Electron microscopy of carbides in annealed 28 wt% Cr - 1 wt% (Mo/W) cast irons

Phase identification and the microstructure of carbide phases present in annealed 28 wt% Cr-2.6 wt% C high chromium cast irons with the addition of Mo or W have been characterized using optical microscopy, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. Electron energy-loss spectroscopy was also used to identify M7C3 and M23C6 carbides in these irons. Annealing was performed by holding at the temperature of 800 °C for 4 h, followed by slow cooling at 20 °C/s to 500 °C, and then furnace cooling. Multiple eutectic carbides including, M23C6 and M6C were observed in the as-cast iron containing 1.42 wt% Mo. In contrast, the addition of 0.99 wt% W led to the change from hypoeutectic to hypereutectic iron and only M7C3 carbide was found in the as-cast condition. The post-annealing microstructures of all irons contained eutectic carbides, coarse secondary carbides next to the eutectic carbides, fine secondary carbide precipitated within the ferrite matrix, and some lamellar pearlite at the central areas of prior austenite dendrites. The results of TEM observation via selected-area electron diffraction and energy-dispersive X-ray spectrometry confirmed that the coarse and fine secondary carbides and lamellar pearlitic carbides, formed during the annealing treatment, were M23C6 carbide. In addition, the fine structures in C K-edge spectra were found to be useful for distinguishing between M7C3 and M23C6 carbides.