Machinability is rated as fair in the annealed state and poor/difficult in the +QT state.
Annealed (~200 HB):
Use positive rake angle tools (carbide or HSS).
Moderate cutting speeds, adequate feed to get under the work-hardened layer.
Use generous coolant/lubrication.
Quenched & Tempered (~300 HB):
Hard Machining: Requires rigid machine tools, sharp carbide or CBN (cubic boron nitride) inserts.
Reduced Parameters: Lower cutting speeds and feeds compared to annealed state.
Important: Avoid letting the tool dwell on the workpiece to prevent excessive heat generation, which can locally temper and soften the material or cause thermal cracking.
Grinding is often the preferred finishing method after heat treatment.