Temperature is the main factor. Total length of temper time also figures into it. Two hours of tempering time will get you just about all the tempering effects at a given temperature that you're going to get. Some of our hypoeutictic and eutectic steels can get by with one tempering cycle. Your hypereutictic steels may need multiple cycles to get retained austenite to convert. Of course the best thing to do with these steels is not to overheat them and put too much carbon into solution in the first place.
Doug
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