What enables wind turbine gears to withstand enormous pressures while maintaining long-term reliability? Beyond precise material selection and heat treatment processes, surface enhancement technologies—particularly shot peening—play a crucial role in improving their service performance. This article examines how shot peening coverage affects the mechanical properties, residual stress, and microstructure of 18CrNiMo7-6 carburized steel, revealing the scientific principles and engineering practices behind this critical process.

18CrNiMo7-6 steel is a low-alloy carburizing steel renowned for its exceptional comprehensive properties, making it widely used in heavy-duty transmission components such as wind turbine gears, bushings, bearings, and cam followers. After carburizing and quenching, this steel achieves high core strength and toughness while attaining surface hardness up to HRC 58-62. This combination of surface hardening and core toughening allows it to endure high loads and impacts while maintaining excellent wear resistance.

To further enhance fatigue life and wear resistance, shot peening is commonly employed as a surface strengthening technique. This process bombards the metal surface with high-velocity pellets, inducing plastic deformation that improves material performance.

The study utilized 18CrNiMo7-6 steel with specified chemical composition. The material underwent carburizing, quenching, and tempering heat treatments to achieve the desired hardness and residual stress distribution. Post-treatment specimens showed surface hardness of approximately HRC 60, with lower core hardness to maintain adequate toughness.

Shot peening is a surface enhancement technique that uses high-speed projectile impacts to create plastic deformation on metal surfaces. By controlling parameters like peening intensity, pellet size, impact angle, and coverage, engineers can modify surface residual stress distribution to improve fatigue strength, wear resistance, and corrosion performance.

This research applied two different coverage levels (100% and 200%) to carburized steel specimens. Coverage refers to the degree of complete surface impact by pellets, with 200% coverage indicating the surface received double the pellet impacts, creating deeper plastic deformation layers and higher residual stresses.

• Hardness Testing: Vickers hardness tests (HV) measured surface hardness before and after peening. Each specimen underwent at least five measurements for statistical reliability.
• Residual Stress Measurement: X-ray diffraction analyzed surface residual stresses by measuring lattice spacing variations. Researchers measured axial and tangential stress components on 17 rollers, taking three measurements at 90° intervals around each roller's circumference.
• Microstructural Observation: Scanning electron microscopy (SEM) examined surface microstructure changes, revealing plastic deformation and grain refinement effects from peening.

Shot peening slightly increased surface hardness from 690 HV to 740 HV, with 200% coverage specimens showing higher values than 100% coverage samples. This demonstrates that greater coverage induces more plastic deformation, thereby enhancing surface hardness and wear resistance.

After heat treatment and grinding, roller surfaces exhibited compressive stress. Shot peening significantly increased this compressive stress, with 200% coverage specimens showing higher values than 100% coverage samples. The induced compressive stresses effectively inhibit crack initiation and propagation, substantially improving fatigue strength.

SEM observations revealed pronounced surface plastic deformation and grain refinement after peening. The process created a nanocrystalline or ultrafine grain layer with exceptional strength and toughness. Specimens with 200% coverage showed more extensive grain refinement than 100% coverage samples, indicating greater mechanical property enhancement.

This investigation into shot peening coverage effects on carburized 18CrNiMo7-6 steel rollers yielded several key findings:

• Shot peening moderately increases surface hardness (690 HV to 740 HV), with 200% coverage producing higher values than 100% coverage.
• The process significantly enhances surface compressive stress, with 200% coverage generating greater stress levels.
• Peening creates nanocrystalline surface layers, with 200% coverage inducing more extensive grain refinement.

The results confirm shot peening as an effective surface strengthening technique for carburized 18CrNiMo7-6 steel. Practical applications should select appropriate coverage levels based on operational requirements to optimize performance. Future research could explore additional parameters like peening intensity and pellet size to further refine this critical industrial process.