This is complex to respond to. I can help if you give me more details of exact composition and thickness of material.
The % chrome and Mo would be helpful. You can use Tig, but need preheat and possibly post heat treatment.
I will be welding 1 1/4 x.062 or .083 tubing. I will just be welding a 3/16" plate on end. This is to make a grinding peg for my son's bike. I can make it out of 304 s/s, but he wants to try the 4130 chromoly. Thanks for the help.
I am going to assume you have a chromemoly steel of no more than 1.5% chrome and 0.5% moly.
Preheat to 100 - 150 Deg F. Use filler wire to ER80X-B2L Or for this small job if you have 309 or 310 stainless that filler could be used. Argon gas is OK.
For these thin sections as welded joints should be OK without post heat treatment.
Joint design -Fillet welds should only be used in welds of low stress! Butt joins are preferred and rapid changes of section should be avoided. Ensure full penetration.
Source Welding Habndbook AWS, 7th Edition Vol 4.
