Mitch, here is Crucible's method.
Hardening
Note: Full hardness will only be attained in sections less than about 3 inches thick.
Critical Temperature: 1325F(720C)
Preheat: 1250/1350F(675/730C)
High Heat: 1500/1550F(815/845C), hold 10/30 minutes at temperature. Quench: Quench in oil to hand warm, 150F(65C). For minimum distortion, parts may be removed from oil at about 400F(205 C), and air-cooled to hand warm.
Temper: 350/600F(175/315C); hold one hour per inch of thickness, two hours minimum, four hours preferred.
Cryogenic treatments: Refrigeration treatments may improve long-term dimensional stability by transforming retained austenite. Refrigeration treatments should be performed after at least one temper, to minimize risk of cracking, and should always be followed by a temper.
Typical Properties
Hardness and Impact Toughness Data
Oil quenched from 1500F(815C)
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Tempering
Temperature
Hardness Toughness,
Charpy C-notch
°F °C HRC ft-lbs Joules
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As quenched 63-65 - -
300 150 63-65 15 20
400 205 60-62 43 58
500 260 58-60 38 52
600 315 56-58 68 92
700 370 53-55 - -