Hi Brian,
If you are going to use disc harrows too...
If there is any warpage it will because there was stresses in the metal during production, such as stamping them out, or the metal being bowed or warped slightly from the production mill, usually after heat treating is when it shows up...for example 0-1 steel will warp easily if it contains internal stresses.
Usually annealing them or tempering them most often does not alter a bow or warp in the metal...however as you approach its hardening temperature it can "awaken" those internal stresses. However on your end the discs are probably already heat treated and you don't need to worry too much about how much they will warp....in other words, if the discs you have are already heat treated, the damage has already been done so to speak in regards to warping.
But to soften the metal I would first try heating them up to a dull red but not a cherry red color...somewhere in between will be fine. If you go by the "colonial" method, just build a good hot fire, with plenty of hot coals, place the discs in there flat , pile some more wood on them and let the thing burn good and hot, keep working the coals onto and under the metal discs, even pile them on top, ...a good garden rake is good tool for this...periodically rake back the hot coals and look at the color of the discs, if they are a dull red in ambient light then you are probably good to go...just mound some more coals on top and let the whole thing burn down nice and slow until you can fish them out by hand It important that you don't rush the cooling process....no buckets of water or oil or anything like that! Just let it cool off slowly.
I'm basing this on that the metal is something like 4140, or another steel which is heat treated by flame and oil or water/brine quenched. These types of steels are usually what is encountered in these types of situations... they easier to manufacture and produce than more expensive tool steels. With that in mind am I 100 percent sure you have an oil or water quenched metal in your hands? No I'm not, but I could venture a guess with 90 % certainty. However if I'm wrong about the metal type, you will not do any damage to it by drawing or tempering the metal back by the above method...it will either soften up or will not...but you have nothing to loose by trying.
Try to keep the color and heat as even as possible...steels such as 0-1 will warp when quenched if heated uneven, but in the above temering method you are bring them upt to a drawing tempreture slowly and evenly and with out the rapid quenching or hardening phase to worry about it warping
All the best,
Gavin
