Study of Microstructure and Phase Composition of Creep Resistant Cast Steel

The study presents the results of the identification of the phase composition of creep resistant cast austenitic 30Ni-18Cr steel with the addition of Ti and Nb after the annealing process conducted at 900°C for 10 and 500 hours. The microstructure of the as-cast steel showed mainly the presence of simple carbides. Depending on the alloy chemical composition, those were the TiC and NbC carbides. In the initial phase of annealing process, the nucleation of small secondary precipitates of chromium carbides Cr23C6 was observed to take place in cast steel microstructure at the matrix / simple carbides interface. After a long-time annealing, a new phase (G phase) was developing; the symbols of the phase were Ni16Ti6Si7 and Ni16Nb6Si7, respectively. The G phase was enclosing simple carbides forming with them the complexes of primary and secondary precipitates of a very intricate morphology. Chromium carbides Cr23C6 were also present there.