Alloying and trace elements can influence the machinability of steel by changing the composition or by forming lubricating or abrasive inclusions. In the following, the influence of the most often used elements on the machinability of steel materials will be described.
Manganese
Manganese improves hardenabillity and increases the strength of steels (ca. 100 N/mm2 per 1 % alloying elements). Because of its high affinity to sulphur, manganese forms sulphides with sulphur. The influence of these manganese sulphides is described by the alloying element sulphur. Manganese contents up to 1.5 % facilitate machinability in steels with low amounts of carbon due to good chip formation. On the other hand, in steels with higher carbon contents, the alloying of manganese influences machinability negatively because of higher tool wear.
Chrome, Molybdenum, Tungsten
Chrome, molybdenum and tungsten improve hardenability, thereby influencing the machinability of case-hardened and heat-treated steels in terms of structure and strength. In steels with higher carbon/alloying contents, these elements form hard special and mixed carbides, which can deteriorate machinability.
Nickel
By adding nickel, the strength of steel materials increases. This generally leads to unfavourable machinability, especially in the case of austenitic Ni-steels.
Silicon
Silicon improves the ferrite strength of steels. With oxygen, it forms, in absence of stronger deoxidation agents like aluminium, hard Si-oxide (silicate) inclusions. Increased tool wear can result from this during chip removal.
Phosphorus
Alloying phosphorus, which is carried out only in some free-cutting steels, leads to segregations in the steel. Even with subsequent heat treatments and heat deformations, the segregations cannot be removed and lead to an embrittlement of the а-mixed crystals (ferrite embrittlement).
Titanium, Vanadium
Titanium and vanadium, even in small amounts, can increase strength considerably due to the extremely dispersed carbide and carbon nitride precipitations. Furthermore, they lead to enhanced grain refinement.
Sulphur
Sulphur is only slightly soluble in iron, but it forms, depending on the alloying components of the steel, various stable sulphides. Iron sulphides (FeS) are undesirable, as they have a low melting point and form deposits primarily at the grain boundaries. This leads to the unwanted “red brittleness” of steel. What are desirable on the other hand are manganese sulphides (MnS), which have a much higher melting point. The positive effects of MnS on machinability are the short comma chips, improved workpiece surfaces and the decreased proneness to clogging the grinding wheel. With an increased inclusion length, MnS exerts a negative influence on mechanical properties like strength, strain, area reduction and the impact value, especially when it is included transversely to the strain direction.
Non-metallic Inclusions
The elements added to the steels for deoxidation, aluminium, silicon, manganese or calcium, bind the oxygen released by steel solidification. The hard, nondeformable inclusions then found in the steel, e. g. as aluminium oxide and silicon oxide, diminish machinability, especially when the oxides exist in the steel in larger amounts or in linear form [WINK83]. However, by choosing a suitable deoxi- datising agent, the steel’s machinability can also be positively influenced. For example, under certain machining conditions, wear-inhibiting oxidic and sulphidic protective layers can form after deoxidation with calcium-silicon or ferro-silicon [OPIT67].