Contact fatigue behavior of plasma-nitrided carbide-free bainitic gears produced by controlled forging: a process-energy-efficient alternative to carburized ferritic-pearlitic gears

This study presents an integrated experimental, analytical, and numerical investigation of the contact fatigue behavior of forged gears produced through an alternative, process-energy-efficient manufacturing route combining controlled forging, continuous cooling, and plasma nitriding. Plasma-nitrided DIN 18MnCrSiMo6-4 carbide-free bainitic gears were compared with conventionally carburized DIN 20MnCr5 ferritic–pearlitic gears using FZG back-to-back contact fatigue tests. Pitting damage evolution was quantified by image analysis and statistically assessed using Weibull distributions, while subsurface stress fields were investigated through Hertzian contact analysis and finite simulations. The plasma-nitrided carbide-free bainitic gears exhibited approximately 75% longer pitting fatigue life than the carburized gears at a 95% confidence level. Despite the significantly thinner hardened layer, plasma nitriding produced higher surface hardness and compressive residual stresses—approximately 27% and 45% greater, respectively. Numerical results revealed that the nitrided diffusion zone closely matches the depth of maximum cyclic shear stress, effectively delaying subsurface crack initiation. In addition, the proposed manufacturing route reduced total energy consumption by approximately 45% and overall production time by about 10% compared with the conventional carburizing-based route. These findings demonstrate that aligning hardened layer characteristics with subsurface stress distributions, rather than maximizing case depth alone, enables plasma-nitrided carbide-free bainitic gears to outperform conventional carburized gears in contact fatigue while offering a more energy-efficient manufacturing alternative.