A sub-critical anneal works well with 52100 to obtain machinability.
We recommend a series of thermocycling then austenitizing to 1550F and quenching in the quench medium of your choice to form fresh/untempered martensite or bainite. Of course tempering is neccesary if slack-quenched (when martensite is the goal).
We recommend Parks 50 or low temperature salt as a quench medium.
52100 works well for Austempering. Hardnesses of 60HRC can be obtained with Austempering 52100.