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To Mig or Tig?

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  • To Mig or Tig?

    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.

  • #2
    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".


    • #3
      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.


      • #4
        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?


        • #5
          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.


          • #6
            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.


            • #7

              Thank you all for the help!


              • #8
                Hi 600micro. This is a great father/son project. Please involve your son as much as possible as this will be a great learning experience that he will never forget. You can do this. Welding 4130 is not difficult. For what it's worth I will explain how I would weld this project and maybe it will help you.

                I would suggest using the Tig welding process. I have not welded 4130 with the Mig process (NHRA requires 4130 to be tig welded) however I am confident that this project could be mig welded with satisfactory results.

                I will start with the basics as I do not know what you know. Set polarity to DCEN. Use 100% argon shielding gas set to 20 CFH. I prefer lanthanated tungsten but thoriated or ceriated tungsten would work well also. Use 1/16 tungsten ground to a point, ground back 1/8 to 3/16 (2 to 2.5 times the diameter). Be sure to use a new or uncontaminated grinding wheel. I prefer to use a gas lens to allow the tungsten to protrude up to 1/2 inch or so from the edge of a no. 6 nozzle. If a gas lens is not used I limit the tungsten to stick out past the nozzle around a 1/4 inch. Wipe the tungsten with a clean rag with acetone before installing into the torch. Although 70 series rod would work well I prefer to use ER 80S-D2 tig rod in .035 and .045 diameters. Take a clean rag with acetone and wipe off rods to be used. Now you are ready to weld.

                I would set up a flat and level table or jig to build the frame upon. Cut and fit all your tubes so they fit together. Sand off the millscale on all tubing where they are to be welded. Wipe all areas with acetone and clamp well to the jig table. I drill 1/16 holes in all intersecting tubes on the inside tube perpendicular to the intersecting tube (no holes on the ouside of the finished frame and all tubes vent into each other on the inside). Wipe all areas to be welded with acetone and tack weld all pieces together before final welding.

                Since it can take several days or longer to fit all your tubes together I like to take a small propane torch and slightly heat all joints right before final welding to remove any moisture, condensation, etc that may have accumulated. Wipe all joints again with acetone and scrub with a stainless steel bristled tig brush and weld as much as possible before releasing frame from jig. When frame is removed from jig wire brush and wipe all remaining areas to be welded with acetone and finish welding. You are done.

                Now that it is getting colder here in the northern part of the country there may be some issues that will affect welding performance. You need to have your work area and frame up to room temperature (70 degrees) while you are welding and kept there until the frame has cooled. Another item to be alerted to would be your shop heater. If it has a fan and blows air around your shop it can cause drafts that can affect shielding gas coverage thus possibly ruining your welds.

                Lastly I will say you must have sufficient tig welding skills BEFORE you begin as the tig welding process demands more skill to be proficient at than other processes require. The safety of your son is at stake as well. Having said that, Tig welding is a skill that almost anyone can acquire if you practice.
                Please teach your son to tig weld as well for the young are many times more quick to learn than an old dog such as I.

                I hope this helps, Rascal


                • #9
                  Good points. The moisture issue is a big one, and in non climate controlled shops its good practice to warm the area up before welding as mentionedn not forgeting to draw down the weld afterwards as well. I personally never liked the idea of using the .035 size fillers as I thought it helped promote undersize welds. In teaching we use 1/16 for that material. I have seen more than my share of failed welds in this exact application on production kart frames.


                  • #10
                    I am just stating how I would approach this welding project. Others are obviously free to do as they see fit. You are correct in saying that the welds can be undersized when using smaller diameter rods. The (well my) cardinal rule of all welding is WATCH THE PUDDLE. I Have also seen many struggle with rod diameters that are too large and promote cold lap in the toes of the welds. Either way watch the puddle as you are welding and if something is not right something must be changed. Rascal


                    • #11
                      Thanks for the advice.

                      I shouldn't have a problem getting my shop up to temperature for the welding and I can keep the air flow to a minimum to keep the shielding gas from being washed away.

                      I've been MIG welding for several years now, and when I had access to a TIG welder I did a bit of welding on mild steel. I'm not totally new to the process but I definately am in the learning stages.

                      You are right about setting it up on a jig and starting from there. I'm going to get a piece of 1/4" plate and stiffen the back with sq tube and legs make it all flat then start by putting tabs on the plate for holding the tubes in place.

                      My Son will definately be involved. He loves the Plasma cutter and he is starting to learn to weld. One of the reasons we got him into racing is to gain hands on mechanical experience with motors, electrical, welding, fab etc. Plus the fact is he loves to race and is pretty good at it.

                      I'll post pics as I get going on the project.

                      Thanks again!


                      • #12
                        Pics of Kart

                        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,
                        Attached Files


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