Ferritic and martensitic steels are seldom particularly difficult to machine, although ferritic steels can give rise to problems of stickiness and the formation of built-up edges. Martensitic steels can, of course, be difficult to machine if they have been hardened to high hardness, although this is nothing unique to stainless steels in particular but applies to all materials with high hardnesses.
 
The fact that the austenitic and ferritic-austenitic types of steel give problems is due to the combination of properties of these relatively soft materials. These properties are such that the hardness can no longer be used as an indication of the material's machinability.

Machinability is a complex concept that embraces several factors, and not just the question of tool wear. In addition to the effect of the material on tool wear, i.e. on tool life, it is also necessary to consider its effect on the magnitude of cutting forces, chip shape and surface quality of the machined surface when considering the overall machinability of a material.

The special material properties of stainless steels affect all four machinability factors: in general, it can be said that the higher the alloy content of a stainless steel, the more difficult it is to machine. The special properties that make stainless steels difficult to machine occur to a greater or lesser extent in all grades of stainless steels, but are most marked in the austenitic and ferritic-austenitic grades. They can be summarised in five points:

• Stainless steels work-harden considerably
• Stainless steels have low thermal conductivity
• Stainless steels have high toughness
• Stainless steels tend to be sticky
• Stainless steels have poor chip-breaking characteristics