Effect of alloy modification on the wear protection coatings made of Ni- and Co-based materials and surface machinability via ultrasonic milling process

Wear-resistant coatings are required for highly efficient and economical steel components in equipment, process, and power plant engineering to withstand high corrosive, tribological, thermal, and mechanical stresses. Co-alloys are used as wear-resistant coatings for steel components, tailored to the specific application. The substitutability of Co alloys with Ni-based wear protection systems, in addition to price and supply uncertainties, is facilitated by the combination of innovative welding and machining processes such as ultrasonic-assisted milling. The aim of the study is to improve the machinability of two different hard-facing alloys while maintaining the same wear protection potential. Therefore, the wear-resistant alloys NiMoCrSi (Colmonoy C56) and CoMnCrSi (Tribaloy T400) were modified by the alloying additions Nb, Hf, and Ti and then applied to a carbon-manganese steel S355 using the plasma transfer arc (PTA) welding process. The influence of the alloying additions on the microstructure as well as on the formation of the hard phases of the build-up welds is compared. For example, the inclusion of the alloying element Nb results in the formation of a more refined hard phase and reduces the machining force required for C56 and T400. In most cases, the wear resistance potential has been maintained. In order to improve the machinability of the hard facings, the optimization of the demanding machining conditions by alloy modifications of the Co- and Ni-based alloys is also presented. It is shown that some of the modified alloys have significantly better machinability than the conventional alloy.