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 « 600A dia.). In y-iron, the boron segregates uniformly to the grain boundaries, the degree of segregation reducing with increasing temperature. However, in cx-ironprecipitation 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 cx-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.The improvement, in the mechanical properties of iron and steel brought about by trace quantities of boron has been recognized commercially for over 20 years.l,2 Properties well known to be affected by boron include hardenability of ferritic steels, 3 hot-working characteristics of austenitic steels, 4 and creep strength of both austenitic and ferritic steels. 5 In all these instances, boron exerts its most significant effects at levels < 120 ppm, and indeed above this level some properties, e.g. ductility, deteriorate again.The mechanism by which boron' influences these properties is in many cases not clearly understood, but it seems certain that the predominant effects are interaction with lattice defects and grain boundaries. 6 In alloys containing carbon or nitrogen there is also an effect on precipitation' behaviour which in' many cases may be ascribed to the defect-seeking nature of the boron atom. 7 In view of the extremely low concentrations of boron employed, an understa.pding of the solubility of boron in these materials is' essential to the elucidation of the mechanism of these effects. A survey of the literature S ,17 shows that while a number of workers have investigated the nature of the solution of boron in iron, rather fewer attempts have been made to determine the
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