I would like to try making a 40 taper spindle shaft out of 4140 and use a kilt to harden it. If you have some experience in this area some tips will be greatly welcomed. Basically one of these:
How much will is warp after the heat treating about? Any special grinding tricks to it?
The kiln goes to 2000F
Is prehardened not sufficiently hardened for you? I suspect you will cause decarb of the surface if you do not have a protective atmosphere while the part comes up to temperature, so you might need a grinding allowance of .010 to get to the fully hardened surface.
If you began with annealed material, then it should not warp too much in the furnace if you can hang it up, rather than lay it down during the heating.
Quenching is best done in oil, and this is probably going to be more of a PITA than heating the part up. You might need a rather large quantity of oil, and a circulation system because oil is not really fast at absorbing heat. So either you circulate the oil or the part, and I have no idea how big a part you are thinking of making. Exactly what type of oil is used for quenching, I am not sure, but it needs to have a high flash point, and even then, the possibility of a fire during immersion is possible.
I would recommend that you use the services of a heat treater who is in the business. And if you consult one of them, he might recommend using C8620 material, rough machine it to within .010 of final size, get it carburized and hardened. This material will maintain a softer core, and yet harden better than C4140 with greater safety in hardening.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)