Increasing the Reuse Potential of High‐Strength Steels by Extending the Fatigue Life through Surface Treatments

To enhance the circularity of engineering steels, this study investigates a surface reconditioning approach to extend the fatigue life of damaged tempered martensitic 42CrMo4 steel. Fatigue performance is evaluated through a four‐phase methodology. In the first phase, specimens are cyclically loaded at 720 MPa and 5 Hz until 75% of their fatigue life is consumed, inducing high‐cycle fatigue damage characterized by surface microcracks and subsurface persistent slip bands (PSBs). In the second phase, fatigue damage is analyzed using scanning electron microscopy, magnetic Barkhausen noise, and cross‐sectional lamella extracted via plasma focused ion beam milling, revealing PSBs ≈5 μm below the surface. In the third phase, surface reconditioning through combined mechanical polishing and electropolishing is performed to remove about 100 μm of material and eliminate surface and near‐surface damage. In the final phase, reconditioned specimens are retested at 630 MPa and 1000 Hz. Results show a 12‐fold increase in fatigue life compared to non‐reconditioned specimens; however, reconditioned specimens retain only about one‐third of the fatigue life of pristine material. These findings demonstrate that surface reconditioning of fatigue‐damaged steel significantly enhances remaining fatigue life and enable safe reuse, particularly in applications where surface‐initiated damage governs failure.