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Abstract. The dissolution and precipitation of boron have been studied in a high-carbon steel. Boron was found in different states: boron oxides, boron carbonitrides and iron-borocarbides Fe23(B,C)6. The dissolution of ironborocarbides in austenite is complete at 1100 ~ and precipitation along 7 grain boundaries of this boron-bearing phase was observed after water-quenching from high austenitizing temperature. Therefore, boron precipitates along 7 grain boundaries before pearlite nucleation.Key words: boron, carbon steel, borocarbides.Boron is an important microalloying element in steels because even small quantities can have a strong effect. This effect has been studied mainly in low-carbon steels in which it is well known that boron retards the nucleation of ferrite and bainite and thereby increases hardenability [1,2]. It is generally agreed that the effect of boron on hardenability results from a segregation of boron atoms at grain boundaries which lowers the strain energy in these regions and thereby retards the formation of high temperature transformation products I-3]. Nevertheless, boron steels tend to retain a reputation for inconsistency and there is still a lack of knowledge about the mechanisms by which boron imparts these remarkable improvements in mechanical properties of steels.In general, the boron treatment of hardenable steels is restricted to compositions containing up to about 0.4~o C because it was shown that as the carbon content increases the effect of boron is reduced and above certain critical levels of carbon, boron has a detrimental effect on hardenability [4]. Accordingly, very few studies have been carried out concerning high-carbon steels.The present investigation was initiated on the basis that a knowledge of the behaviour of boron in high-carbon steels might provide interesting information for understanding the mechanism by which minute percentages of boron improve the hardenability of steels and more generally to clarify various aspects of the boron

Natural or forced fluid flow of the bulk melt can induce several changes in the solidification history of a casting : it may change the heat transfer conditions, the as-cast grain distribution, and the segregations. On the other hand, solidification can result in intense fluid flow due to natural solutal convection and in severe local segregations. The two following practical cases illustrate the variety and complexity of interactions between the fluid flow and the solidification of alloys : the effects of the stirring of the bulk liquid on the formation of the equiaxed zone during the continuous casting of steels, the formation of freckles and related segregations during the directional solidification of nickel-base alloys. Informations are given about, either new experimental observations, or updated physical and numerical models related to each case chosen here as examples. In the case of the influence of stirring on the formation of the equiaxed zone, emphasis is put on the importance of the interaction of the fluid flow with the dendrites in the coIumnar zone and in the stirred region itself. In the case of the freckles, evidence is brought forward that the dendritic nature and structure of the mushy zone are not the only causes of the dependence of the freckling on the heat transfer conditions during directional solidification.