Many years ago a postdoc from Chemistry? Material Science? had some powders he'd synthesized and wanted to do epma on them and I gave him to standard gospel that we can't do epma on powders...but he was clever and knew more than I did... the trick is to have a small (cm diameter I think he used) press with the bottom surface with a mirror polish on it. Given that the grains were small enough, it worked. The reflected light image, while not perfect, showed a smooth, reflective surface, close enough for what he needed. So if the cementite grains are small enough, you might be able to "make do". (I don't now recall what materials the postdoc was mounting and they mave have been soft or ductile enough to flow; not sure what material properties of cementite are....)