Emergency Repair of an EN-GJS-800-2 Ball Mill Ring Gear: An Industrially Validated Framework Integrating Dissimilar Welding, Thermal Distortion Control, and Vibration-Based Operational Validation

The unplanned failure of the drive ring gear of a horizontal ball mill is one of the most severe unscheduled shutdowns a cement plant can face, since replacement lead times of 9–18 months make on-site repair the only viable option for maintaining production continuity. This paper reports an industrially validated emergency repair framework applied to a two-piece EN-GJS-800-2 nodular cast-iron ring gear (6531.50 mm pitch diameter, 216 teeth) affected by multiple cracks, including a 750 mm through-crack. Because the material combines high strength with markedly low ductility, five shielded metal arc welding (SMAW) procedure specifications (WPS 1–WPS 5) were developed and evaluated sequentially in the field over approximately thirteen months (December 2024–January 2026) while progressively varying preheating temperature (150–600 °C), joint geometry, filler metal chemistry, and mechanical restraint strategy. WPS 1 and WPS 2 were limited by, respectively, transverse post-weld cracking under the lower-preheat condition and severe geometric distortion (up to −2.8 mm radial run-out) from excessive restraint at 600 °C. WPS 3 and WPS 4 progressively introduced a root land, dissimilar ASTM A36 reinforcement plates, and post-weld heat treatment. WPS 5, a hybrid procedure combining nickel buttering (ENi-CI), a nickel–iron dissimilar filler (ENiFe-CI), and controlled interpass peening, produced no reported surface-breaking crack indications on post-weld liquid penetrant testing (PT) and preserved the corrected gear concentricity. During the distortion period, broadband root mean square (RMS) vibration velocity at the main bearing housings was reduced from 20 mm/s to 7 mm/s by temporarily lowering the ball-charge filling degree from 37% to 29% and production from 90 to 55 t/h. Following WPS 5, the repaired ring gear operated continuously for approximately three months, from October 2025 until the replacement gear was installed in January 2026. The results provide field-scale evidence, rarely documented in the open literature, on how welding procedure variables, geometric distortion, and operational vibration interact during the emergency repair of large gear components incorporating dissimilar materials.