Characterisation of the wear and friction behaviour of laser deposited cobalt and iron-based hardfacing alloys for nuclear applications

Valves and pumps in nuclear plants operate in a particularly aggressive environment of a high neutron flux, high contact loads and in superheated water up to 300 °C. Traditionally there has only been one material that can survive these conditions: cobalt based hardfacing alloys called Stellites. There is an urgent need to replace cobalt-based alloys because of the formation of cobalt-60 radioisotope, which causes radioactive exposure. In this study an iron-based hardfacing alloy, Tristelle 5183 was compared with Stellite 6. The hardfacings were manufactured via laser cladding. Wear testing was performed using a pin-on-flat configuration, with like-on-like couples, from room temperature up to 300 °C with loads of 50–500 N. The friction coefficient for the Tristelle was higher (typically 0.45) than the Stellite (typically 0.35) across all temperatures and loads. The wear rate of the Tristelle was higher than the Stellite, but the exact difference varied with both load and temperature. The specific wear rate increased with temperature, but only gradually. The operative wear mechanism was a combination of adhesive and oxidational wear, but there were significant differences found between the two alloys. For the Tristelle, the oxide became mechanically mixed with the heavily deformed surface metal. In contrast, the oxide on the Stellite was detached from the surface without any mechanical mixing. The extent of surface deformation was much greater for the Tristelle than the Stellite. The large (5–20 mm) NbC in the Tristelle fractured a considerable distance (up to 40 mm) below the worn surface and became increasingly fine as the worn surface was approached as a result of high strain deformation in the matrix. In the Stellite no carbide fracture was observed. The reasons for the marked difference in behaviour between the two alloys, each with the same mechanical properties, are discussed in detail.