FREE-CUTTING STEEL
How they “work”
The free-cutting steel are extremely machinable for high speed machine: this is their main purpose.
To achieve this purpose, it is necessary that:
- during machining, the chip is broken up so that it does not wrap around the tools;
- the tools have a long service life;
- the surface of the finished part has an adequate roughness.
To achieve this, it is necessary to act on the chemical analysis of the steel.
In all free-cutting steels, sulphur is present as an alloying element.
This element below 769°C is poorly soluble in iron and in the presence of manganese (also always present in automatic steels) forms manganese sulphides.
Manganese sulphides improve machinability because they act as discontinuities in the metal matrix, allowing chips to break during machining. They also act as self-lubricants and reduce the adherence of the material on the tool.
Like manganese sulphides, lead increases the machinability of steel and also melts locally at the interface between tool and workpiece during machining, thus acting as a lubricant and consequently improving surface finish and tool life.
There are other elements that improve the machinability of free-cutting steels: above all, tellurium and bismuth.
Tellurium greatly influences the shape of manganese sulphides by making them globular, which improves machinability and cold deformability.
Bismuth has a very similar behavior to lead. It has a low solubility in liquid steel and forms intergranular precipitates during solidification.
Having a low melting temperature, bismuth forms a lubricating layer between workpiece and tool, which improves tool life.
Along with sulphur, lead is also often present, an element that does not remain in solution in the steel, but disperses finely and homogeneously, forming inclusions that are often associated with manganese sulphides.
Types of free-cutting steels available at Gieffeacciai
Free-cutting steels not intended for heat treatment
This is the most common and wide family, which can be divided in two groups: the sulphur-only steels, such as 11SMn30 and 11SMn37, characterized by good machinability, and their versions with lead, 11SMnPb30 and 11SMnPb37 with a significant increase in machinability , up to steels with tellurium and bismuth, such as 11SMnPb30+Te+Bi and 11SMnPb37+Te+Bi, which take machinability to the highest levels with the addition of cold deformability, thanks to the presence of tellurium.
For those who want highly machinable free-cutting steels but with a lead content below the limits set by the REACH regulation and the RoHS Directive, we recommend GFast30 and GFast37 steels.
Free-cutting steels for case hardening
The 10SPb20 is a free-cutting steel designed for all those parts that need a good machinability and that have to be subsequently case-hardened.
Free-cutting steels for quench and tempering
46S20 is the ideal free-cutting steel to be quenched and tempered after machining; for those who need more machinability due to the presence of lead, 36SMnPb14 can be used.
If higher mechanical properties are required, it is advisable to use 44SMnPb28 steel.
