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Influence of boron on the surface microstructure of sintered AISI 316L austenitic stainless steel

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Boron is one of the most suitable element investigated with the aim to activate sintering of stainless steels powders, whose properties are strongly influenced by residual open porosity. Due to it high affinity for oxygen, boron reduces the surface oxide of the powders. Furthermore, boron is α-stabilizer element and it forms different types of borides (depending on the composition of the base material) which react with the matrix producing eutectic liquids at lower temperatures than those usually adopted for the sintering of stainless steel, what in consequences increases density [1÷7]. However there are many factors, beside chemical composition, which influence liquid phase sintering: particle size, green density, sintering atmosphere, temperature, time of isothermal holding and heating rate. Their combined effect makes it more difficult to envisage and control this process. For small components the control of the heating rate is sufficient to ensure uniform densification, but for complex components the densification variation are influenced first of all by the amount of liquid, which is related to the mutual solubility of the base and additive. As far as boron is concerned, depending on its concentration and heating rate, a transient or persistent liquid phase during sintering is induced and densification variation and microstructure are changed. During persistent liquid phase sintering, both primary and secondary rearrangement occur, which promote a considerable densification. Near full density can be easily obtained, but the effect on the mechanical properties is partially (sometimes significantly) reduced by the boride particles resulting from the solidification of the eutectic liquid, which remain as an almost continuous network surrounding the ferrous based grains. Whilst hardness and strength are increased, ductility and toughness are lower than it could be expected, since this network constitutes a low energy path[...]

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