Solidification of iron-base alloys

Abstract

The solidification process of iron-base alloys can start with either primary precipitation of ferrite or primary precipitation of austenite. The segregation sequence during precipitation of ferrite is quite different from the segregation sequence during primary precipitation of austenite. This depends mostly on the difference in diffusion rate between ferrite and austenite but also on differences in the partition coefficient between ferrite and liquid or austenite and liquid for different alloying elements. The differences in segregation processes during a primary precipitation of austenite/ferrite are discussed in the paper. Many alloys starting the solidification process by a primary precipitation of ferrite go through a peritectic reaction and transformation. This reaction will be influenced by the diffusion rate of the different alloying elements in austenite as well as in ferrite. The peritectic reaction in binary alloys is discussed in the paper. Most iron-base alloys contain one or more austenite-stabilizing elements (C, Ni, Mn, for instance) and one or more ferrite-stabilizing elements (P, Cr, Mo, for instance). The austenite-stabilizing elements favour a peritectic reaction during the solidification process and the ferrite-stabilizing elements favour a eutectic reaction. In ternary alloys with one ferrite-stabilizing element and one austenite-stabilizing element, the peritectic reaction can change to a eutectic one with increasing content of the ferrite stabilizing element. This transition is discussed for Fe–P–C, Fe–Ni–Cr alloys and also for stainless and high-speed steels. Some ferrite-stabilizing elements give rise to a metatectic reaction, for instance Fe–S and Fe–P–C alloys. In some alloys, the metatectic reaction occurs together with a peritectic reaction. This transformation sequence is discussed in the paper for Fe–Ni–S alloys as well as for Fe–P–C and high-speed steel alloys.