The Role of Boron in Low Copper Spheroidal Graphite Irons

The effects of boron at concentrations ranging from 5 to 525 ppm in low copper spheroidal graphite iron (SGI) has been studied. At 130 to 140 ppm, no particular effect of boron was observed on the size distributions, number densities, or morphologies of the microparticle populations in the material. Neither was there observed any effects on the size distributions or number densities of graphite nodules. However, boron was observed to lead to a rough surface morphology of the graphite nodules at concentrations as small as 24 ppm. Intercellular carbides were found to form in alloys containing more than 70 ppm boron. Additionally, the graphite shape began to degenerate in alloys with more than 300 ppm of boron. Mass spectrometry analyses revealed these carbides contain relatively high amounts of boron. In an alloy containing 74 ppm boron, it was inferred by using electron backscatter diffraction that these were of the type M-23(C,B)(6) borocarbides, where M = Fe, Mn, V, or a combination of them. Mass spectrometry analyses also revealed elevated concentrations of boron in the surface layers of the graphite nodules.