42CrMo is an excellent substrate for surface hardening due to its hardenable core.
Induction Hardening or Flame Hardening: The surface is rapidly heated and quenched, creating a hard martensitic case (55-60 HRC) while the core remains tough. This is ideal for wear surfaces like gear teeth or bearing journals. The core must be in the quenched & tempered state first to support the hard case.
Nitriding (Gas or Plasma): Diffuses nitrogen into the surface at ~500°C, forming a very hard, wear-resistant compound layer and diffusion zone with high surface compressive stresses. It improves fatigue strength and wear/scuffing resistance with minimal distortion. The core must be in the +QT state prior to nitriding.
Carburizing: Not typically used for 42CrMo, as it already has sufficient carbon. Used more for low-carbon steels.