Some aspects of the occurrence of boron in alloy steels

Abstract

High-resolution boron autoradiography and a literature survey have enabled an assessment to be made of the location of the small amounts of boron (0.002–0.005%) added to alloy steels. It has been shown that boron readily segregates to fcc carbides to give boro-carbides such as M₂₃(BC)₆ and V₄(BC)₃, and that metal borides are not formed in steels with this level of boron addition. Boron also segregates to austenite grain boundaries on slow cooling low-alloy steels below 980°C. Grain-boundary segregation thus occurs in ½%Mo–B bainitic steels as predicted by some harden ability theories. However, quenched and tempered Cr-Mo-V creep-resisting steels do not have boron at the austenite boundaries and the boron does not segregate to the boundaries even after creep testing for > 16 000 h at 550°C. The observed microstructural effects have been related to mechanical properties and interpreted in terms of vacancy–boron interactions.