Study Note on Clad Repair of a 55 kW Motor Shaft

Background and Problem Statement

Electric motor shafts are critical rotating components in industrial power transmission systems, and their failure can lead to unplanned downtime and significant production losses. The literature under study describes the repair of a 55 kW motor shaft that suffered severe wear and dimensional loss at the bearing seat and coupling keyway areas. Rather than replacing the entire shaft, a weld overlay repair approach was adopted to restore the shaft to its original dimensions and performance specifications. This study note examines the technical approach, material selection, welding parameters, and quality assurance measures employed in this repair, providing valuable insights for engineers dealing with shaft restoration challenges.

Failure Analysis and Repair Strategy

Root Cause Analysis

The failed shaft exhibited a bearing seat diameter reduction of approximately 0.8 mm from the nominal dimension, accompanied by surface scoring and localized material loss at the keyway. Metallographic examination revealed that the wear was primarily due to fretting corrosion caused by relative micro-movement between the shaft and bearing inner ring, exacerbated by inadequate lubrication and possible misalignment of the motor coupling. The shaft material was identified as 45 steel (equivalent to AISI 1045), a medium-carbon steel with moderate strength and good machinability but limited wear resistance.

Repair Approach Selection

Three repair strategies were evaluated: (1) replacement of the entire shaft, (2) sleeve fitting with interference fit, and (3) weld overlay cladding followed by machining. The literature reports that option (1) was rejected due to long lead times and high cost; option (2) was considered but deemed less reliable due to potential fretting at the sleeve interface under dynamic loading; option (3) was selected as the optimal solution, offering rapid repair, full dimensional restoration, and improved surface properties.

Material Selection and Welding Process

Overlay Material Selection

The overlay material selection was based on several criteria: compatibility with the base 45 steel, sufficient hardness for bearing seat service, good machinability for post-weld finishing, and resistance to cracking during welding. The literature reports the use of a nickel-iron-chromium alloy welding consumable (analogous to AWS A5.5 ERNiFe-CI type) for the transition layer, followed by a high-carbon chrome-molybdenum steel welding consumable (analogous to AWS A5.5 ER80S-D2 type) for the functional layer. This two-layer approach ensures metallurgical compatibility at the base-overlay interface while providing the required hardness and strength in the functional layer.

Welding Process Parameters

The overlay welding was performed using gas tungsten arc welding (GTAW) with direct current electrode negative (DCEN) polarity, which provides deep penetration and good bead profile control. The literature specifies the following process parameters: arc current 120–150 A, arc voltage 18–22 V, travel speed 5–8 mm/min, shielding gas argon at 15–20 L/min, and tungsten electrode of thoriated tungsten (WT20) with 3.2 mm diameter. The base shaft was preheated to 200–250°C using an induction heater, and the interpass temperature was maintained below 250°C to control the cooling rate and prevent cracking.

Surface Preparation

Surface preparation is critical for achieving sound metallurgical bonding. The literature describes a rigorous preparation sequence: (1) mechanical grinding of the worn surface to remove all oxide, scale, and contaminated material; (2) degreasing with acetone or equivalent solvent; (3) final grinding to achieve a surface finish of Ra ≤ 3.2 μm; and (4) immediate welding within 1 hour of preparation to prevent re-oxidation. The literature emphasizes that any deviation from this preparation sequence can result in lack of fusion or porosity defects.

Quality Assurance and Inspection

Non-Destructive Testing

The repaired shaft was subjected to a comprehensive NDT program to ensure repair quality. Magnetic particle testing (MT) was performed on the entire overlay area to detect surface and near-surface cracks, with no indications exceeding the acceptance criteria of NB/T 47013. Ultrasonic testing (UT) was conducted on the overlay thickness to verify uniform deposition and detect subsurface defects such as lack of fusion and internal porosity. The literature reports that the overlay thickness was measured at 12 locations around the circumference, with a variation of less than ±0.1 mm, indicating excellent process control.

Mechanical Property Verification

Hardness testing was performed on the overlay layer and the heat-affected zone (HAZ) to verify that the material properties meet the design requirements. The overlay layer hardness was measured at 450–480 HV, which is 1.5 times the base material hardness and provides adequate wear resistance for bearing seat service. The HAZ hardness was measured at 280–320 HV, indicating no excessive hardening or embrittlement. Tensile testing of overlay coupons (weld overlay test coupons prepared per ASTM A263) showed a tensile strength of 720 MPa and elongation of 18%, confirming good ductility and strength.

Dimensional and Geometric Verification

After machining, the repaired shaft was inspected for dimensional accuracy and geometric tolerances. The bearing seat diameter was verified to be within ±0.01 mm of the nominal dimension, the roundness was better than 0.005 mm, and the surface roughness was Ra ≤ 0.8 μm. The keyway dimensions were verified against the original drawing specifications. The literature reports that all dimensional and geometric tolerances met or exceeded the original manufacturing specifications.

Engineering Practice Implications

The successful repair of this 55 kW motor shaft demonstrates that weld overlay cladding is a viable and cost-effective alternative to component replacement for shaft restoration applications. The key success factors identified from this case study include: (1) thorough failure analysis to identify the root cause and ensure the repair addresses the underlying issue; (2) appropriate material selection with a multi-layer approach for metallurgical compatibility; (3) rigorous process parameter control and surface preparation; (4) comprehensive quality assurance through NDT and mechanical testing; and (5) post-repair dimensional verification to ensure functional performance. The literature also highlights the importance of addressing the root cause of the original failure, such as improving lubrication and alignment, to prevent recurrence of the same failure mode.

Study Insights and Conclusions

This study provides a practical and detailed account of the weld overlay repair of a motor shaft, demonstrating the versatility of cladding technology in industrial maintenance and repair applications. The GTAW-based two-layer overlay approach, combining a nickel-based bonding layer with a high-carbon chrome-molybdenum functional layer, offers a reliable solution for restoring worn shafts to their original dimensions and performance. The systematic quality assurance program, encompassing NDT, mechanical testing, and dimensional verification, provides confidence in the repair quality. For engineers dealing with similar shaft restoration challenges, this case study serves as a practical reference for material selection, process parameter optimization, and quality control, reinforcing the principle that weld overlay cladding can extend component life and reduce maintenance costs when applied with proper technical rigor.