Effects of Niobium on Microstructures and Hardness of 15Cr-14Ni Heat Resisting Steels

Abstract

The effects of niobium on the microstructures and hardness of 0.15C-15Cr-14Ni heat resisting steels, after solution treatment and isothermal aging, have been studied by means of optical and electron microscopy, electrolytic isolation technique, X-ray diffraction method, and Vickers hardness testing. Theresults are as follows:
The carbon solubility for the Nb free steel, and the solubility products of niobium carbide for the 1.03%Nb steel and the 2.02%Nb steel are determined by the electrolytic isolation method. The undissolvedcarbides, mainly NbC and M₂₃C₆, tend to become coarse with the increase of solution temperature or withthe increase of the niobium content. The distribution density of undissolved particle has a maximum at1.0%Nb, and decreases with the increase of the solution temperature. The distribution density of undissolvedcarbide affects the hardness and the austenite grain size of the steels solution treated. On agingthe steels with niobium less than 1.03% in the range 600°C to 750°C, massive, cubic and libbon-like M₂₃C₆, and thread-like NbC precipitate. In the 2.02%Nb steel, only thread-like NbC precipitates. From theresults of the microstructure observation and hardness testing, we find that there is a direct relationshipbetween the distribution density of cubic M₂₃C₆ and the increase in hardness. The measurements of thechange in the diameter of cubic M₂₃C₆ and the change in the width of thread-like NbC during isothernalaging show that the coarsening process of cubic M₂₃C₆ is controlled by the diffusion of chromium in matrix, and the coarsening process of thread-like NbC is controlled by the diffusion of niobium.