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 characterized in the as-cast conditions to have comparable parameters with the heat treated samples. Destabilization heat treatments were undertaken at temperatures of 825, 900 and 975ºC for 25 minutes; each sample was air cooled after this soaking time. Characterization was undertaken by optical and electronic microscopy, image analysis and EDS microanalysis; hardness and microhardness were also evaluated. It was found that the volume fraction of secondary carbides precipitated is always higher for the lowest destabilization temperature (825ºC) 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 destabilization temperature was also noticed. For the iron containing boron, a density of 23 carbide particles per square micron was measured when destabilized at 825ºC, and it decreased to about 10 particles per square micron when destabilized at 975ºC. In the case of the alloy without boron additions, about 10 carbides per squared micron were counted when destabilized at 825ºC and about 5 when destabilized at 975ºC. Higher volumes of carbide precipitation implies higher values of bulk hardness and microhardness in the alloys. The results suggest that boron works as nuclei 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.