The main parameters affecting machinability are:

• Chemical composition
• Shape and distribution of the manganese sulphides
• Presence of stringers
• Strain-hardening and mechanical features

Let’s see each of them in detail:
Chemical composition

Carbon C
The European standard for cold-finished products settles a carbon content C ≤ 0,14 %.
Steel is usually delivered with a carbon content C ≤ 0,10 %; the disadvantage of excessive ductility, typical of steels with low carbon content, is corrected by suitable drawing reductions.

Silicon Si
The European standard for cold-finished products settles a silicon content Si ≤ 0,05 %.
This element is extremely deleterious to machinability and should be kept in an extremely reduced content.
As a matter of fact, silicon tends to fix with the oxygen contained in steel leading to the formation of abrasive inclusions reducing the tool life.
Moreover it has a negative influence on the manganese sulphide morphology. The best machinability results are obtained with a silicon content Si ≤ 0,02 %.

Manganese Mn
It can be found in steels in the shape of manganese sulphides or dissolved in the ferritic basic structure.
Its content can vary from 0,90 % up to 1,50 %.

Phosphorus P
It is dissolved in the ferritic basic structure; it contributes to hardness and brittleness increase, favouring the chip fragmentation.
The European standard for cold-finished products settles a phosphorus content ≤ 0,11 %.

Sulphur S
It is present in steel in the shape of manganese sulphides; it is one of the determining parameter for machinability. We will deal with sulphide nature and morphology in the relevant chapter.
The sulphur content can vary from 0,27 % up to 0,40 %.

Lead Pb
It is insoluble in iron; it tends to bind to the non-metallic inclusions present in steel or it can be found in the shape of elementary particles.
It acts as a lubricant, reduces the friction coefficient between tool and chip and the formation of the built-up edge.
The European standard settles a lead content from 0,20 % up to 0,35 %.

Oxygen O:
If present in steel in the “free” condition (as oxygen not bound to Si and Al) it favours the formation and preservation of round sulphides, essential requirement for a good machinability.

Aluminium Al:
It is extremely deleterious to machinability and must be kept in a very low content. On the contrary, this will result in the formation of alumina inclusions with extremely detrimental effects on tool life.

Hardening elements:
The content of elements like Ni, Cr, Mo etc. must be limited.
Their cumulative value should preferably be below 0,30 %.