The Distribution of Boron in Pure Iron

Abstract

The solubility of boron in pure iron has been determined over the temperature range 500–1300° C using the technique of boron autoradiography, which can distinguish between boron in solution, boron-rich precipitates, and boron segregated to grain boundaries. However, the solubility limits determined should be regarded as upper limits, as the technique cannot differentiate between soluble boron and very fine boride particles (< 600Å dia.). In γ-iron, the boron segregates uniformly to the grain boundaries, the degree of segregation reducing with increasing temperature. However, in α-iron precipitation of iron boride at grain boundaries was observed, with no detectable segregation between the particles. A Snoek peak was not detected in internalfriction measurements, indicating that boron is not in interstitial solid solution in α-iron. The segregation to austenite boundaries delays the nucleation of ferrite leading more readily to an acicular ferrite morphology. The structures obtained on quench-ageing were also examined.