It is unlikley that a spring maker would specify a material incorrectly as this type of activity is their core business so I would rule out this as a possible resason for a problem to occur.
It is possible that heat treatment has been carried out incorrectly but it is unlikley that the bar material used for the manufacture of a torsion bar has ever been annealed as there is no reason to carry out this operation.
Bars of the diameter used for torsion bars are likely to be processed in a 'as rolled'or possibly a normalised condition.
The ends of the bar will then be upset which can be considered as a forging operation amd then heat treated.
A typical Carbon/Manganse/Silicon Spring Steel have ruling sections which are substantially greater then the diameter of a 911 torsion bar it is likely that the centre of the bar is not a strong as the outer diameter.
To establish the variation is strength thorugh the bar's cross-section is easy to determine using basic hardness tests.
The potential errors that can occur during heat treatment are straightforward.
The tempertur from which the bar would be quenched could be too low and the material may not be fully austenetised. This would mean that not all of the carbon in the steel would be taken into solution and the quenching process would not be fully effective.
The second type of problem is that the bar may not have been held at temperature for a sufficiently long time and the centre of the bar will be too cold and again quenching will not produce the desired result.
In the first case a simple hardness test will reveal that the correct strength level has not been obtained.
The second case is more difficult to determine BUT as the outer layers of the bar could be correcly heat treated the influence of this problem would limited.
The outer diameter of the bar will be the forst part of the component to yield and the greater the angle of twist the deeper the plastic deforamtion.
As bars would never be designed to yield in a duty cycle the fact that the outer layer had sufficient strength would mean that the bat would still be fit for purpose.
Again the heat treatment of springs is such a routine process for any spring manufacturer and modern process control is so advanced it is difficult to understand how this would happen.
The process of yielding in a torsion bar is much more complex than in tension. In the torsion member yielding starts at the outside and progresses inward with increasing torque. As the torque increases we finally reach a fully plastic condition where the strain (and thus angle of twist) is increasing while the torque (TFP) is approximately constant. At the fully plastic torque the entire cross section is yielding.
There are problems with this theoretical state of fully plastic torsion because it calls for a discontinuity of stress at the centre of the specimen. In spite of this defect in our model, we find we can in fact approach a fully plastic torque. At the fully plastic condition the torque will remain essentially constant while the angular deformation increases. If we continue to twist the specimen the torque will again begin to increase. The increasing torque which follows the plastic action is directly related to the increased stress which follows yielding in the uniaxial loading of carbon steel (i.e. after sufficient yielding at essentially constant stress, the stress again increases with additional deformation).
In a fully plastic condition the relationship between torque and shear yield stress is given by
There is never a compressive stree developed in a torsion memeber this would only occur in bending.
The pre-setting or scragging of springs is carried out on all production springs and is designed to stabilise the spring.
The heat treatment process may result in the formation of retained austenite and subsequent elastic deforamtion may cause this phaes - which is metastable - to transform and this caises a dilation which can allow some initial 'settling' of the spring.
Prestetting eliminates this issue and means that the ride height of new cars is unlikely to change.
If new torsion bars have not been preset then there is a possibility that they could sag during initial use but as you correctly state once the bar has settled it will stabalise and not continue to yield.
It is also very unlikley that retained austeite would result in the steeling of ride of 70-80mm as rwsudual austinite is usually significanlty less than 2% of the structure and the dilation effects would be very small.
Presetting also increases the strength of the material and a reasonable explation of the process can be found here:
http://www-mdp.eng.cam.ac.uk/web/librar ... pendix.pdf
As I disccussed in a previous thread having supplied spring testing and presetting machines to most of the leading spring manufacturers in Europe for more than 20 years it is just hard to unnderstand how errors occur.
