I am a novice welder myself, but have been around racecars for many years. You may want to check with the sanctioning body your customer races under. Many times they will state the rules in which certain materials need to be welded. I myself am a drag racer and have to follow the NHRA rulebook. For mild steel cages (almost all are ERW mild steel) MIG or TIG is allowed. For 4130 cages, NHRA requires the TIG process. I have spoken with several chassis builders and they used a mild steel filler rod (either ER70-S2 or S6). The moly cages can be heat normalized by using heat "crayons" at the correct temp.

In NHRA, mild steel cages are required to be .118 wall thickness min. and most builders use .135 wall to make sure they pass sonic testing on the outside of bends. With moly tubing, we are allowed to drop down to .085 wall thickness. I'm not sure why your customer wants 4130 unless he wants to save weight if you use a heavy wall DOM. It is my understanding that NASCAR uses .085 or .095 DOM, so weight savings of moly would probably be nill.

BTW, what type of racing are you building cages for?

Todd Geisler
http://www.MalibuMuscle.cjb.net
Malibu Muscle Racing

Hi Gabe.
My day job here in Australia is the manufacture and repair of sprintcar chassis. We TIG weld 4130 Cr/Moly all the time and we never normalise or heat treat the chassis after welding as it is not cost effective.
Weld failures are extremely rare and after seeing some of the extreme crashes these monsters perform we are very happy with our chassis.
Even in the USA the majority of sprintcar manufacturers do not stress relieve by heat treating. One company, Avenger, supposedly stress relieves using vibratory "Meta Lax" methods, but we are yet to be convinced of any benefit.
Hope this helps. Regards Andrew.

There is alot of conflicting information about welding 4130. Here is what I've learned. As Goose has said, your sanctioning body probably has welding specifications.

Brazing 4130 is not recommended due to the relatively coarse grain structure of the material. Apparently the braze penetrates the grain and can cause "splintering".

TIG Welding 4130 does not require any type of post weld treatment. To properly "Normalize" a roll cage/chassis the complete structure would have to be put in an oven and heated to about 1600+ degrees F, (which would be impracticle and marginally beneficial). Any other type of stress relief hasn't really been proven to be of any significant benefit. Also, copper coated filler rod is not recommended, the copper can have the same effect as brazing.