Study on W-rich M3B2 borides in a 9Cr3W3CoB heat-resistant steel

Abstract

In a 9Cr3W3CoB heat-resistant steel with 150 ppm B, the evolution of W-rich M3B2 type borides during aging at temperatures of 650, 700, 750 °C for different times was investigated. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA) and secondary ion mass spectroscopy (SIMS) were employed to characterize the morphology evolution of the W-rich M3B2 type borides. Most of B was discovered to be present in M3B2-type (M = Fe, W, Cr, V and Nb) borides which were unevenly distributed in the matrix and could not be eliminated by normal heat-treatment. However, these borides could dissolve during prolonged isothermal exposure, where the dissolution process was accelerated by increasing aging temperature. It is indicated that the W-rich M3B2 type borides are thermodynamically metastable at high temperature. Their temporary existence postpones the nucleation and growth of Laves phase and prevents B from segregating in M23C6 carbides. W-rich M3B2 type borides are not responsible for cleavage fracture initiation when they are dissolved after aging at high temperatures for 2000 h.