Formation of carbonitride precipitates in hardfacing alloy with niobium addition

Abstract

Niobium, as the most effective second-phase forming element, was added in the Fe–Cr13–C–N hardfacing alloy to get carbonitride precipitates. Morphology and composition of carbonitride in the hardfacing alloy were studied by optical microscopy, scanning electron microscopy, and electron probe microanalyzer. The thermodynamics and the effect on the matrix of the formation of carbonitride were also discussed. It was found that niobium carbonitrides are complex Nb(C, N) precipitate distributed on grain boundary and matrix of the hardfacing alloy. Under as-welded condition, primary carbonitride particles were readily precipitated from the hardfacing alloy with large size and morphology as they were formed already during solidification. Under heat treatment condition, a large number of secondary carbonitrides can precipitate out with very fine size and make a great secondary hardening effect on the matrix. As a result, addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase on grain boundaries and intergranular chromium depletion. The Fe–Cr13–C–N hardfacing alloy with niobium addition has microstructure in which hard, fine niobium carbonitrides are homogeneously distributed in the matrix, and thus make a great secondary hardening effect on the matrix. And addition of niobium in the hardfacing alloy can prevent the formation of chromium-rich phase in grain boundaries and intergranular chromium depletion.