CLADDING TECHNOLOGY SHANXI CO., LTDControlling Cladding Deformation of Axial-Flow Turbine Blades Using Rigid Fixing Method
Literature Overview
This 1997 publication by Tong Ximin, published in the journal Water Power, presents a practical engineering solution for controlling welding-induced deformation during the overlay cladding of axial-flow turbine blades at the Qingtongxia Hydropower Plant. The study addresses a long-standing challenge in hydraulic machinery maintenance: how to apply protective or repair cladding to thin-walled turbine blade components without introducing geometric distortions that compromise hydrodynamic performance. The rigid fixing method described represents a straightforward yet effective approach that has been validated through field application.
Core Technical Content and Analysis
Axial-flow turbine blades are precision-machined components with tight geometric tolerances, typically requiring surface flatness deviations to remain within 0.1 to 0.3 millimeters to maintain optimal hydraulic efficiency. The application of overlay cladding for corrosion or erosion protection introduces significant welding thermal cycles that cause differential expansion and contraction, leading to angular distortion, bowing, and warping of the blade structure.
The rigid fixing method described in this study involves clamping the blade firmly between two rigid platens or fixtures during the cladding operation. The fixture design constrains the blade in all directions of movement, preventing the thermal deformation from manifesting as geometric distortion. Instead of allowing the blade to deform freely, the rigid constraint forces the thermal stresses to remain as residual stresses within the material, which can then be relieved through a controlled post-weld stress relief heat treatment.
The key design parameters of the rigid fixing system include the clamping force magnitude, the contact area between the fixture and the blade, and the fixture material properties. Insufficient clamping force allows partial deformation to occur, while excessive force may cause fixture marks or localized yielding at the contact surfaces. The fixture material must have sufficient rigidity to resist elastic deformation under the applied clamping loads, typically requiring steel platens with thickness ratios to the blade span of at least 3:1.
| Parameter | Recommended Value | Purpose |
|---|---|---|
| Clamping force | 15-30 MPa contact pressure | Prevent angular distortion |
| Fixture thickness ratio | ≥ 3:1 vs blade span | Minimize fixture deflection |
| Contact area coverage | ≥ 80% blade surface | Uniform constraint |
| Interpass temperature | ≤ 100 °C | Limit thermal accumulation |
| Post-weld stress relief | 550-650 °C, 2-4 h | Relieve residual stresses |
| Acceptable distortion after relief | ≤ 0.2 mm | Maintain hydraulic efficiency |
The cladding process parameters are also critical to the success of the rigid fixing approach. Lower heat input welding processes such as GTAW or pulsed GMAW are preferred to minimize the thermal load per pass, and multi-pass cladding with low deposition rates is recommended to distribute the thermal input more evenly across the blade surface. The welding sequence must be carefully planned to ensure symmetric thermal loading, with passes applied in a balanced pattern that minimizes directional distortion tendencies.
Engineering Practice Implications
The rigid fixing method is particularly advantageous for maintenance and repair applications where specialized equipment is not available and the blade must be cladded in a workshop environment. Unlike induction heating preheat methods or advanced robotic welding systems, the rigid fixing approach requires only simple mechanical fixtures and conventional welding equipment, making it accessible to field maintenance teams.
However, the method has limitations that must be acknowledged. The residual stresses trapped within the blade during constrained welding can be substantial, potentially reaching values approaching the yield strength of the base material in the heat-affected zone. Without adequate post-weld stress relief, these residual stresses can lead to delayed cracking, particularly in high-strength steels or stainless steel overlay deposits. The stress relief heat treatment must be carefully controlled to avoid sensitization of the base material or the overlay deposit, which would compromise corrosion resistance.
For blades made of austenitic stainless steels such as 304 or 316, the stress relief temperature must be limited to below 450 degrees Celsius to avoid chromium carbide precipitation and intergranular corrosion susceptibility. For carbon steel or low-alloy steel blades, higher stress relief temperatures of 550 to 650 degrees Celsius are appropriate but must be carefully controlled to avoid temper embrittlement in certain alloy grades.
Key Questions and Reflections
An important question that arises from this approach is the long-term fatigue performance of the blade after rigid fixing cladding and stress relief. The trapped residual stresses, even after partial relief, can act as bias loads that reduce the fatigue life of the blade under cyclic hydraulic loading. Fatigue analysis of the cladded blade should consider the combined effect of the service loads and the residual stress field to ensure adequate design life.
Another consideration is the effect of the rigid fixing method on the overlay deposit quality. The constraint imposed by the fixture may increase the residual tensile stresses in the overlay deposit, which can promote cracking in high-strength martensitic or austenitic overlay materials. The bonding strength between the overlay and the base metal should be verified through tensile or shear bond testing to ensure that the rigid fixing approach does not compromise the cladding integrity.
Study Insights and Implications
This practical engineering solution demonstrates that effective deformation control in cladding operations does not always require sophisticated equipment or advanced process technology. The rigid fixing method leverages fundamental mechanical principles of constraint and stress redistribution to achieve acceptable geometric accuracy in overlay cladding applications. For engineers working in hydropower maintenance, the approach provides a validated methodology that can be implemented with minimal capital investment.
The broader lesson from this work is that deformation control in welding operations is fundamentally about managing thermal inputs and mechanical constraints in a balanced manner. Whether through rigid fixing, induction preheat, back-gas cooling, or multi-axis robotic welding, the goal is to minimize the thermal gradient and mechanical asymmetry that drive distortion. The rigid fixing method represents one point in the spectrum of available strategies, and its effectiveness depends on proper implementation and complementary process controls.