Investigation of M6C and M7C3 carbides in SiMo ductile cast iron alloyed with Al and Cr

The SiMo ductile cast irons (DCI) present usable plasticity and retain significant strength at high temperatures (HT). However, an austenite to ferrite transition at similar to 900 degrees C resulting in volume change is one of the main reasons of degradation of exhaust manifolds and alike made of these materials. Alloying SiMo DCIs with Cr and Al allows to rise this temperature, but its effect on carbides deciding on hardness of these materials has not been thoroughly studied as yet. The present experiments were aimed at assessment of an effect of Cr (up to 0.8 wt%) or Cr and Al (up to 1 and 1.8 wt%, respectively) additions into SiMo5-1 on the microstructure, phase and chemical composition of the eutectic carbides. The light, scanning and transmission electron microscopy (LM, SEM, TEM) including electron diffraction (ED) and X-ray energy dispersive spectroscopy (EDS) methods were applied. The investigation helped to establish that in both alloys the nucleation of dendrite-shaped M6C has to take place within the liquid in the so called intercellular areas. The start of crystallization of the austenite abruptly rises the Mo concentration in the remaining liquid, what causes that the outer parts of the growing M6C also undergo an abrupt enrichment in this element. In the SiMo DCI alloyed with higher amount of Cr, i.e. 1.0 wt% and 1.9 wt% Al, the precipitation of Mo rich M6C was accompanied by co-precipitation of Cr rich M7C3 carbides. The ratio of alloying elements in the M6C carbides was found precisely defined in each alloy, i.e. at (Fe2.5,Mo2,Si0.5)C and to (Fe2.5,Mo1.5,Si0.5Cr0.5)C for SiMo DCI alloyed only with Cr and both with Cr and Al, respectively. It means, that the Cr is capable to partially substitute Mo in M6C carbides provided that its concentration in these alloys approaches 1 at%.