Physics-Guided and Sustainability-Oriented Design of Carburized Steel Cases by Integrating Thermodynamic Kinetic Coupling with Alloy-Controlled Carbon Diffusion

Gas carburization is widely used to improve the surface durability of load-bearing steels, yet alloy-dependent thermodynamic and kinetic effects often cause large variability in case development and component life. This study presents a physics-guided comparison of carburization behavior in EN3, 20MnCr5, and EN353 steels by integrating hardness profiles, mechanical and wear testing, and thermodynamic–diffusion analysis. Under an identical boost–diffuse–equalize cycle at 930 °C, distinct surface carbon levels of 0.764 wt.% (EN3), 0.792 wt.% (20MnCr5), and 0.822 wt.% (EN353) were obtained. These corresponded to effective case depths of ~1 mm in EN3 and ~2 mm in 20MnCr5 and EN353. Near-surface hardness reached ~12–13 HRC in EN3, ~33 HRC in 20MnCr5, and ~35–36 HRC in EN353. Ultimate tensile strength increased from ~620 MPa (EN3) to ~870 MPa (EN353), while wear mass loss decreased from ~28 mg to ~21 mg. Thermodynamic interpretation showed alloy-dependent moderation of carbon activity and sustained chemical-potential gradients, promoting deeper diffusion. Reconstructed carbon profiles confirmed diffusion-controlled case growth. By enabling longer service life and reduced material replacement, the results support durability-oriented and resource-efficient surface engineering for engineering steels.