Zhuojin Pipe Fitting Co., LtdFailure Analysis of Fan Transmission Shaft Fracture After Overlay Welding Repair
Literature Overview
The paper by Feng Xiaoliang et al. from Guangzhou Special Pressure Vessel Inspection Research Institute (2021, Physical Testing, Vol. 57, No. 11) presents a thorough failure investigation of a fan transmission shaft that fractured shortly after being repaired by overlay welding. The shaft had suffered severe wear at the journal-sleeve interface during long-term operation, prompting an overlay welding repair to restore the journal diameter. Despite the repair, the shaft failed in service within a short period. This case is highly instructive for engineers dealing with shaft repair, fatigue management, and the quality control of overlay welding operations on critical rotating components.
Core Findings and Failure Mechanism
The investigation employed a comprehensive suite of analytical methods including macroscopic observation, chemical composition analysis, tensile testing, hardness testing, microstructural examination, and fractographic analysis. The results revealed that the shaft experienced low-cycle (high-stress) multi-source fatigue fracture. A particularly notable feature on the fracture surface was the presence of tire-shaped impressions at the ratchet steps along the fracture edge, which indicate repeated crack arrest and re-initiation events during the fatigue process.
The root cause analysis identified several contributing factors. First, the microstructure and hardness of the overlay-welded surface layer at the shaft shoulder exhibited anomalies, suggesting non-uniform weld metal composition or improper heat input during the overlay process. Second, the un-machined weld toes at the diameter transition (step change) significantly exacerbated stress concentration at the shaft shoulder. Under cyclic loading, the keyway and the weld toes served as fatigue crack initiation sites, and once initiated, the cracks propagated rapidly under working loads until catastrophic fracture occurred.
Key Technical Parameters and Defect Analysis
| Parameter / Defect | Observation / Value | Engineering Implication |
|---|---|---|
| Fracture type | Low-cycle multi-source fatigue | High stress amplitude, limited cycle life |
| Tire-shaped impressions | Found at ratchet steps on fracture edge | Indicates repeated crack arrest/re-initiation |
| Overlay weld microstructure | Abnormal at shaft shoulder surface | Non-uniform composition or excessive HAZ softening |
| Hardness at repaired area | Anomalous distribution | Potential for reduced fatigue strength |
| Weld toe geometry | Un-machined at diameter transition | Severe stress concentration factor |
| Crack initiation sites | Keyway and weld toes | Geometric discontinuities dominate fatigue life |
The tire-shaped impressions on the fracture surface are a distinctive feature that merits special attention. These impressions form when multiple fatigue cracks initiate at different locations and propagate until they interact and merge. The ratchet steps between merged crack regions record the sequence of crack coalescence. In this case, the multi-source nature of the fatigue damage indicates that the entire shaft shoulder region was under significant stress concentration, not just a single critical location.
Process Analysis of Overlay Welding Repair
Overlay welding repair of shaft journals is a common industrial practice for restoring worn dimensions, but it carries inherent risks if not executed with proper engineering controls. The following table summarizes the critical process considerations:
| Process Step | Recommended Practice | Common Failure Mode |
|---|---|---|
| Pre-weld preparation | Thorough surface cleaning, grinding to remove all wear damage | Residual oxide layers causing lack of fusion |
| Welding procedure | Controlled heat input, appropriate filler matching | Excessive HAZ softening, microstructural anomalies |
| Post-weld machining | Grind weld toes to match parent metal contour | Un-machined toes causing stress concentration |
| Post-weld heat treatment | Stress relief annealing if applicable | Residual stress accelerating fatigue crack growth |
| Post-repair inspection | MT/PT of entire repaired area, dimensional verification | Undetected defects serving as crack sources |
The failure in this case was fundamentally caused by the neglect of post-weld machining and stress management. The un-machined weld toes created a geometric discontinuity that acted as a potent stress concentrator, with a theoretical stress concentration factor potentially exceeding 2.5 to 3.0 depending on the weld toe radius. Combined with anomalous hardness and microstructure in the overlay layer, the fatigue life of the repaired shaft was drastically reduced.
Engineering Practice Implications
For engineers involved in shaft repair and overlay welding, this case underscores several critical lessons. First, overlay welding repair must never be considered a simple dimension-restoration operation; it is a metallurgical intervention that fundamentally alters the stress state and material properties at the repair location. Second, post-weld machining of weld toes to a smooth transition with the parent metal is non-negotiable, especially at geometric discontinuities such as shaft shoulders and keyway junctions. Third, the keyway itself represents an inherent stress concentration that must be accounted for in fatigue assessment; the combination of a keyway with poorly finished weld toes creates a synergistic failure mechanism.
A systematic approach using FMEA (Failure Mode and Effects Analysis) should be applied to any overlay welding repair of rotating shafts. The FMEA should evaluate each process step for potential failure modes, assign severity, occurrence, and detection ratings, and establish control measures. For the shaft repair scenario described in this literature, the most critical control points are: weld toe finishing quality, hardness verification of the overlay layer, and post-repair non-destructive examination.
Study Insights and Reflections
This failure case highlights a recurring theme in industrial practice: the tendency to view welding repair as a straightforward dimensional restoration rather than a complex metallurgical process that demands the same rigor as new component fabrication. The overlay weld deposit introduced not only new material but also new residual stresses, potential microstructural heterogeneity, and geometric discontinuities. Without careful management of these factors, the repair itself becomes the weakest link in the component's fatigue life.
The presence of multi-source fatigue initiation suggests that the stress concentration was distributed across the entire shoulder region, not localized to a single defect. This implies that even if the weld toes had been ground, the keyway and the inherent shoulder geometry would still constitute significant fatigue risk factors. A comprehensive repair strategy should therefore include fatigue assessment of the entire repaired geometry, not just the weld deposit itself.
The tire-shaped impressions on the fracture surface provide valuable forensic evidence for understanding the crack propagation sequence. In future failure investigations, careful fractographic analysis of impression morphology and step height can help reconstruct the loading history and identify the dominant crack initiation sites. This information is essential for developing effective corrective actions that address the true root causes rather than merely treating symptoms.
In conclusion, this literature serves as a powerful reminder that overlay welding repair of fatigue-critical components demands comprehensive process control, thorough post-weld finishing, and rigorous quality verification. The failure was not caused by the overlay welding process alone but by the combination of metallurgical anomalies in the weld deposit, un-machined weld toes, and the inherent stress concentration of the keyway. Engineers must adopt a holistic approach to repair design, execution, and inspection to ensure that repaired components achieve service life comparable to their original condition.