Influence of B and Cu on microstructure and eutectoid transformation kinetics in spheroidal graphite cast iron

The influences of boron (B) and copper (Cu) on the morphology of the graphite nodules and fraction of ferrite in spheroidal graphite iron (SGI) have been studied. It was observed that B promotes ferrite in a fully pearlitic SGI alloyed with 0.5 wt% Cu and 0.7 wt% Mn. For this particular alloy and casting shape the ferrite promoting effect of B saturated at approximately 20 ppm, resulting in a ferrite fraction of 20%. The carbon rejected from the matrix during the ferrite growth was deposited on the graphite nodules as sharp graphite protrusions. For an alloy with a fixed concentration of B (25 ppm), a fully pearlitic matrix could not be obtained by increasing the concentration of Cu to 1.2 wt%. Transmission electron microscopy (TEM) showed that a nanometer thick layer of Cu is present at the graphite-ferrite interface. B is detected in close proximity to the graphite-iron interface by using microprobe analysis. In-situ synchrotron X-ray experiments were conducted to analyze the eutectoid transformation kinetics. It shows that both B and Cu retards the eutectoid phase transformation kinetics when they are added individually. However, B accelerates the kinetics in alloys where Cu is present.