welding a roll cage

[38Chevy454]

 The problem with using Mig is that the lower heat input results in a narrow HAZ and faster cooling rates, which in turn can lead to a hard and brittle HAZ.

That is what I'm talking about. I've always thought that MIG puts the heat into the filler so the base tends to act as a heat sink. Tig puts more  heat into the base metal. All other things being equal I think the MIG should be more brittle due to a faster cooling rate, in any metal. 

Mig works well on mild steel because the low carbon equivalency, a product of carbon and other elements, doesn not promote the formation of martensite even at extremely rapid cooling rates. 1020 steel can be water quenched and it will remain ductile.

Exactly right, although water quench will make martensite if thin sections.  The main reason 4130 is not allowed to MIG weld is because the rapid cooling combined with the low alloy chrome moly material causes martensite to form in the HAZ.  Marteniste is brittle.  Gas or TIG has slow enough cooling you do not form martensite.  The problem is not the size of the HAZ, it is the cooling rate.  There is a region in the HAZ that is in the austenitic temp and will cool through transition, you want that transition to be slow and not form martensite.  I am a metallurgical engr, the basic answer is what I stated.  Of course there can be outside circumstances and other variables, but in general, MIG and chrome-moly will produce a brittle HAZ region.  Post weld heat treating can be done, but it has to be done on the whole assy in a controlled furnace, but then you get distortion coming into the picture.  Not possible to do effective PWHT with a torch.

[fastman614]

Being that I was never a full time chassis or roll cage builder, I too pondered the questions of which methods of welding may be best. In the end, I went with what I knew I was most experienced and capeble of doing..... MIG...

In the last few years, with the new career path that takes me away from home for extended periods of time, I weld even fewer roll cages and chassis' now. I would heartily endorse the sugggestions to weld scraps and pieces together and then cut them apart to see how the welds turned out. I do this a practise for sometimes several hours prior to doing real welding work on anything nowadays.... (and I now have to use a "cheater" lens in my helmet too- but that is another story)..... experiment with a bit of preheat too.... especially if you are working in a cold season of the year like we in Canada and many other parts of North America have to do.... NOT lots though.... 200 to 250 degrees F is usually enough.....

Another item to seriously consider too.... FROM THE RULE BOOK SECTION 3.B:

Low carbon (mild) steel tubing is recommended for the construction of roll cage structures.

Chrome moly tubing is widely used in drag racing as a method of weight saving. A mild steel dragster chassis would not allow a car to be built to the minimum weights allowed in the various classes. In land speed racing, weight is not generally a critical factor in the competitiveness of a car- unless perhaps there is not enough weight.

I wish you well in your chassis building adventure and I hope to see you in tech.