The chromoly (4130) alloy is also stronger than mild steel so less is needed to achieve the same strength. However, all steel has the same modulus of elasticity so regardless of alloy, stiffness is dependent upon the size of the tubing. Since most car components are designed for stiffness rather than strength, chromoly is a waste unless you redesign for it. In terms of shape, you can get it in whatever shape you want; they key is finding a supplier.A.Moore
I've seen people go bonkers about heat treating and doing all sorts of other complicated stuff to 4130 post weld and quite frankly, I've never seen a reason. All of the FSAE frames (and suspension components for that matter) I've done were 4130 and aside from some post welding stress relieving with a torch on the arms, they were used as-is - nothing has failed to date.
Albeit they were all TIG welded; the only reason I wouldn't MIG 4130 would be the available filler wire. ER80S2 should be used on 4130. For some reason (probably lack of demand), I can only find ER70SD6 for the MIG.
for the full discussion.....http://locostusa.com/forums/viewtopic.php?t=3877
But it really comes down to WHY BOTHER? 4130 is so expensive, and for what? You might be better off spending that money on extra go-fast goodies for your power plant. There are a lot of guys racing their Locosts and I haven't heard a single one yet say "If only I had used 4130...."
Just my 2 cents,