Redetermination of the Fe-rich portion of the Fe−Ni−Co phase diagram

Abstract

The iron-rich portion of the Fe-Ni-C phase diagram has been determined in the composi-tion range from 0 to 20 wt pct Ni and 0 to 6.67 wt pct C for four temperatures, 773, 873, 923 and 1003 K. Long term heat treatments were used to grow the ferrite plus austenite assemblages, while slow cooling heat treatments (25 K/h) were used to grow the metal plus carbide assemblages. Other types of heat treatments produced metal plus graphite. The two phase tie-lines and three phase tie-triangles were measured using electron mi-croprobe techniques. In samples where bulk equilibration had not been achieved, tie-lines were obtained by using extrapolated interface compositions, on the assumption of local equilibrium at the interface. The tie-lines lie at higher Ni contents than the equilibrium tie-line through the bulk composition. The tie-line shift was required to produce match-ing growth rates of Ni and C for the carbides. The addition of Ni slightly reduces the solubility of carbon in austenite and decreases the stability of the carbide phase. In addi-tion, the carbide is always Ni-poor relative to the coexisting metal phase(s).