Slitting 4140 - Work hardened - How to recover

Hello;

I'm having trouble slitting 4140. Being a novice, I overheated the cutter by running to fast an rpm and the slot has work hardened. Hack saw skates in slot, rear of slot is discolored. I suspect I need to anneal the piece to get past this but am concerned it might damage the part..

Any suggestions on how to complete the slot?

Details:
Hand milling on RongFu Mill - traditional hand fed - not climb milling
Slot 1/6" wide 3/8" deep, 5/16" long in 4140 (Initially non hardened)
Have about .100" left to cut

Saw was HSS 3" x 1/6 x 1" arbor no key, 30 teeth - flat tooth, on a china universal size arbor.

Even though was to fast, It cut fine .020 per pass with a little machine oil coolant till I got to the tooth depth. Started to make some awful noise...etc.

My current plan is to pickup a few saw blades with key, try to cut through at a very slow feed with an alternate keyed arbor which I have somewhere. If that fails anneal and use one of the spare saws.

Any suggestions on blades, speeds, etc to complete cut would be greatly appreciated.
Thanks
Mike

Well I was able to finish the slot by hand with a diamond needle file. It appears suitable for now.
The best suggestion I've found for the saw is 70rpm with a warning for feed calcs that not all teeth are engaged with these saws
Sorry if I posted in the wrong Forum
Thanks
Mike