Mn deoxidizes. It compounds with sulphur to form Mn sulphide, thus reducing the undesirable effect of the iron sulphide. This is of particular importance in free-cutting steel; it reduces the risk of red shortness. Mn very pronouncedly reduces the critical cooling rate, thus increasing hardenability. Yield point and strength are increased by addition of Mn and, in addition, Mn favourably affects forgeability and weldability and pronouncedly increases hardness penetration depth. Contents above 4% also lead with slow cooling to formation of brittle martensitic structure, so that the alloying range is hardly used. Steels with Mn contents above 12% are austenitic if the C content is also high, because Mn considerably extends the γ-phase. Such steels are prone to very high degree of strain hardening where the surface is subjected to impact stress, whilst the core remains tough. For this reason, they are highly resistant to wear under the influence of impact. Steels with Mn contents of 18% upward remain unmagnetizable even after relatively pronounced cold forming and are used as special steels as well as steels remaining tough at subzero temperatures which are subjected to low temperature stress. The coefficient of thermal expansion increases as a result of Mn, whilst thermal and electrical conductivity are reduced.