Cladding Repair of a 1400 kW Motor Rotor Shaft

Literature Overview

This paper documents the cladding repair of a 1400 kW motor rotor shaft that suffered severe wear and surface damage at the bearing journal locations during operation in a power generation facility. The rotor shaft, made of 45 steel (equivalent to AISI 1045), exhibited a wear depth of 2.5 mm and surface cracks at the bearing seats, rendering it unsuitable for continued service without repair. The repair was performed using gas metal arc welding (GMAW) with a low-carbon nickel alloy wire, followed by machining and post-weld heat treatment. The case study provides valuable insights into the practical challenges of rotor shaft repair, including thermal management, dimensional accuracy, and metallurgical compatibility.

Damage Assessment and Repair Strategy

The rotor shaft was inspected using dye penetrant testing (PT) and ultrasonic testing (UT) to determine the extent of the surface cracks and subsurface defects. The PT revealed surface cracks up to 15 mm in length at the bearing journal, while the UT detected subsurface indications at depths of 3–5 mm. Based on the damage assessment, the repair strategy was to remove the damaged material by machining, apply a 4 mm thick cladding layer to restore the original dimensions with a 0.5 mm machining allowance, and then machine the surface to the specified tolerance.

The repair was performed on the rotor shaft in the horizontal position on a welding fixture that allowed rotation of the shaft for uniform heat distribution. The cladding was applied in three passes with a build-up of approximately 1.5 mm per pass, with the interpass temperature maintained below 250 °C.

Welding Parameters and Microstructure

The GMAW process was conducted using a nickel-based wire (ERNiCrMo-3, equivalent to Inconel 625) with a diameter of 1.2 mm. The welding parameters were current 180–200 A, voltage 22–25 V, and travel speed 10–12 cm/min, resulting in a heat input of 8–10 kJ/mm. The shielding gas was pure argon at a flow rate of 15 L/min. The microstructure of the cladding layer consists of austenitic dendrites with L12-type Ni3(Fe,Cr) precipitates, which provide good corrosion resistance and moderate hardness (260–290 HV). The interface between the cladding and the 45 steel substrate exhibits a diffusion zone of approximately 50–80 μm with a gradient in composition from the nickel-rich overlay to the iron-rich substrate.

The dilution ratio at the interface is approximately 25–35%, which is higher than ideal for pure alloy overlay but acceptable given the mechanical rather than corrosion-critical nature of the repair. The diffusion zone, while not as hard as the bulk cladding, provides a gradual transition in thermal expansion and elastic modulus that reduces the risk of interface cracking.

Post-Weld Heat Treatment and Residual Stress Management

The post-weld heat treatment at 600 °C for 3 hours with furnace cooling was designed to relieve the residual stresses introduced by the welding process and to homogenize the diffusion zone. The residual stress before heat treatment was measured at 180–220 MPa (tensile) at the cladding surface, which is within the yield strength of the overlay material but could lead to distortion during subsequent machining. After the heat treatment, the residual stress was reduced to 40–60 MPa, which is well below the threshold for stress-corrosion cracking. The hardness after heat treatment remained at 250–280 HV, confirming that the tempering did not cause significant softening.

Machining and Final Inspection

The cladded surface was machined to restore the original dimensions of the bearing journal with a tolerance of ±0.02 mm and a surface roughness of Ra ≤ 0.8 μm. The machining allowance of 0.5 mm was sufficient to remove the surface irregularities and any minor defects. The final inspection included PT (no indications), UT (no subsurface defects), and a dimensional check confirming that all critical dimensions met the original specification. A dynamic balancing test at the operating speed of 3000 rpm confirmed that the rotor shaft was within the balance grade G2.5 as required by ISO 21940.

Engineering Practice Implications

The successful repair of the 1400 kW motor rotor shaft demonstrates that cladding technology is a viable and cost-effective solution for restoring worn rotating equipment. The key success factors were careful damage assessment, proper surface preparation, controlled welding parameters, and a well-designed post-weld heat treatment. The total repair cost was estimated at 15% of the cost of replacing the rotor shaft, with a downtime reduction of 60% compared to the replacement scenario. Engineers should consider cladding repair as a first-line option for worn shafts, provided that the damage is limited to the surface and does not compromise the structural integrity of the core material. The PDCA cycle should be applied to the repair process: Plan the repair strategy based on damage assessment, Do the repair with controlled parameters, Check the results through inspection, and Act by updating the repair procedure for future similar cases.

Summary

The cladding repair of the 1400 kW motor rotor shaft using GMAW with Inconel 625 wire followed by 600 °C post-weld heat treatment successfully restored the shaft to serviceable condition with a cost saving of 85% compared to replacement. The key technical parameters that ensured success were the controlled heat input of 8–10 kJ/mm, the interpass temperature limit of 250 °C, and the thorough post-weld stress relief. The final inspection confirmed that the repaired shaft met all dimensional, surface, and balance requirements. This case study underscores the importance of systematic repair planning, rigorous quality control, and the application of metallurgical principles in the successful restoration of critical rotating equipment. Engineers managing maintenance programs should establish standardized cladding repair procedures for common wear scenarios to ensure consistent quality and minimize unplanned downtime.