What are the primary factors that affect the material machinability?

The chemical composition of steel is very important. The higher alloy composition of steel, the more difficult to machining. Metal cutting performance will be decreased when carbon content increase.

Steel structure is also very important for metal cutting performance. Different structures included: Forged, cast, extruded, rolled and machined. Forging and casting parts surface are very difficult to processing.

Hardness is another factor that affect metal machinability. Regularly the steel is more hard, it is more difficult to machining. High speed steel(HSS) can be used to processing material with a hardness up to 330-400 HB. But for material with hardness 65-70 HRC, it muse use carbide, ceramic, metal ceramic and Cubic Boron Nitride (DBN ).

Non-metallic mixing will have bad affect on tool life. Sunch as Al203, which is pure ceramic, have a strong abrasive.
The last one which can affect metal cutting performance factor is residual stress. It is often recommend to do stress relief after rough machining.