Welding Tips and Tricks

Hey gents, new to the forum, looks good.
I have some welding experience from years ago, but I'm relatively new to MIG. Looking to use a Lincoln 140 and ER70S-6 wire with an Argon/CO2 mix to weld 4130 chrome-moly tubing. Tubes are 5/8" in diameter, walls are .035 thick.
I welded with no pre or post heat, low voltage and very slow wire speed, to make a weld on some practice tubing that held very well. However, the metal itself sheared at about 1/2 the weld-width distance above the weld. So I didn't break the weld, but instead the metal failed 'near' the weld. I understand that pre and post heating will fix this, as it tempers the steel to make it less brittle.
Only documentation I could find regarding pre/post heating is for TIG welding, and was for .058 to .720 thickness. That suggests preheating to 250 F for an area 6" on either side of the weld, welding, then post-heating to the same temp for the same area. Seems excessive to me for fillet welds that MIGHT cover 1/4 ". Not to mention that my metal is not as thick as .058. I'm thinking of preheating to 250 F for about 2" on either side of the weld, welding, then post-heating to 250 F and letting it cool on it's own. Surely the 250 pre-heat temp isn't going to be TOO hot? Any thoughts as to whether I'm an idiot, or am I on the right track? Any experience out there?

Well, all that being said begs the question, What are you fabricating out of the 4130 first of all? Second, you really should be using an ER80S-D2 wire preferrably an .030 dia. max. With the tubing being an .035 wall, you should not need any pre or post heat with that depending on what your application is, although the tube must be at least 70 deg. F before you start welding on it. DO NOT weld on cold 4130. Also, 1/4" fillet welds are too much for that size pipe. Is there any way you can tig these??

Fabricating an aicraft frame. Yes, one that will eventually fly my butt around. Most advice was to gas weld this, but 1) I haven't done that since 1989 and 2) I have a MIG welder, not an acetylene setup. I guess I also didn't put it down, but my wire was .025.

Well, gas welding in not a necessity, tig welding is the preferred method. In my opinion, unless you are really good with a mig, i wouldn't mig weld this, I would tig, especially with your life in the balance. Also, DO NOT use ER70s-6, use the ER80s-D2, especially if you are not going to heat treat the entire frame after welding.