C influences hardness (higher content increases abrasive wear). Low carbon content <0.2%, increases the adhesive wear, which will lead to built-up edge and bad chip breaking.
Cr, Mo, W, V, Ti, Nb (carbide formers) – increase abrasive wear.
O has a large influence on machinability: it forms non-metallic,
oxidic and abrasive inclusions.
Al, Ti, V, Nb are used as fine-grained-treatment of steel ; they make the steel tougher and more difficult to machine.
P, C, N in the ferrite, lowers the ductility, which increases adhesive wear.
Pb in free machining steel (with low melting point) reduces friction between chip and insert, lowers wear and improves chip breaking.
Ca, Mn (+S) form soft lubricating sulphides. High S-content improves machinability and chip breaking.
Sulphur (S) has a beneficial effect on machinability. Small differences, such as those between 0.001% and 0.003% can have substantial effects on machinability. This effect is used in free machining steels. Sulphur content of around 0.25% is typical. Sulphur forms soft manganese sulfide (MnS) inclusions that will form a lubricating layer between the chip and the cutting edge. MnS will also improve the chip breakage. Lead (Pb) has a similar effect and is often used in combination with S in free machining steels at levels of around 0.25%.
Both positive and negative
Si, Al, Ca form oxide inclusions that increase wear. Inclusions in the steel have an important influence on the machinability, even though they represent very small percentages of the total composition. This influence can be both negative and positive. For example, aluminium (Al) is used to deoxidize the iron melt. However, aluminium forms hard abrasive alumina (Al2O3), which has a detrimental effect on machinability (compare the alumina-coating on an
insert). This negative effect can, however, be counteracted by adding Calcium (Ca), which will form a soft shell around the abrasive particles.
- Cast steel has a rough surface structure, which can include sand and slag, and places a high demand on the toughness on the cutting edge.
- Rolled steel exhibits a fairly large grain size, which makes the structure uneven, causing variations in the cutting forces.
- Forged steel has a smaller grain size and is more uniform in structure, which generates fewer problems when cut.
Predominant uses include: constructional steel, structural steel, deep drawn and stamped products, pressure vessel steel, and a variety of cast steels. General uses include: axles, shafts, tubes, forgings and welded constructions (C<0.25%).
Difficulties in chip breaking and smearing tendencies (built-up edge) require special attention in low carbon steels (< 0.25%). High cutting speeds and sharp edges and/ or geometries, with a positive rake face and thin coated grades, will decrease the smearing tendencies. In turning, it is recommended that the depth of cut remains close to or bigger than the nose radius to improve chip breaking. In general, the machinability is very good for hardened steels, however, they tend to generate relatively large flank wear on the cutting edges.
Mo- and Cr-alloyed pressure vessel steels are used for higher temperatures. General uses include: axles, shafts, structural steels, tubes and forgings. Examples of components for the automotive industry are: con rods, cam shafts, cv-joints, wheel hubs, steering pinions.
Machinability for low alloyed steels depends on the alloy content and heat treatment (hardness). For all materials in the group, the most common wear mechanisms are crater and flank wear.
Hardened materials give higher heat in the cutting zone and can result in plastic deformation of the cutting edge.