Abstract
The tribological challenge of balancing sliding wear and corrosion resistance is of particular interest in the plastics industry during the extrusion of fluorine containing resins or plastics. Highly corrosive environments containing hydrofluoric acid can limit the selection of corrosion resistant alloys that are also appropriate for metal-on-metal sliding wear. It is well known that austenitic Ni-based alloys that are resistant to hydrofluoric acid are prone to galling. In-depth research on Co-based alloys in recent years has shown that these alloys are a good candidate for such applications. The sliding wear behaviour of two laser deposited Co-based weld overlays, commercially available Stellite 21 (Co-27Cr-5Mo-3Ni-0.3C) and a custom alloy, Apollo-Clad 4024 (Co-31Ni-18Cr-15Mo-1.6Nb-0.4W) are evaluated by self-mated pin-on-disk testing. Stellite 21 is designed to resist metal-on-metal sliding wear, and Apollo-Clad 4024 has significant Ni and Mo additions for improved corrosion resistance in the presence of hydrofluoric acid . The overlays are evaluated in the as-deposited condition, a mechanically treated condition, a chemically treated condition, and a combination of mechanically and chemically treated condition. The mechanical treatment elicits a strain induced martensitic transformation from metastable FCC to HCP in the near surface region. The chemical treatment consists of emersion in hydrofluoric acid for 50 hours to deposit in-service corrosion products on the wear test coupon surfaces prior to testing. For the combined treatment, the mechanical treatment is performed prior to the chemical treatment. Wear test results are evaluated to understand the role of corrosion products and near surface crystallographic structure on resulting tribological properties of the laser deposited cobalt-based overlays.