Microstructural evolution and functional performance of microwave-synthesised stellite-6 claddings on SS-316L alloy

In the present study, a cobalt-based alloy (Stellite-6, S-6) was deposited on low-carbon austenite steel (SS-316L) (base metal, BM) using a hybrid microwave heating (MH) process to improve its surface performance. The clad layers were characterized through X-ray diffraction (XRD), scanning electron microscope (SEM) coupled with energy dispersive spectroscopy (EDS) and microhardness testing. XRD study revealed dominant presence of Co (fcc), eutectic-based carbides (Cr 23 C 6 , (Fe, Cr) 7 C 3 , Cr 7 Mn 7 C 6 ), solid solution of CoCr, Ni-Fe-Cr, along with minor presence of Co (hcp) phases. The microstructural result confirmed a metallurgically sound interface between clad and substrate layer, free from pores and cracks. A distinct microstructural gradient was observed in the clads, including a planar zone at the BM clad interface, a columnar dendrite zone away from the planar zone and equiaxial dendrite structure near the surface of the clads. The formation of hard phases within S-6 deposit leads to significant increase in its microhardness value compared to the BM. The maximum microhardness of clad zone was found as 510 ± 20 HV at top of the clad area, roughly 2.5 times that of the substrate. The mechanical performance of the S-6 deposit was evaluated through a 3-point bend test, which yielded an average flexural strength of 231.5 ± 6 MPa accompanied by a deformation of 0.23 mm. The fractography of the tested S-6 deposit showed a mixed mode of failure of the fractured specimen.