Bubbles when TIG welding case-hardened 8620

by Joe
(Enfield, CT)

Hi! I'm having a friend who is good at TIG welding help me with filling in a hole around the firing pin port of a black-powder revolver. After a great deal of consultation with metallurgists and heat-treatment experts, we settled on using ER120S-1 rod (.045"). We ground out part of the case (to expose the inner metal), cleaned it thoroughly,baked it at 175 degrees overnight to remove all oil, soaked it in acetone, and TIG'ed it using argon shielding gas. The problem we're having is bubbling, which results in zillions of little holes. Any recommendations?
I forgot to add that we preheated to 300 degrees.

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could be one of the following.

the metal gets red hot and gets oxidized to the point where it causes porosity

too small a tig cup is used

too much argon flow out of the cup is causing turbulence and loss of shielding..


If I am picturing the application correctly, I think I would use an oversized tig cup with added diffuser material with a gas lens .

I would also use either a backup purge box or a piece of aluminum or copper sheet to trap the argon on the back side of the part.

here is a link to the kind of cup I am describing.

oversized tig cup