Effects of Isothermal Aging Process on Microstructure and Mechanical Properties of Stellite 6 Coatings by Plasma Arc Cladding

Abstract

Stellite 6 is widely used in high-temperature environments; the evolution of its microstructure and mechanical properties during high-temperature exposure is essential for long-term service. In our work, Stellite 6 claddings were manufactured by plasma arc cladding, followed by a long-term isothermal aging process at 700 °C to imitate high-temperature service. The microstructures were characterized using a scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS) and electron backscattered diffraction (EBSD). It was observed that the microstructure of the as-clad coating mainly consisted of γ-Co (cobalt) and carbide eutectics. During isothermal aging at 700 °C, the γ-Co was found to transform to ε-Co. Moreover, fine M₂₃C₆ particles were observed to precipitate along the stacking faults of ε-Co. The eutectic carbides also experienced remarkable change; M₂₃C₆ grew along the edge of the M₇C₃ block, indicating the decomposition of M₇C₃, which might result in the dispersal of eutectic after aging for 500 hours. The microhardness and wear tests indicated that the aging process was beneficial in enhancing the mechanical properties both at room temperature (RT) and 700 °C. According to the morphology of the worn surface, the enhancement of wear performance mainly resulted from the restriction effect of dispersal eutectic carbides and timely removal of oxide. It was also found that the wear mode gradually transformed from adhesive wear to abrasive wear with aging time, which mainly resulted from the increase in the amount of precipitations cut from the matrix, and they acted as the friction pair during the wear test.