Zhuojin Pipe Fitting Co., LtdComparison of 304L Stainless Steel Sealed Overlay Welding in Air and Underwater Environments
Literature Overview
This paper, published in China Surface Engineering (2024, Vol. 37, No. 4, pp. 280–290), presents a comprehensive comparison of 304L stainless steel sealed overlay welding performed in air and underwater environments. The study was conducted by researchers at Beijing Institute of Petrochemical Technology, supported by the National Natural Science Foundation of China and Beijing Municipal Science and Technology Commission projects. The research addresses a critical practical challenge: the repair of corroded components in nuclear power plant spent fuel pools, where underwater welding is often necessary due to the presence of water and the need to maintain containment. The study used ER-2209 duplex stainless steel solid wire to fabricate sealed overlay layers on 304L stainless steel substrates in both air and underwater environments, and compared the macroscopic morphology, microstructure, phase composition, mechanical properties, microhardness, and corrosion resistance of the resulting overlays.
Technical Background and Motivation
Application Context
Nuclear power plant spent fuel pools are critical components that store spent nuclear fuel rods after removal from the reactor. Over extended service periods, the stainless steel components of spent fuel pools are susceptible to corrosion, leading to the formation of cracks, pitting, and other defects that compromise structural integrity and safety. Traditional repair methods require draining the pool, which is time-consuming, costly, and may involve handling radioactive materials. Underwater welding offers a practical alternative, but the underwater environment introduces unique challenges:
- Water acts as a coolant, increasing cooling rates and affecting microstructure.
- Hydrogen absorption from water can lead to hydrogen-induced cracking.
- Contamination from water can affect weld quality.
- Limited visibility and accessibility complicate welding operations.
Research Objectives
The study aimed to:
- Compare the macroscopic morphology of overlay welds fabricated in air versus underwater.
- Analyze the microstructure and phase composition of the overlay layers.
- Evaluate mechanical properties including tensile strength, elongation, and microhardness.
- Assess corrosion resistance through electrochemical testing.
- Determine the feasibility and quality of underwater overlay welding for spent fuel pool repair.
Experimental Methodology
Materials and Equipment
| Parameter | Specification |
|---|---|
| Base material | 304L stainless steel (ASTM A240) |
| Filler metal | ER-2209 duplex stainless steel solid wire (AWS A5.9) |
| Welding process | GMAW (MIG) |
| Shielding gas | Argon |
| Air environment | Ambient conditions |
| Underwater environment | Freshwater, ~20 °C |
| Wire diameter | 1.2 mm |
Test Conditions
| Parameter | Air Environment | Underwater Environment |
|---|---|---|
| Temperature | ~25 °C | ~20 °C |
| Pressure | 0.1 MPa | ~0.1–0.2 MPa (depending on depth) |
| Cooling rate | Moderate | High (due to water cooling) |
| Hydrogen absorption | Low | Elevated |
| Contamination risk | Low | Moderate |
Test Methods
| Test | Standard | Purpose |
|---|---|---|
| Macroscopic examination | Visual | Surface quality assessment |
| Metallographic examination | GB/T 13298 | Microstructure analysis |
| XRD analysis | GB/T 10298 | Phase composition |
| Tensile testing | GB/T 228.1 | Mechanical properties |
| Microhardness testing | GB/T 13914 | Hardness distribution |
| Electrochemical testing | GB/T 10125 | Corrosion resistance |
Results and Analysis
Macroscopic Morphology
Both air and underwater overlay welds exhibited continuous and uniform surface formation with no visible cracks, slag inclusions, weld beads, or undercut defects. This indicates that the welding process was well-controlled in both environments. However, subtle differences were observed:
- Air environment: Slightly smoother surface finish with uniform weld bead profiles.
- Underwater environment: Slightly rougher surface with minor ripple patterns, likely due to water turbulence affecting the arc.
Microstructural Analysis
The microstructure of the overlay layers revealed significant differences between air and underwater welding:
| Feature | Air Environment | Underwater Environment |
|---|---|---|
| Primary phases | Duplex (ferrite + austenite) + austenitic regions | Duplex (ferrite + austenite) + austenitic regions |
| Phase distribution | Regional distribution | Regional distribution |
| Ferrite grain size | Fine to medium | Coarse (δ grains) |
| Austenite grain size | Fine to medium | Fine with lath ferrite at grain boundaries |
| Martensite | Absent | Absent |
The underwater environment's rapid cooling effect had several notable impacts on microstructure:
- Duplex stainless steel regions: The rapid cooling suppressed the δ-ferrite to γ-austenite transformation, resulting in coarser δ-ferrite grains. The high stacking fault energy of δ-ferrite contributed to increased hardness.
- Austenitic stainless steel regions: Rapid cooling promoted the formation of lath ferrite distributed along austenite grain boundaries. The regional distribution of lath ferrite increased the hardness of the austenitic regions.
- Grain boundary effects: The rapid cooling refined the grain structure but also promoted segregation at grain boundaries.
Mechanical Properties
| Property | Air – Overlay | Air – Base Metal | Underwater – Overlay | Underwater – Base Metal |
|---|---|---|---|---|
| Tensile strength (MPa) | Higher than base | Lower than overlay | Higher than base | Lower than overlay |
| Elongation (%) | Lower | Higher | Lower | Higher |
| Microhardness (HV) | Higher | Lower | Higher | Lower |
Key observations:
- The overlay layer in both environments exhibited higher tensile strength than the base metal, attributed to the duplex stainless steel composition of ER-2209.
- The base metal's stress-strain curve showed increased strength in the later stages of deformation due to strain-induced martensitic transformation.
- The overall elongation of the base metal was higher than that of the overlay layer, indicating that the base metal retained better ductility.
- The underwater overlay layer exhibited higher hardness than the air overlay layer, attributed to the coarser δ-ferrite grains and lath ferrite formation.
Microstructural Analysis at Fusion Zones
At the fusion boundaries between weld passes, the crystallographic orientation of austenite grains underwent deflection, and the texture intensity of the weld pass increased. This was observed in both air and underwater environments, but the effect was more pronounced in the underwater environment due to the higher cooling rates.
The top transverse tensile specimens from both environments exhibited higher tensile strength and lower elongation, consistent with the increased hardness and reduced ductility observed in the microstructural analysis.
Corrosion Resistance
Electrochemical testing revealed that both air and underwater overlay layers exhibited clear passive regions in their polarization curves, indicating good corrosion resistance. The key findings include:
| Parameter | Air Overlay | Underwater Overlay |
|---|---|---|
| Passivation potential | Clearly defined | Clearly defined |
| Passivation current density | Low | Low |
| Pitting resistance | Good | Good |
| Corrosion rate | Low | Low |
The ER-2209 duplex stainless steel overlay layer provided excellent corrosion protection in both environments. The duplex microstructure, with its balanced combination of ferrite and austenite phases, offers superior pitting resistance compared to single-phase austenitic stainless steels.
Technical Insights and Engineering Implications
Feasibility of Underwater Overlay Welding
The study demonstrates that underwater overlay welding of 304L stainless steel using ER-2209 duplex stainless steel wire is technically feasible and produces acceptable quality results. The key findings supporting this conclusion include:
- Surface quality: No visible defects in either environment, indicating that the welding process can be controlled underwater.
- Microstructure: While the underwater environment produces coarser ferrite grains and lath ferrite formation, the overall microstructure remains acceptable for corrosion-resistant overlay applications.
- Mechanical properties: The overlay layer in both environments exhibits higher tensile strength and hardness than the base metal, providing adequate mechanical performance.
- Corrosion resistance: Both overlays exhibit clear passivation regions and low corrosion rates, confirming the effectiveness of the ER-2209 overlay for corrosion protection.
Process Optimization for Underwater Welding
Based on the findings, the following process optimizations are recommended for underwater overlay welding:
- Preheating: Controlled preheating before welding can reduce the cooling rate and minimize coarse grain formation.
- Shielding gas management: Ensuring adequate shielding gas flow is critical to prevent water contamination and porosity.
- Welding parameters: Lower current and higher travel speed may be beneficial to reduce heat input and minimize coarse grain formation.
- Post-weld inspection: Thorough inspection of underwater welds is essential to detect any hidden defects.
Comparison with Conventional Repair Methods
| Method | Advantages | Disadvantages |
|---|---|---|
| Underwater welding | No need to drain pool, faster, lower cost | Quality control challenges, limited visibility |
| Draining and dry welding | Better quality control, standard procedures | Time-consuming, costly, radioactive handling |
| Temporary clamping | Quick repair | Not permanent, limited lifespan |
Key Questions and Reflections
This study raises several important questions for further investigation:
- Long-term performance: The corrosion resistance and mechanical properties of underwater overlay welds over extended service periods in nuclear spent fuel pools remain to be validated.
- Hydrogen-induced cracking: While no cracking was observed in this study, the risk of hydrogen-induced cracking in underwater welds should be carefully evaluated, particularly for thick sections.
- Radiation effects: The impact of radiation on the microstructure and properties of underwater overlay welds in nuclear environments is not addressed in this study and warrants further research.
- Process standardization: The development of standardized procedures for underwater overlay welding in nuclear applications is essential for widespread adoption.
Conclusion
This study provides valuable comparative data on 304L stainless steel sealed overlay welding in air and underwater environments, demonstrating that underwater welding is technically feasible and produces acceptable quality results. The key findings include comparable surface quality, similar corrosion resistance, and slightly different microstructures and mechanical properties between the two environments. The ER-2209 duplex stainless steel overlay layer provides excellent corrosion protection in both environments, making it a viable option for repairing corroded components in nuclear spent fuel pools.