Influence of boron on the decomposition of austenite in low carbon alloyed steels

The influence of boron on the isothermal decomposition of Fe-Ni₆-C_0.12 (wt pct) steels has been investigated. The isothermal γ→ pro-eutectoid ferrite reaction was studied by quantitative metallography and dilatometry. It was clearly shown that boron slows down considerably the nucleation rate of ferrite on γ-grain boundaries. End-quench experiments performed on C_0.18-Cr-Mn industrial steels emphasized the changes in hardenability with thermal history. Particular attention was devoted to the study of the state and location of boron in the microstructure of the steels studied. Ion microscopy, alphagraphy and transmission electron microscopy were used to this effect. It was confirmed that boron segregates easily to γ-grain boundaries during cooling, which results in the precipitation of iron boro-carbides. This precipitation was shown to occur both in stable and metastable austenite, prior to the γ → pro-eutectoid ferrite reaction. The precipitates were identified as Fe₂₃(B, C)₆ (FCC structure with a ≈ 10.6?). The grain boundary Fe₂₃(B, C)₆ were shown to have a parallel cube-cube orientation relationship with one of the neighboring grains. The role of the Fe₂₃(B, C)₆ precipitates with respect to the γ → proeutectoid ferrite reaction is discussed.