<p>This study compares the effects of two surface strengthening technologies (ionitriding and Quench-Polish-Quench, QPQ) on the properties of 42CrMo4 steel gears and identifies significant differences in the microstructures and properties of their modified layers. The ionitrided surface layer is dominated by <i>γ</i>-Fe₄N and <i>α</i>-Fe phases, with nitrogen atoms mostly in a solid solution state and no obvious bright white layers. The QPQ-treated 13.2&#xa0;μm-thick composite layer is composed of Fe₃O₄ and <i>ζ</i>-Fe₂N phases and divided into a top nitride/carbide zone, a middle diffusion zone, and a bottom ferrite zone. In terms of performance, QPQ treatment is a superior technology. The wear rate (0.8 × 10⁻¹⁵&#xa0;μm³/(N&#xa0;m)) of the QPQ treatment is only one-fifth that of ionitriding, with a lower friction coefficient (0.15–0.18) and a narrower wear scar width (127&#xa0;μm). For corrosion resistance, the self-corrosion current density of the QPQ treatment (5.27 × 10⁻⁸&#xa0;A/cm², with a passivation zone) is superior to that of ionitriding (3.54 × 10⁻⁶&#xa0;A/cm², no passivation). Additionally, the impact absorption energy of the QPQ treatment (102 J) is greater than that of ionitriding (96 J), which can be attributed to the lower brittleness and residual compressive stress of the QPQ layer. However, ionitriding is advantageous in terms of flexibility for local nitriding and compatibility with special materials, including stainless steel. In conclusion, QPQ is suitable for harsh scenarios demanding high wear resistance, corrosion resistance, and impact toughness, whereas ionitriding is more appropriate for local strengthening needs. Future combinations of multiple technologies are expected to enhance the performance of 42CrMo4 steel gears under extreme working conditions.</p>