Boron effect on the precipitation of secondary carbides during destabilization of a high-chromium white iron

Abstract

This work analyses the secondary carbides precipitation during the destabilization of a 17% Cr White iron containing 195 ppm boron. The experimental iron was characterised in the as-cast conditions to have comparable parameters with the heat-treated samples. Destabilisation heat treatments were undertaken at temperatures of 825, 900 and 975 °C for 25 min; each sample was air-cooled after this soaking time. Characterisation was undertaken by optical and electronic microscopy, image analysis and energy dispersive spectroscopy microanalysis, hardness and microhardness were also evaluated. It was found that the volume fraction of secondary carbides precipitated is always higher for the lowest destabilisation temperature (825 °C) due to the lower carbon solubility in austenite at low temperatures. A much higher precipitation for the irons containing boron than that for the iron without boron at any destabilisation temperature was also noticed. For the iron containing boron, a density of 23 carbide particles per square micron was measured when destabilised at 825 °C, and it decreased to about 10 particles per square micron when destabilised at 975 °C. In the case of the alloy without boron additions, about 10 carbides per square micron were counted when destabilised at 825 °C and about 5 when destabilised at 975 °C. Higher volumes of carbides precipitation implies higher values of bulk hardness and microhardness in the alloys. The results suggest that boron works as a nucleant for the precipitation of secondary carbides; this is discussed in terms of the limited solubility of boron in iron and the formation of boron-rich precipitates found in the iron in the as-cast conditions.