sec:unclassified Coil Spring Metallurgy (a little long)

["Reid, Warren" <Warren.Reid@domain.elided>]

It's probably time to clear up a few misconceptions people seem to have about the heat treatment of steel, especially in regard to the recent topic on lowering coil springs by heating them (alfa digest V9 #276).
 
First. You cannot alter the stiffness (ie Force versus Deflection) of a coil spring by heating, heat treating or whatever, it is only a function of the wire thickness and its overall length. From the steel's point of view its stiffness is purely a function of Youngs Modulus, which for most steels remains constant at around 220 x 10E9 Newtons per square metre or  30 x 10E6 psi for our metric impaired friends. What you can do however is to either stretch or compress a spring so that rather than staying within the elastic limit of a spring, you move (temporarily) into the plastic zone and the spring takes up a different set point when the load is released. This technique is sometimes referred to as "scragging"
 
Second. As Ian Lomax points out you do have to be careful heating springs, however I'm guessing that he is actually referring to when an oxy torch is waved over the top of the spring and it reaches a dull cherry-red temperature (approx. 700 Celcius for steels). Again this has the effect of taking the steel into its plastic zone and it takes up a new "set" position. The danger of this "technique" is that it can alter the metallurgical structure of the steel, where in a Spring steel (which has a relatively high carbon content) you can get diffusion of carbon to the grain boundaries which amongst other things can make the steel brittle and more likely to fail.
 
Third. The heating procedure that Frank Smith described is actually a reasonable way of altering the set point of a coil spring. Heating to around 200 Celcius (392F) or more will more or less allow the steel to take up a new set point without altering the metallurgical structure to any extent. Essentially this lowers the force required to take the spring past it's set point so that it can take up a new overall length. This is usually referred to as annealing and can be done up to temperatures of about 500 Celcius (932F) without detrimental effect on the steel's properties, however it can lower the point at which an applied force will cause the spring to again move into it's plastic regime. The spring still has the same stiffness, however, care must be taken at elevated annealing temperatures as the spring may "relax" quite quickly..
 
Alfa content: Alfa 105's were notorious for the rear ends sagging over many years. This I believe is probably due to the fact that the springs were over annealed after forming. In operation the force/deflection was often enough to take the springs almost into the plastic regime, the consequence of which was that over the years the springs slowly take up lower set points and the cars sag a bit. This may have been compounded (me speculating again) by the fact that Alfas engineers would have been trying to make the rear springs as soft (from a Force/deflection view point) as possible due to the heavy rear axle.  Remember its a fine compromise between annealing a spring so that it is not to brittle, and will therefore snap and lowering the elastic/plastic transition point (causing sagging), especially if the spring is relatively soft to start with. Most of what I've written here is from memory and unfortunately I'm about 12 years out of practice in the metallurgical game, so apo!
logies if I suddenly discover any mistakes in the aftermath!
 
Warren Reid (MApp Sc - Physical Metallurgy)
Melbourne, Australia
68 1750 GTV (did have a droopy rear end)
86 Spider Veloce (not droopy yet)
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