Surfacing Welding of Turbine Bottom Ring at Maogai Power Station Using Submerged Arc Tape Electrode

Literature Overview

This paper, published in the journal Welding (2013, Vol. 7, pp. 67–69), authored by Li Mingwei, Yao Lijia, Hu Hongwei, and Zhao Ting from Zhejiang Fuchun River Hydropower Equipment Co., Ltd., presents a practical engineering case of applying submerged arc tape electrode surfacing to the bottom ring of the guide vane mechanism in the Maogai Power Station turbine. The base material is Q235B carbon steel, and the objective is to build up a wear- and corrosion-resistant overlay layer that meets the demanding hydraulic service conditions of a large hydroelectric turbine. The study employs a two-layer surfacing strategy using Soudotape wire electrodes with matched fluxes, and the results demonstrate excellent metallurgical bonding, absence of internal defects, and superior surface finish.

Core Technical Approach

Two-Layer Surfacing Strategy

The engineering challenge here is straightforward in concept but demanding in execution: how to deposit a durable, corrosion-resistant overlay on a plain carbon steel substrate without cracking, delamination, or excessive dilution. The authors adopt a two-layer approach, which is a well-established practice in surfacing metallurgy for dissimilar material transitions.

• Transition layer (intermediate layer): Soudotape 309L wire electrode with Record INT109 flux. The 309L composition (austenitic stainless steel with low carbon) serves as a metallurgical buffer between the ferritic Q235B base metal and the final overlay, reducing the risk of cracking due to coefficient of thermal expansion mismatch and limiting carbon pickup at the fusion line.
• Surface layer (final overlay): Soudotape 430 wire electrode with Record RT152 flux. The 430 composition (ferritic stainless steel, high chromium) provides the desired surface hardness and corrosion resistance for the hydraulic environment.

Process Configuration

The use of a submerged arc tape electrode (SAW-T) process is significant. Unlike conventional submerged arc welding with solid wire, the tape electrode is a continuously rolled strip that allows for very wide, uniform bead deposition with minimal overlap. This is particularly advantageous for large flat or slightly curved surfaces such as turbine bottom rings, where covering a large area with consistent quality is essential. The process was performed on a positioner (turning table), which ensures uniform heat input around the circumference of the ring and minimizes residual stress concentration.

Key process considerations include:

1. Preheating and interpass temperature control: Q235B has low carbon equivalent, so cracking susceptibility is relatively low, but preheating to approximately 150–250°C is still recommended to reduce thermal gradient and minimize the risk of cold cracking, especially at the fusion boundary where the microstructure transitions from ferrite-pearlite to austenite.
2. Travel speed and wire feed speed: These must be carefully matched to maintain a stable arc and uniform bead width. Tape electrode processes typically operate at lower travel speeds than solid wire SAW to achieve the desired bead width and penetration profile.
3. Flux coverage: Adequate flux coverage is critical to prevent gas porosity and spatter. The flux also plays a role in alloying the weld metal through addition of alloying elements during the melting process.

Interpretation of Technical Points

Metallurgical Bonding and Dilution

One of the key findings highlighted in the paper is the excellent metallurgical bonding between the overlay layers and the base metal, with no internal defects detected. This is a direct result of the careful selection of the transition layer composition. The 309L layer, being austenitic, has a lower thermal contraction coefficient than the ferritic base metal, which accommodates the thermal mismatch during cooling. Furthermore, the low carbon content of 309L prevents the formation of hard, brittle martensite at the fusion line, which would otherwise be susceptible to cracking under residual tensile stress.

Dilution is an inherent challenge in surfacing welding. In the first layer, dilution from the base metal into the 309L weld metal is significant, potentially altering the composition away from the nominal 309L specification. However, the austenitic nature of 309L provides a wide composition window for maintaining ductility and crack resistance. In the second layer, dilution from the first layer is lower because the 309L layer already has a composition closer to the 430 overlay. The authors' results confirm that the final surface hardness meets the design requirement, indicating that dilution was within acceptable limits.

Surface Hardness and Corrosion Resistance

The surface hardness of the 430 overlay is expected to be in the range of 200–260 HV, depending on the exact composition and heat treatment history. This hardness level provides adequate resistance to erosive wear from high-velocity water flow while maintaining sufficient ductility to resist cracking under cyclic loading. The high chromium content (approximately 17–22%) in the 430 composition provides good general corrosion resistance in water environments, although it should be noted that ferritic stainless steels are generally less resistant to pitting and crevice corrosion than austenitic grades.

Integration with Engineering Practice

Applicability to Turbine Components

The Maogai Power Station turbine bottom ring is a critical component that operates under continuous hydraulic loading, with water velocities that can exceed 20 m/s in certain flow regimes. The wear and erosion patterns on such components are typically characterized by cavitation erosion, impingement erosion, and general corrosion. The two-layer surfacing approach addresses all three degradation mechanisms:

• The 309L transition layer ensures mechanical integrity under cyclic loading.
• The 430 surface layer provides hardness for erosion resistance and chromium for corrosion resistance.

Quality Control Measures

For a component of this criticality, the quality control protocol should include:

Lessons Learned

From my own experience in turbine component surfacing, I would emphasize several additional points that this paper does not explicitly address but which are critical in practice:

1. Surface preparation: The base metal surface must be thoroughly cleaned and machined to a consistent flatness before surfacing. Any surface irregularities can lead to arc instability and inconsistent bead profile.
2. Post-weld stress relief: While the paper does not mention post-weld heat treatment, for turbine components subject to cyclic loading, a stress relief annealing at 550–650°C for 1–2 hours is often recommended to reduce residual stresses that could contribute to fatigue cracking.
3. Dimensional control: The surfacing buildup must be controlled to meet the final dimensional tolerances specified in the turbine assembly drawings. This requires careful control of the number of passes and the bead height.

Key Questions and Reflections

One question that arises from reading this paper is the long-term durability of the 430 overlay in the specific water chemistry of the Maogai reservoir. Ferritic stainless steels can be susceptible to intergranular corrosion if sensitized during welding, and while the 430 composition is less prone to sensitization than austenitic grades, it is not immune. In service, if the water contains chlorides or sulfates above certain thresholds, localized corrosion could initiate at grain boundaries or inclusions within the overlay. A more comprehensive study might include accelerated corrosion testing in simulated reservoir water conditions.

Another reflection concerns the cost-benefit analysis. Soudotape wire electrodes and the associated fluxes are imported products with relatively high costs. For a large turbine component, the total surfacing cost can be significant. The paper does not discuss alternative approaches such as cold spray, thermal spray, or laser cladding, which might offer competitive performance at different cost points. However, the submerged arc tape electrode process has the advantage of being well-understood, with extensive qualification data available, and it can be performed with relatively simple equipment compared to laser cladding systems.

Study Insights and Implications

This paper is a valuable engineering case study that demonstrates the practical application of submerged arc tape electrode surfacing for large hydraulic components. The two-layer approach with a 309L transition and 430 surface is a sound metallurgical strategy that balances crack resistance, hardness, and corrosion resistance. The use of a positioner to ensure uniform heat input around the circumference is a practical engineering solution that addresses the challenge of welding large ring-shaped components.

For fellow engineers working on similar turbine component refurbishment projects, the key takeaways are:

• The two-layer surfacing approach with austenitic transition and ferritic surface is a reliable strategy for carbon steel substrates requiring wear and corrosion resistance.
• Submerged arc tape electrode welding is well-suited for large, flat or slightly curved surfaces where uniform bead width and high deposition rate are desired.
• Positioner-assisted welding ensures uniform quality around the circumference and minimizes residual stress concentration.
• Comprehensive NDT and hardness verification are essential for acceptance of critical turbine components.

The paper, while concise, provides a solid foundation for engineers to design their own surfacing specifications for turbine components, provided that the specific service conditions (water chemistry, flow velocity, temperature) are carefully considered in the material selection process. The results are encouraging and confirm that this technology can deliver high-quality overlays that meet the demanding requirements of large hydroelectric turbine applications.