snip

However, it would surprise me if a sway bar designed for a high-performance
car would even approach the elastic limit of untreated, cold-rolled steel.
It shouldn't deflect a great deal. Much more likely to need fancy steel and
heat treatment is an ordinary street sedan, in which the bar would be less
stiff, and therefore would need more elasticity.

In any case, the difference in elastic limit between the normalized
condition and the hardened condition is not that great for 4130. It's fairly
strong as it is, and it isn't tremendously strong even when heat treated.

???????????

The modulus of elasticity is the same whether it is heat treated
or not. ie for a force of x, it will deflect y. The question is:
does the deflection make it exceed the yield strength? Annealed
4130 is about half again stronger (higher yield strength) than
equivilent cold rolled mild steel. (80-90kpsi versus 50-60kpsi)
The heat treated version of 4130 can easily push past 200kpsi or
more than double its unheateated version.

I agree that a flat track car might get along with a non heat
treated bar, but an off road racing vehicle needs all it can get
from the sway bar.