Abstract: Process for joining by diffusion welding a part made of steel having a high carbon content and low carbide-forming elements content with a part made of steel or of nickel alloy having a low carbon content and a high carbide-forming elements content, each of the parts comprising a surface to be joined in which process an intermediate material is placed between the surfaces to be joined, then diffusion welding is carried out to join the two parts, and the assembly obtained is cooled, characterised in that the intermediate material is an alloy, with a matrix made of nickel and optionally of iron and/or cobalt, having an austenitic micro-structure at the welding temperature, and comprising 2 to 25% by mass of molybdenum. Also disclosed is an assembly obtained by this process.