Avoiding or Promoting Graphite in Carbon-Rich Chromium-Containing CoNiFer Cast Alloys—Part 2: Microstructures of the Elaborated Alloys

Nine M-xCr-yC ternary alloys, three cobalt based, three nickel based and three iron based, were elaborated by foundry, from chemical compositions previously selected by the mean of thermodynamic calculations. They were metallographically characterized, using electron microscopy, image analysis, and X-ray diffraction. The as-cast microstructures are in rather good agreement with the ones predicted at 500 and/or 600°C, despite that the elaboration conditions did not meet any thermodynamic equilibrium criteria. Indeed, the obtained carbides and graphite fractions were close to the calculated ones, and the new chromium contents previously chosen effectively led to the expected microstructure modifications, notably almost total suppression of graphite in the nickel alloys and obtaining large fractions of carbides in the cobalt alloys. This allowed specifying the hardness evolution resulting, for these alloys, from the presence or absence of the soft graphite phase, and from the lowering or the enhancement of the carbides presence.