Denny,
I am not sure what material GM uses but according to Hellwig's website, they use 4140 alloy steel, a very common carbon steel shaft material. There are lots of different heat treatments that can be done to 4140. A simple hardness check of one can tell you a lot of information about what kind of heat treatment process was used and the resulting yield strength of the bar. More important than the strength or harness of the 4140 is the actual bar diameter. Since the bar acts like a torsion spring, the spring rate varies with the bar radius raised to the 4th power.
Here is some specific information on the various heat treat properties of 4140 from MetalSuppliersOnline:
General Information
Principal Design Features
This is one of the chromium, molybdenum, manganese low alloy steels noted for toughness, good torsional strength and good fatigue strength.
Applications
4140 is used in a tremendous variety of applications, too numerous to mention here.
Machinability
Machinability of this alloy is good in the annealed condition. In the heat treated and quenched condition machining is best limited to finish grinding.
Forming
As with all the low alloy steels forming may be done by conventional methods with the alloy in the annealed condition. These alloys have good ductility, but are tougher than plain carbon steel and thus usually require more force, or pressure, for forming.
Welding
Weldable by all of the conventional methods. Note that welding with the alloy in the heat treated condition will affect the mechanical properties and a post weld heat treatment may be needed.
Heat Treatment
This alloy is hardened by heating to 1550 F and quenching in oil. It is best to normalize the alloy by heating at 1675 F for a long enough time to permit thorough heating, followed by air cooling, prior to the hardening treatment.
Forging
4140 may be forged at 2200 F down to 1700 F.
Hot Working
Hot working, if required, may be done in the range of 1900 F to 1500 F.
Cold Working
The alloy readily cold works in the annealed condition by conventional methods.
Annealing
Annealing is done at 1600 F followed by slow furnace cooling.
Aging
Not applicable to this alloy.
Tempering
Tempering temperatures range from 400 F to 1200 F depending upon the hardness level desired. The lower the tempering temperature the greater the hardness of the alloy. For example tempering at 600 F gives a tensile strength of 225 ksi while tempering at 1000 F will give 130 ksi.
Hardening
Hardens by cold working, or heating and quenching - also see "Heat Treatment" and "Tempering".
