I'm making a fullering jig.  For the fullering arms, I've used leaf spring steel.  I cut a 7" length in half and put the two pieces in the forge so I could straighten them for the jig.  Air cooled them.  Then I tried to drill holes so I could mount them in the jig.  Steel had hardened.  So I tried to anneal them in two 1-hour cycles in an oven at 400 degrees.  Still too hard to drill.  What have I done wrong?

In my opinion you never annealed them.  You kind of normalized by heating to forging temperature and air cooling.  The 2 cycles in the oven were at tempering temperatures, not annealing.

You can do a sub-critical annealing if you heat them to dull red (still magnetic) and allow them to cool slowly or you can bring them up past critical and again allow them to cool slowly.  This is usually done by burying the still glowing piece in something insulating, such as ashes or vermiculite. That's for fairly simple alloys.  More complex alloys have very specific temperatures, soak, and ramp times needed to accomplish proper annealing.

However, if all you are trying to do is get the pieces soft enough to drill, then a sub-critical annealing should do what you need.  A dull drill bit, too much speed, and/or not enough pressure on the bit can still lead to work hardening though.

Thanks, Buzzkill.  I've been doing some on-line searches and it seems I can put a rosebud on my Oxy-Acetylene rig and heat it up to a blue color and then let it air cool.............or cool in vermiculite as you suggested.  That should make it soft enough to drill.  Will see what that accomplishes.  Just don't have any vermiculite at the moment.

As long as you stay below critical temperature, air cooling should work ok if you're just trying to get a couple holes drilled.  It won't be the softest/most easily machined state of the steel, but it should be good enough.

Waiting for them to cool down.  We'll see what happens.

Your leaf spring is most likely 5160 or 9160, which I believe are notorious for experiencing a degree of air hardening and/or potential formation of lamilar plates of iron carbide if kept at elevated temperatures for extended periods.  Your cycle at 400 degrees would have tempered the steel, but not addressed these issues.  Buzzkill has given you a good recommendation to do a subcritical tempering cycle (bringing the stock up to just below the transformation temperature, do not enter non-magnetic, and letting them air cool slowly (better in vermiculite, but that will take a while).  Then you should be able to drill, provided you haven't destroyed the edge on your drill bits.

Of course then the steel will be soft, so you might want to go through the whole normalizing, hardening and tempering process again.  I would temper for a full spring hardness, which should be bringing it up to well in excess of 400 deg F.  I'm not sure exactly what you will need, but would expect in the 500 - 600 degree range, or a temper color of deep blue on clean steel.

Dry sifted wood ashes works for annealing.  Easy to source too.

This is getting far too "compilated".    I'm just going to take everyone's suggestion and just go get mild steel for these fullering arms.

It's really not that bad.  If you deal with much high carbon steel in the future it's something you will probably run into again.  A blue temper color is in spring range, so that would still be pretty hard.  When this happens to me I go past all the temper colors to at least a "black heat" or the dull red I referenced earlier.  With decent cobalt bits it works every time for me.  The cheap HF bits are not your friend for much of anything, but especially not for high carbon steels.

On the other hand mild steel might be the easier option for this particular project.  It will probably just have to be replaced sooner or more frequently.

I've got good bits............they aren't from HF.  I'll just get mild steel for now and quit messing with it.

Good Morning Chris,

"Just don't have any vermiculite at the moment".  Use garden Lime from your local Feed store or Garden Supply. Simple and basic. Put the lime in a bucket with a lid. Lime also works as a Flux for forge-welding, except it doesn't upset your fire. K.I.S.S.

Neil