To Katana(And the rest of you fine folks on this forum,this being my first post-Greetings,and my great pleasure to have joined you here).
You're correct in that an abrasive grinder wheel will ruin the temper of a piece of leafspring.Here're a couple of thoughts,the expressed opinion,of course,being strictly from my own experience(which is not great).
(1)A used vehicle spring is somewhat ill-suited for such a project,primarily,because it may contain a microscopic structural flaw stemming from it's long,hard past service.However,it probably is a good choice of an alloy in general.A section of the alloy brand new,from the mill(complete with it's datasheet,detailing the HT schedule),is very inexpensive.Also,very probably,a shop specializing in spring work would give one a chunk free,or nearly so(as well as being capable to competently and reliably heat-treat(HT)your finished object for you afterwards,for a small fee).
(2)There are methods of reduction of steel that would preserve the original HT,water-jet being one.
(3)Vehicle leaf-spring alloy,often 5160 in the U.S.,and similar elsewhere,(a Cr alloy),is not the most difficult alloy to properly HT with some rudimentary backyard set-up.However the shape of the work,the prod,being as long and slender as it must be,may pose some difficulty.Far from insurmountable,but,depending on one's experience and access to the proper equipment,could be a bit of a headache.
Sounds like fun,though!One could experiment with both the cross-section of the arm of the prod and the temper,using a scale,and measuring the deflection distance,and it may get to be quite a controlled,and interesting process.The very best of luck to you,if ever you shall so embark!
P.S.Having written all that,belatedly,and with dismay,i realised that it very well may fall ouside the scope of the preferred discussion on the forum,as set out in a thread by Mr.McDougall,that i've read immediately before this one.If so,my apologies,and i'll make an effort not to be carried away by my empiricism in the future.
Having failed in an attempt to erase it,would the implied safety warning about the use of post-industrial steel deem it even marginally acceptable?Or it's questionable benefit to an archeometallurgical studies?