To Mig or Tig?

[600micro]

Hey all

I'm getting ready to start a new project making a racing kart frame and cage for my son. I will be welding 1 1/4" .095 wall and 1" .065 wall 4130.

I am wondering is Tig the best way to go or is mig acceptable? Seems like depending on what you read one guys says this another guy says that.

ESAB had a link in the welding journal indicating that MIG was okay for NASCAR. Except they predominantly use mild steel tube since there cars are so heavy. The kart complete with the driver ways about 425 lbs. So I need to keep it light to be competitive.

The chassis flexes a lot to compensate for no suspension so I was thinking I would use ER70s wire / rod to keep the welds ductile.

These are open wheel karts and they go about 60 mph. Not uncommon to see them flip.

I currently have a millermatic 210, but I know I would have far better control with the TIG. I'm concerned with the small tubing about the cold starts associated with MIG most frames that I see built typically have about three starts around the tube.

Any thoughts appreciated.

[Aerometalworker]

Tig and post heat or O/A are what the aviation authorities and engineers expect. Without proper testing and analaysis for your EXACT application and joint, any other method on that material is a "crap shoot".

[Fortyfords]

I would say you should chk with your kart association or the track officials on their recommendation/rules on type of frames used.
Tig will give better control ,but mig can be okay also too.

[600micro]

The rules are pretty vague... Must be of kart configuration, no suspension.

I'm thinking I will TIG weld it up better safe than sorry.

Also with the thicknesses I'm using all of which are .095 and under I had read

that PWHT was not required. Is that the case?

What filler would you guys recommend?

Also should I drill a small gas relief hole in the members to help keep the weld from purging at close up?

Any other tech tips?

[Aerometalworker]

No thats not the case. Where that number came from I cant put my finger on exactly, but basicly it was a mis-representation from a cooling rate calculation. Long story short whoever did that "calculation" failed to take into account the "material thickness equivalancy" calculations for actual joint configurations. i.e. if it was a butt weld on .095 sheet of a infinite length, then technically the cooling rate would be slow enough.Thus is never the case, and by all engineering standards, PWHT or at least tempering is normally suggested. For filler I cant imagine needing anything over a E70 series.

[Blondie_486]

When I built my dragster frame back in the late 70's I TIG welded most of it and what I didn't TIG I O/A welded. Then I hauled it 1,300 miles to have the entire chassis stress relieved in an oven. I'd at least perform a post weld heat and if you can insulate after the post weld heat I'd do that as well. Cooling 4130 too quickly will make for a brittle joint and premature joint failure.

I think I'd folow Aero's advice as he works with this material in an aviation application, which should also be good enough in your application as well.

[Sammy]

Here is some pics of a kart that a friend made. I believe he used mig, but i am not sure since i am just getting into welding. Right now i'm working on my car interior (Floor pan is rusty so i'm replacing it). It's a 1964 volvo p1800. Just shows what can be made (the kart pics)

So long,