Spark Surfacing Repair of Worn Generator Rotor Shaft Journal

Literature Overview and Background

The paper by Huang Xiaou and Jiang Ruijun, published in the journal Welding in 2000, addresses a critical maintenance challenge in power generation: the repair of worn shaft journals in the sealing section of large generator rotors. Large turbo-generators, typically rated above 300 MW, operate under extreme mechanical and thermal loads, and the shaft journal surfaces in the seal area are particularly susceptible to wear due to the combined effects of high-speed rotation, thermal cycling, and the abrasive action of seal components. When wear exceeds the permissible tolerance specified by the manufacturer—typically on the order of 0.1 to 0.3 mm for precision journals—the entire rotor assembly may require either replacement or repair. Replacement of the entire rotor is often economically prohibitive and logistically impractical, making in-situ repair the preferred solution. The authors investigated spark surfacing (also known as electric spark deposition or electro-spark surfacing) as a viable repair methodology and reported satisfactory results from field implementation.

Core Technical Approach and Process Parameters

Spark surfacing is a solid-state welding process that deposits material onto a substrate using repeated electrical discharge pulses. Unlike conventional arc welding, the process does not involve full melting of the base metal; instead, each spark discharge creates a localized molten pool that rapidly solidifies, resulting in a deposit with minimal dilution of the base material. This characteristic is particularly advantageous for repair applications where the mechanical integrity and dimensional accuracy of the base component must be preserved. The process parameters typically include pulse current amplitude, pulse frequency, electrode material and geometry, and the working distance between the electrode and the substrate.

The authors selected an appropriate electrode material to match or slightly exceed the hardness and strength requirements of the original shaft journal. The repair procedure involved the following sequence: surface preparation through grinding to remove the worn layer and expose sound base metal; application of preheat to reduce thermal stresses; multi-pass spark surfacing to build up the required dimensional restoration; post-weld machining to achieve the final journal geometry within tolerance; and finally, hardness and dimensional verification. The key advantage of spark surfacing in this application is the low heat input, which minimizes distortion and avoids the risk of cracking in the high-strength rotor steel. The deposit typically exhibits a hardness in the range of HRC 35–45, providing adequate wear resistance for the seal area while maintaining compatibility with the rotor shaft material.

Engineering Practice Insights and Quality Control

From an engineering practice perspective, this case study highlights several important lessons. First, the selection of the repair method must account for the operational environment of the component. The generator rotor shaft journal operates at high rotational speeds and is subject to centrifugal stresses; therefore, the repair deposit must be free of porosity, cracks, and insufficient fusion. Post-repair non-destructive testing, including magnetic particle inspection and ultrasonic testing, is essential to verify the integrity of the deposit. Second, the dimensional accuracy of the repaired journal must meet the original manufacturing tolerances, which typically require surface roughness below Ra 0.8 μm and concentricity within 0.02 mm. Achieving these tolerances through post-weld machining is feasible but requires careful control of the deposit build-up to avoid excessive material removal.

The study also underscores the importance of process parameter optimization. The authors found that excessive pulse current leads to excessive dilution and potential cracking, while insufficient current results in poor bonding and low deposition efficiency. A systematic trial-and-error approach, supplemented by metallographic examination of test coupons, was employed to establish the optimal parameter window. This approach is consistent with the PDCA cycle: Plan the repair strategy, Do the trial surfacing, Check the results through hardness and microstructural analysis, and Act by refining the parameters for the production repair.

Study Insights and Implications

This literature provides valuable reference for engineers dealing with large rotating equipment repair. The use of spark surfacing for shaft journal restoration demonstrates that advanced welding repair techniques can extend the service life of critical components without the need for complete replacement. The low heat input and minimal dilution characteristics of spark surfacing make it particularly suitable for repair of high-strength alloy components where thermal sensitivity is a concern. However, engineers should note that the deposit thickness achievable by spark surfacing is limited, typically to 0.5–2 mm per pass, and multi-pass building requires careful control of interpass temperature to avoid cracking. For deeper wear or damage, a combination of plasma arc surfacing or laser cladding for bulk restoration followed by spark surfacing for final finishing may be a more practical approach. The economic benefits of such repair strategies are substantial, as they can reduce downtime and spare parts costs by a significant margin compared to rotor replacement.