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Differential quenching of the whole one-piece wheel is shown to be more effective than quenching of only the contact surface at the wheel rim. Hybrid strengthening by differential bulk quenching and surface plasma hardening is very promising in increasing the crack resistance of steel. The martensitic structure of the treated layer in hybrid strengthening is very disperse, thanks to decrease in size of the initial austenite grain. In bulk-quenched components, surface plasma strengthening creates a composite working layer characterized by high wear and crack resistance, whereas the core is relatively soft and plastic. [ABSTRACT FROM AUTHOR]