4130 Chromoly tubing is used for all kinds of applications, but 2 of the main ones are for airplanes and for racing just about anything from motorcycles, to mountain bikes, to go-carts.
AISI 4130 tubing is strong. It's real strong. So the reason it is so popular among race car and airplane builders can use thinner tubing and that translates into speed and less weight.
Welding 4130 steel tubing is not that hard. There seems to be some mystique around it but it actually welds better than plain carbon steel.
So then whats all the fuss about? Well there is one thing. If it cools too quickly, 4130 chromoly tubing will be hard and brittle in the heat affected zone of the weld.
Thats the main thing to be concerned about. So how do you prevent the weld area from cooling too quickly?
Some frequently asked questions about tig welding 4130 steel tubing are:
Q. Do I need to heat treat the weld when I am done welding?
A. No, the term heat treat is often misused. What is usually meant when it comes to welding chromoly is "stress relief". A stress relief can be done with an oxyfuel torch and a 900 degree F temp stick or a digital pyrometer. But it is usually unnecessary unless the thickness exceeds one eighth inch.
Q. Do I need to back purge my Chromoly welds with argon gas?
A. It always helps, but think about how many oxy acetylene welds there are out there doing fine that were done with no purge.
Q. What about using 308 stainless for welding chromoly?
A. there is a bunch of discussion on Baja Racing forums on this subject. I can tell you that lots of welders are using er308 tig welding rod with success, but for airplanes, E70S2 is the way to go. One thing that might not occur to you that you should consider is that if you ever needed to weld repair something and all you had was a gas welding torch, the E70s2 would be easily welded over with an oxyfuel rig. The 308 stainless welds? Not so much.