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Long as your set up right its a piece of cake. 70-s-6 is good filler. I used to run s-2. The old timers will tell you to gas weld it, but seems most tig it. I went to a airframe shop last year. 1 guy gas welding a frame, 10 guys tigging parts. That was all 4130. The parts I made all had to be stress relieved-and believe me they needed it.
What is the wall thickness? I have not welded 4140, I would think for a sled it would be 4130 ? I have a special torch that I used, running under 100 amps,dc, welding .035 up to .o68 wall tube. If you are going to be doing tubing you will probably be notching it, the fit is very critical (shooting for a fit with in .005 at the weld area) and be sure to grind then clean up all the weld area with acetone. Donít quote me my memory sucks, but I believe to stress relieve it you need to bring the area up to 900 degrees with oxy-acct carburizing flame, then allow it to cool at temp over 55 degrees. I think that is pretty close to accurate. You can get a sheet on tig welding it that was put out by lincoln from your local prax air supply if there is one around you, or probably find a down load somewhere.
Yes chromoly can be welded with mig, however Tig and/or gas are the preferred methods. I have welded some with Mig but I generally try to persuade the customer to have it Tig welded. I weld quite a bit of 4130 in sizes from 1/2 inch to 1 1/2 inch .049" to .095" wall thicknesses and have never stress releived or had a problem with stress being a factor. With that said I will say that I agree with Dan, fitup is crucial, the tighter the better and cleanliness. I sand back about 1 inch with emery cloth and wipe down with acetone before ever striking an arc. When welding try to have the shop at a comfortable temp. I don't like welding this stuff in a cold environment and keep the door closed so there is no chance of a breeze blowing across the hot joint. That will stress crack quickly. When welding a chassis or a structure with multiple joints jump around the part as to not put a lot of heat into one area of the structure. I have posted some pics of an ATV chassis on the Ask Andy forum if you would like to do a search and view them.