floating on the top.

Is this being caused by contamination of metal from the area surrounding the weld as it heats up I always grind the edge around the weld to 6 or 7 mm back, but my tutor says that 12mm is prefferable and on both the weld side and the rear. (we are using 1.6mm steel) and using a rig to send purge gas to the back of the weld.

Would any TIG heads on the forum say that grinding 12mm back from the edges on both the front and rear of the weld is correct, or is my tutor being a little too, belt and braces?

PS: TIG is excelent. I am saving up from now on. Too bad ill need over a grand to get a decent ac/dc kit coz now im out of work it'll take forever and a day to save

 

[oceanobsession]

Re contaminated weld pool beginer question

Hi yardoz. you need to keep the filler rod close to the cup at all times with
mild/stainless. also dont move the torch away until the post gas has cooled
the job. if you come out from the weld pool to far with a hot filler rod its the
rod that is introducing contamination back into youre weld pool cheers ocean

 

[rodofgod]

Hi All!

zardoz, First of all, well done in taking a level 2 course!

You will get natural contamination of the Tungsten by the TIG welding process, without 'touching down'.This is normal!

The 'scum' that you see is actually, most likely oxides that have been introduced into the weld by ineffective cleaning of the parent material or (as oceanobsession correctly points out), taking the still molten end of the filler wire out of the protective gas shield, allowing it to be contaminated by atmosphere and then re-introducing it into the molten weld pool!

These contaminates will transfer across the arc to the tungsten and make your welding much more difficult!

So, to answer your question 12mm is not an unreasonable amount to clean back! I would request this as a minimum, both sides! Heat input will naturally draw any oils/contaminates towards it,so any paint or surface finish will need cleaning back at least 12mm to prevent these entering the weld! Also, keep in mind your filler wire, It needs to be protected from atmosphere as well,even when not being used, but still hot!

Regards

 

[zardoz]

Cheers guys. I didnt realize that the tip of the filler wire could be contaminated just by taking it out of the gas shield area. I will give it a try and post some pics.