Investigations on the thermal stability of microstructures generated by different thermal electron beam surface treatments

Surface treatments are frequently used to improve the wear and/or corrosion resistance of metal components. In the case of cast iron, the material‐specific graphite limits both its treat‐ability and load‐bearing behaviour. A promising option for overcoming these limitations is provided by combination processes, in which near‐surface graphite is first removed in an initial liquid‐phase surface treatment – such as, e. g., remelting, alloying or cladding using electron beam (EB) – before application of thermochemical processes or hard coatings. A prerequisite for this is sufficient thermal resistance of these microstructures. This was investigated by means of annealing tests. The ranges of temperature used for annealing are based on those typically used for hard coating (250 °C–500 °C), nitriding (400 °C–600 °C) and boriding (600 °C–860 °C). The metastable microstructures produced as a result of rapid solidification during the electron beam liquid‐phase treatments differ in their alloy content and, therefore, in their microstructural components. Hardness measurements after annealing provided an initial indication of thermal stability. Based on these measurements, interesting treatment conditions were analysed in more detail using scanning electron microscopy and x‐ray diffraction. The focus of interest was on the formation of secondary graphite and the dissolution of ledeburitic carbides and other intermetallic phases. Electron beam surface treatments are excellently suited to increase the surface hardness of cast iron by a factor of 2–4, which provide a good basis for improved support of thinner layers, as produced during coating, nitriding or boriding. The variants of the combination treatments possible depend on the thermal resistance of the unbalanced microstructural constituents after electron beam surface treatments.