Effects of chromium carbide volume fraction on high-stress abrasion performance of NbC-bearing high chromium white cast irons

Abstract

The addition of NbC to high chromium white cast irons (WCIs) has potential to improve component wear lives in mining and ore beneficiation environments, due to the combined presence of Cr-rich and niobium carbides. While the effect of chromium carbide volume fraction (Cr-CVF) on abrasion performance of plain-Cr WCIs is well understood, its effect on NbC-bearing WCIs is unknown. This work studies the effect of Cr-CVF on high-stress abrasion of this newer class of dual-reinforced high-Cr WCIs, using the ball mill abrasion test with quartzite and basalt. The alloys cover a wide range of Cr-CVF, from M7C3-free NbC-reinforced steel to hyper-eutectic WCI. Bulk hardness increased linearly with Cr-CVF, at a rate of 3.9 HV per 1% Cr-CVF. In basalt, abrasion resistance increased linearly with Cr-CVF, at an average life improvement rate of 24% per 1% Cr-CVF. This confirms that performance is maximized by increasing Cr-CVF even in the presence of substantial volumes of cubic carbides. In quartzite the trend is again linear up to 30 vol% M7C3 (3.7% life increase per 1% Cr-CVF), but the hyper-eutectic alloy shows poorer resistance than slightly hypo-eutectic alloy. SEM studies of the worn surfaces demonstrate that micro-fracture of the hard carbides was the controlling damage mechanism. Primary M7C3 was observed to suffer fragmentation in high-stress abrasion in quartzite, confirming the expectation from the quantitative trend.