Precipitation reactions in a Cu-Ni-Al medium carbon alloyed dual hardening steel

Dual hardening steels achieve secondary hardening through the combined precipitation of intermetallic phases and carbides. The early stages of precipitation of a Cu-Ni-Al medium carbon alloyed dual hardening steel with a low alloying ratio of Cu/NiAl were investigated. Phase separation during continuous heating with 20 K min ^{− 1} was identified to occur in the temperature range from 360 °C to 620 °C by differential scanning calorimetry. The precipitates were characterized utilizing atom probe tomography. Different cluster search algorithms were used in order to determine the sequence of phase separation and thus identify the individual precipitation reactions. It was shown that phase separation starts with the formation of Cu (Ni, Al, Mn) clusters. At an ageing temperature of 580 °C, heterogeneous precipitation of VC-clusters at the interfaces between the Cu-precipitates and the iron matrix takes place. A phase separation of the Cu (Ni, Al, Mn) clusters into separate Cu and NiAl particles could not be observed up to an ageing temperature of 620 °C. The sequence of precipitation is thus determined to be Cu-VC-NiAl.