Implications of FCC and HCP cobalt phases on wear performance of weld deposited cobalt-based coating

Abstract

SS316L possesses comparatively low wear and abrasion resistance properties and thus needs proper surface modification to sustain under severe industrial applications. Cobalt-based Stellite 6, a suitable coating for such applications, contains 4–5 % of tungsten that provides solid solution strengthening to the cobalt-based matrix, due to which it shows good wear resistance in abnormal working conditions. Stellite 6 is dominated by face-centered cubic (FCC) and hexagonal close packed (HCP) cobalt along with different carbide phases such as M7C3, M23C6, MC and M6C where M is Cr, W, Mo, Co and Fe. Weld deposited cobalt-based Stellite 6 coating on SS316L substrate, contains FCC structure as primary phases. Pure cobalt contains a stable HCP phase at temperature below 417 °C and FCC phase at and above 417 °C up to melting point of pure cobalt i.e., 1495 °C. Under the frictional stresses caused during wear test, impact more on FCC cobalt than HCP cobalt due to the high slip system of FCC. Because of this, FCC cobalt underwent through high wear losses compared to HCP cobalt in cobalt-based coating. Present study revealed that, high wear losses are seen in samples where FCC cobalt phases are dominant. The observed mechanism by SEM shows abrasive wear in which material removal takes place by ploughing, micro cutting and indentation. The targeted industrial applications where such wear losses are supposed to happen includes slurry transportation applications such as pipes, sea engineering components, submersible pumps, control valves.