Formation and characterisations of S phase in plasma carburised high carbon Stellite 21 CoCr alloy

Our recent work has revealed that a hardened layer (H approximate to 13 GPa, E approximate to 230 GPa) with combined improvements in wear and corrosion resistance can be formed on CoCr alloys during plasma surface alloying with carbon. However, the formation mechanism cannot be explained based on current S phase theory for austenitic stainless steels as CoCr alloys normally have a dual phase (fcc and hcp) structure at room temperature. Hence, the present research is directed at advancing scientific understanding of the hardened S phase layers formed in high carbon cast Stellite 21 CoCr alloy. Detailed characterisation has been conducted by glow discharge spectrometry, XRD, SEM/EDX, TEM and nano-indentation. Results have revealed that while the as received Stellite 21 alloy is mainly composed of fcc and hcp phases, the extremely hard surface layer formed on the CoCr alloy during low temperature plasma carburising consists of a carbon supersaturated fcc structured phase, i.e. an S phase, with numerous entangled dislocations and stacking faults. The formation of the S phase could be attributed to the high capability of carbon in stabilising the gamma phase and the paraequilibrium nature of the low temperature plasma carburising processes.