Overlay Welding Repair Process for 55kW Motor Shaft

Literature Overview

This technical paper by Wang Zhengjian, published in Machinery (1993, Vol. 20, No. 4, p. 46), documents the overlay welding repair of a motor shaft in a 55kW electric motor used in a water pump unit at a phosphate fertilizer and cement plant. The paper, though brief, provides practical engineering insights into shaft repair through overlay welding, with a two-year service validation confirming the reliability of the process. The classification number TM307 indicates its focus on electrical machinery maintenance.

Repair Requirements and Technical Specifications

The shaft repair requirements specified in the paper are as follows: the total shaft length is 1200 mm, the shaft diameter is 110 mm, the overlay welding length is 120 mm, and the overlay welding thickness must not be less than 8 mm. These dimensions indicate a substantial repair operation where the worn region represents approximately 10% of the shaft length, and the required build-up thickness is significant relative to the shaft diameter.

Process Analysis and Engineering Considerations

The decision to repair rather than replace the shaft is economically driven. For a 55kW motor, the shaft is a relatively expensive component, and replacement would require complete motor disassembly, potentially damaging other components such as the armature and commutator. The overlay welding approach allows the motor to remain largely intact while restoring the worn section to functional dimensions.

The 8 mm minimum overlay thickness is a substantial build-up that requires careful multi-pass welding strategy. In my experience, achieving uniform 8 mm build-up on a rotating shaft requires a systematic approach: the first few passes establish the base layer with good metallurgical bonding to the shaft, intermediate passes build up the bulk of the material, and the final pass(es) achieve the precise dimensional tolerance required for subsequent machining.

Welding Process Selection

For motor shaft repair, the choice of welding process is critical. The shaft material is typically a medium carbon steel (such as 45 steel or 40Cr), which is susceptible to hydrogen-induced cracking and hard martensite formation in the heat-affected zone. Therefore, the following considerations apply:

• Preheating temperature should be in the range of 200-300°C to reduce cooling rate and minimize hydrogen cracking risk
• Interpass temperature should be maintained above 200°C to prevent excessive hardness in the HAZ
• Post-weld heat treatment (stress relief) at 550-650°C is recommended to reduce residual stresses
• Low-hydrogen electrodes (such as E7018 or equivalent) should be used to minimize hydrogen pickup

The paper mentions that the overlay welding process was validated over two years of operation, which is a significant service life for a repaired shaft. This suggests that the welding parameters, consumable selection, and post-weld treatment were all appropriately chosen for the application.

Quality Control and Defect Prevention

For shaft overlay welding repairs, the following quality control measures are essential:

1. Visual inspection of the weld surface for porosity, undercut, and excessive spatter
2. Magnetic particle testing (MT) to detect surface and near-surface cracks
3. Dimensional verification after machining to ensure the final shaft meets tolerance specifications
4. Hardness testing of the HAZ to ensure it is not excessively hardened (target: below 350 HB for most shaft steels)
5. Tensile or bend testing of coupon samples welded under identical conditions to verify mechanical properties

Common defects in shaft overlay welding include:

Engineering Practice Integration

From a maintenance engineering perspective, this paper represents a classic example of cost-effective repair versus replacement decision-making. The economic analysis is straightforward: the cost of overlay welding repair is typically 10-30% of the cost of a new shaft, while the downtime is significantly reduced. For a 55kW motor in a continuous production process such as phosphate fertilizer manufacturing, even a few days of downtime can result in substantial production losses.

The two-year service validation is particularly noteworthy. It demonstrates that properly executed overlay welding repairs can restore components to a condition equivalent to new parts, provided that the welding parameters are carefully controlled and the post-weld machining achieves the required surface finish and dimensional accuracy.

Study Insights and Practical Recommendations

This paper, while brief, encapsulates important practical wisdom about shaft repair through overlay welding. The key insight is that overlay welding is not merely a deposition process but a comprehensive repair technology that requires careful planning of the entire process chain: from surface preparation and preheating through multi-pass welding, post-weld heat treatment, and final machining.

For engineers undertaking similar shaft repair operations, I recommend the following systematic approach: first, thoroughly assess the damage extent and determine whether repair is feasible; second, select appropriate consumables and welding parameters based on the shaft material and service conditions; third, implement a rigorous quality control plan including pre-weld, in-process, and post-weld inspections; and fourth, validate the repair through extended service monitoring.

The economic and technical benefits of overlay welding for shaft repair are well-established, and this paper provides a concrete example of successful implementation in an industrial setting. The approach is particularly suitable for high-value rotating equipment where component replacement would cause excessive downtime or cost.