Extrusion mandrels are surrounded by the billet at the extrusion temperature during the entire extrusion process. The thermal loading is correspondingly high. The temperature in the production of light metal tubes is below the tempering temperature. With heavy metals these temperatures are, however, greatly exceeded so that an annealing effect on the mandrel surface over a period of time cannot be completely
avoided, even with internal water cooling.

The Cr-Mo-V steels grades 1.2343, 1.2344, 1.2365 and 1.2867 with good thermal conductivity are used for mandrels with internal cooling, in particular 1.2367. The correct choice of the cooling bore diameter and optimum temperature control during the working process and during the non-working times are nevertheless, particularly, important for achieving the best service life.

Considerable friction occurs on the mandrel surface. A high quality surface is therefore important. Nitrating the surface has proved particularly successful in the case of light metal processing to reduce the friction and adhesion.

Hardly any benefits are achieved with heavy metals, however, as the nitrated layer cannot withstand such high temperatures. A surface oxidation of the mandrel by prolonged heating to approximately 500°C has proved useful in this case. The mandrel should also be lubricated. Bright metallic surfaces on mandrels should be avoided. Surface coatings applied by PVD or CVD processes have so far been unsuccessful, as they cannot withstand the high temperatures. Further research in this area is expected.

Since significant tensile stresses develop in the mandrel during extrusion along with high buckling stresses, if piercing is carried out with the mandrel the tensile strength should be in the range 1500-1700 N/mm².

Mandrel tips for the production of heavy metal tubes are made in the hot-working steel 1.2367 at 1400-1500 N/mm² and to an increasing extent for very high stresses in the Ni base alloys in 2.4973 (SL15). The tensile strengths after age hardening are around 1300 N/mm².