Behavior of Niobium Carbides and Nitrides in Steel

Fundamental study on niobium-added steel-I

Abstract

High purity Fe-Nb-C, Fe-Nb-N and Fe-Nb-C-N alloys containing niobium up to 033% were vacuum-melted and cast into 2-5 kg ingots. The metallographic structure was investigated with these specimens which had been prepared from the samples by quenching in water after long-time holding at respective temperatures of 800 to 1250°C. At the same time, the precipitants were isolated from the3e specimens by 6N-HC1 solution an 1 identified by X-ray diffraction method.The quantitative determination of insoluble niobium was also investigated. Results obtained are as follows:
1) The precipitation of niobium-carbonitrides or -carbides is recognized predominantly on austenite grain boundaries as finely dispersed particles, while the niobium-nitrides precipitate as comparatively large particles.
2) The precipitants of niobium-carbides and-carbonitrides in these samples were identified to be f.c.c.δ5-phases with somewhat different lattice constants.
3) Two types of hexagonal niobium-nitrides were recognized and identified to be ε-and δ'-phases, the ratio of ε- to δ'-phase seemed to be increased as the ratio of N to Nb in sample was increased.
4) These precipitants can be quantitatively isolated from the iron matrix by dissolving the chips in 6N-HCl solution at room temperature.
5) It is concluded from the results of analysis for insoluble niobium content in each specimen that the precipitation of niobium-carbides, -nitrides and-carbonitrides begins at high temperature such as 1200°C and is almost completed at about 900°C in these alloys.