<p>This study examines the structure and hardness of a layer (up to 9 mm thick) deposited by plasma transferred arc (PTA) surfacing of molybdenum high-speed steel (HSS) non-conducting cored wire onto 30KhGSA steel in a nitrogen atmosphere. A skeletal structure was observed in the deposited layer. A single high-temperature tempering of the surfacing sample with the substrate was shown to increase the microhardness of the deposited layer to 5.96 GPa, which is approximately twice the hardness of the substrate (30KhGSA steel). Microhardness remains nearly uniform across the surfacing thickness. Double tempering led to a further increase in the microhardness of the surface layer (100 μm thick) to 7.1 GPa. In this case, microhardness gradually decreases with depth, approaching values observed after single tempering. The formation of a hardened surface layer is attributed to its enrichment with carbon and oxygen atoms during surfacing, followed by the precipitation of oxycarbide particles during tempering.</p>