Effect of Nb and Ti additions on microstructure, and identification of precipitates in stabilized Ni-Cr cast austenitic steels

Microstructures of three austenitic 30%Ni/18%Cr cast steels with additions of Nb and Ti, in the as-cast and annealed (900 °C for 300 h) state have been studied. The additions of Nb and Ti were as follows (wt.%): 1% Ti (no Nb), 1.71% Nb, (no Ti) and 0.83% Ti and 1.75% Nb, respectively. Microstructural analyses were carried out using scanning microscopy, X-ray diffraction and quantitative microprobe analysis. Depending on chemical composition, the following phases have been identified: NbC, TiC, (Ti,Nb)C, M23C6, and Ni- and Si-rich phases (probably G phase). The presence of the Ni–Si-rich phases, found subsequent to the annealing process, appears to be closely associated with the presence of unstable carbide precipitates. Although additions of Ti and/or Nb improved the microstructure, mechanical properties of such modified alloys at temperatures of 20 and 900 °C were markedly lower in comparison to these modified separately by either of these two elements.