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 degreesC 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,N-b)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 degreesC were markedly lower in comparison to these modified separately by either of these two elements. (C) 2001 Elsevier Science Inc. All rights reserved.