CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance Analysis of Underwater Wet Welding Repair Cladding Layer
Literature Overview and Application Context
This study investigates the microstructure and wear resistance of cladding layers produced by underwater wet welding for repair applications. Underwater wet welding is performed in a water environment without gas shielding or dry chamber protection, making it an economical and accessible method for in-situ repair of submerged structures such as ship hulls, offshore platforms, pipelines, and marine equipment. However, the water environment introduces unique challenges including arc instability, hydrogen absorption, incomplete shielding, and rapid cooling, all of which affect the quality and performance of the deposited cladding layer. Understanding the microstructure and wear resistance of underwater wet weld cladding layers is essential for ensuring the long-term durability of repaired submerged structures exposed to abrasive marine environments.
Underwater Wet Welding Process Characteristics
Underwater wet welding operates with the welding arc submerged in water, and the arc is stabilized by the water medium itself. The welding parameters differ significantly from atmospheric welding due to the high thermal conductivity of water and the presence of hydrogen in the arc. Typical parameters include a welding current of 200–350 A (higher than atmospheric welding to compensate for heat loss to the water), arc voltage of 20–30 V, and travel speed of 50–150 mm/min. The effective heat input is reduced due to heat dissipation into the surrounding water, but the local heat input near the arc is concentrated, creating a steep thermal gradient.
The microstructure of underwater wet weld cladding layers is characterized by rapid solidification effects, hydrogen-induced porosity, and the formation of unusual phases. The rapid cooling caused by the water environment results in a fine-grained microstructure with a high density of dislocations and fine precipitates. However, the hydrogen absorption from the water can lead to porosity, hydrogen-induced cracking, and reduced ductility. The study employs metallographic examination, X-ray diffraction (XRD), SEM with EDS, and microhardness mapping to characterize the microstructure of the cladding layer produced by underwater wet welding with various filler materials including nickel-based alloys (Ni-Cr-Mo type), cobalt-based alloys (Stellite type), and high-chromium cast irons.
| Process Parameter | Underwater Wet Welding | Atmospheric Welding | Effect of Water |
|---|---|---|---|
| Welding current | 200–350 A | 150–250 A | Higher current needed to compensate for heat loss |
| Arc voltage | 20–30 V | 18–25 V | Higher voltage due to water resistance |
| Travel speed | 50–150 mm/min | 100–300 mm/min | Slower speed for adequate penetration |
| Cooling rate | Very high | Moderate | Water accelerates cooling significantly |
| Hydrogen content | High (5–20 mL/100g) | Low (< 5 mL/100g) | Hydrogen absorbed from water |
| Grain size | 20–60 μm | 50–150 μm | Rapid cooling produces finer grains |
Microstructure Analysis of Cladding Layer
The microstructure of the underwater wet weld cladding layer varies depending on the filler material composition and welding parameters. For nickel-based alloy cladding (Ni-Cr-Mo type), the microstructure consists of austenite and ferrite phases with a fine cellular dendrite structure. The austenite:ferrite ratio is typically higher than in atmospheric welding due to the rapid cooling, which stabilizes the austenite phase. The presence of austenite improves the wear resistance and corrosion resistance of the cladding layer but may reduce the hardness compared to fully martensitic structures.
For cobalt-based alloy cladding (Stellite type), the microstructure is characterized by a matrix of solid solution with dispersed carbides (Cr7C3, Co3W, and Co2C). The rapid cooling in underwater welding produces finer and more uniformly distributed carbides compared to atmospheric welding, which enhances the wear resistance through the mechanism of fine hard particle dispersion. However, the high hydrogen content can lead to microcracks and porosity that may compromise the integrity of the cladding layer. For high-chromium cast iron cladding, the microstructure consists of a carbide-rich matrix with pearlite and martensite, and the rapid cooling promotes the formation of fine carbides that provide excellent abrasion resistance.
Wear Resistance Performance and Mechanisms
The wear resistance of the underwater wet weld cladding layer is evaluated through pin-on-disk wear tests, dry sliding wear tests, and erosion wear tests simulating marine abrasion conditions. The results show that the underwater wet weld cladding layer generally exhibits comparable or superior wear resistance compared to atmospheric weld cladding of the same composition, primarily due to the finer microstructure and higher density of hard phases resulting from rapid cooling.
The nickel-based alloy cladding shows a wear rate of 5–15 mg/mm² in dry sliding wear, which is 20–30% lower than the atmospheric weld equivalent. The improved wear resistance is attributed to the higher austenite content, which provides better deformation resistance, and the finer cellular structure, which impedes dislocation motion. The cobalt-based alloy cladding exhibits the best wear resistance with a wear rate of 2–8 mg/mm², benefiting from the fine, uniformly distributed carbides that provide effective micro-ploughing and micro-cutting resistance. The high-chromium cast iron cladding shows a wear rate of 3–10 mg/mm² with excellent resistance to abrasive wear due to the high volume fraction of hard carbides.
The wear mechanism analysis using SEM reveals that the primary wear mechanisms are adhesive wear, abrasive wear, and oxidative wear. The fine microstructure of the underwater wet weld cladding layer reduces the contact area between the asperities, thereby decreasing adhesive wear. The high density of hard carbides in the cobalt-based and high-chromium cladding layers provides effective resistance to abrasive wear through micro-ploughing and micro-cutting mechanisms. However, the presence of porosity and hydrogen-induced cracks can act as stress concentrators and initiate wear-related crack propagation, reducing the fatigue life of the cladding layer under cyclic loading conditions.
Engineering Practice and Quality Considerations
For underwater wet welding repair applications, the following engineering considerations are important based on the study findings. First, the selection of filler material should be based on the specific wear conditions expected in service; nickel-based alloys are suitable for general corrosion and wear resistance, cobalt-based alloys for severe abrasive wear, and high-chromium alloys for high-temperature wear applications. Second, the welding parameters should be optimized to minimize hydrogen absorption and porosity; using higher welding current and slower travel speed can reduce the cooling rate and hydrogen content while maintaining adequate penetration. Third, post-weld inspection should include both visual examination and ultrasonic testing to detect porosity and cracking, with acceptance criteria that account for the inherent defects of underwater wet welding.
The study also emphasizes the importance of surface preparation before underwater wet welding, including thorough cleaning of rust, paint, and marine growth from the substrate surface. Incomplete cleaning can lead to contamination of the weld pool, increased porosity, and reduced bond strength. Additionally, the substrate should be preheated to 100–200 °C using an external heat source if possible, to reduce the thermal gradient and minimize hydrogen-induced cracking. The overall quality of underwater wet weld repairs depends on a combination of proper material selection, parameter optimization, surface preparation, and thorough post-weld inspection, all of which must be integrated into a comprehensive quality management plan.
Key Reflections and Study Insights
This study provides a comprehensive understanding of the microstructure and wear resistance of underwater wet weld cladding layers, which is of practical importance for the marine and offshore industries. The findings confirm that underwater wet welding can produce cladding layers with excellent wear resistance, often superior to atmospheric welding due to the fine microstructure resulting from rapid cooling. However, the inherent challenges of the process, including hydrogen absorption, porosity, and arc instability, must be carefully managed through proper parameter selection and quality control. For engineers involved in underwater repair operations, this study offers practical guidance on material selection, process optimization, and quality verification, and reinforces the principle that even in challenging underwater environments, high-quality cladding repairs are achievable with appropriate technical knowledge and disciplined execution.