Hot workability of steels for forgings

Sufficient hot ductility is one of the prerequisites for the successful forming and heat treatment of steels. The influence of chemical composition (including trace elements), of soaking and deformation temperatures, strain rate and duration of deformation was to be studied in hot tensile testing. To distinguish the different embrittling mechanisms from each other, a great number of steels with systematically varied composition were examined. Test conditions were chosen so as to give maximum agreement with actual hot working operations. In the temperature range from 1 200–600°C, which was covered by this study, hot embrittlement was only found on steels containing at least one of the elements N, Nb, Pb or Bi. Embrittlement due to MnS precipitations did not occur, as the soaking temperature was limited to max. 1 315°C and the Mn/S ratio was at least 30. Nitrogen is the main cause of hot embrittlement in commercial steels. The fact that also unalloyed, aluminium-free steels embrittle with sufficiently low strain rate shows that nitrogen in solid solution may cause embrittlement even in the absence of nitride formers. The nitride formers accelerate the embrittling process, provided the nitrides are dissolved at soaking temperature. Aluminium, however, has a retarding effect in the presence of vanadium. Embrittlement is attributed to nitrogen atoms entering into multiple voids and micropores, where they recombine to form molecules which impede the slip of dislocations, thus leading to embrittlement. A sufficient length of the deformation operation and recrystallisation being impeded by precipitations are the prerequisites for this type of embrittlement. Titanium, by binding nitrogen at an early stage, prevents precipitation. Also in the case of embrittlement by lead and bismuth, the most conclusive explanation is that atoms of these elements accumulate in voids. Embrittlement by niobium, however, is attributed to deformation-induced precipitation, as it only occurs on cooling from soaking to test temperature and not on direct heating to test temperature.