First results of a new duplex surface treatment for cast iron: Electron beam remelting and plasma nitriding

Cast irons exhibit very good machinability but very low resistance against wear and corrosion. Furthermore, nitriding of cast irons does not produce satisfactory results. This paper suggests a new duplex surface technology for elimination of these disadvantages. In an initial processing step, the surface of cast iron material was remelted by electron beam (EB) up to a certain depth (0.3 to 1.0 mm). The graphite in this layer is dissolved and, as a result of the rapid solidification process, a very fine ledeburite and interdendritic martensite or perlite forms. This microstructure is nitrided in the second step, and a dense, closed nitride compound layer is produced. The paper will present current results concerning electron beam remelting of different cast irons (GJL-250, GJS-600, GJV-300) followed by plasma nitriding. The experimental results were characterised by visual methods, microstructural investigations, and hardness measurements. As a result of the EB surface remelting, graphite can be completely eliminated from the surface area, and nitrideability was also significantly improved. Unlike the untreated base material, the remelted surface is fully covered by the compound layer. According to processing parameters and chemical composition of the base materials, compound layer thicknesses varied from 8 μm to 14 μm. Surface hardness improved from ~ 200 HV 0.3 (as-cast) to ~ 850 HV0.3 after duplex treatment without preheating, and to 650 HV0.3 with preheating. A considerable improvement in wear and corrosion resistance is expected.