Understanding Machinability
Machinability is the relative susceptibility of a material to the machining process.
Not all materials are equally machinable. (For example, aluminum is more easily machined than stainless steel or titanium.) The chemical makeup and metallurgical qualities of a material affect machinability.
Below are some specific factors which influence the machinability of a material:
Thermal conductivity: Materials that conduct heat well facilitate the dissipation of heat away from the area that is being machined. Therefore, high thermal conductivity generally aids the machining process.
Hardness: Low hardness values make a material easier to machine. However, low hardness is a disadvantage if it is associated with high ductility. (see below)
Ductility: Ductility is a measure of how much deformation a material can sustain before it fractures. The machining process is all about breaking metal into chips. Low ductility therefore facilitates machining.
In this regard, there is an inverse relationship between ductility and hardness. Hard materials are less ductile. This quality makes them comparatively easy to machine. On the other hand, if a material is too hard, its low ductility will be offset by its hardness. So an ideal material should be not too hard---and not too ductile.
Inclusions: An inclusion is a substance that is contained within steel in trace amounts. Some inclusions enhance machinability; others make steel less machinable.
- Aluminum oxide (Al2O3), for example, is very hard and abrasive. (In fact, it is sometimes used as an ingredient in cutting tools.) Steel that contains Al2O3 is difficult to machine.
-Sulfur is sometimes added to steel to enhance its machinability. When sulfur and manganese are both present, manganese sulphides will form in the steel. This makes the steel selectively weaker, and therefore easier to cut.
-When heated during the cutting process, silicates become soft and powdery, thereby making steel more machinable. Silicates are therefore a “good” inclusion in steel.