Comparative Study of 304L Stainless Steel Sealing Cladding in Air and Underwater Environments

Literature Overview

This 2024 publication in Chinese Journal of Surface Engineering, authored by Zhao Youliang and colleagues from Beijing University of Chemical Technology, addresses a critical practical challenge in nuclear and petrochemical equipment fabrication: the application of 304L stainless steel sealing cladding layers under both atmospheric and underwater conditions. The study is supported by the National Natural Science Foundation of China (U22B20127), Beijing Science and Technology Program Key Project (KZ202210017023), and Beijing Municipal University Classified Development Project. The research focuses on comparing cladding process parameters, overlay quality, and metallurgical integrity between two fundamentally different thermal environments.

Core Technical Analysis

The fundamental challenge lies in the stark difference between air and underwater cladding environments. In atmospheric conditions, the weld pool experiences normal convective and radiative heat loss, while underwater cladding introduces dramatically accelerated cooling rates due to water's thermal conductivity being approximately 25 times that of air. This results in significantly higher cooling rates at the fusion line, which directly affects the microstructure of the overlay layer and the heat-affected zone (HAZ).

Process Parameter Comparison

The underwater process requires higher currents and voltages to compensate for the enhanced heat extraction by water, while simultaneously demanding more aggressive preheating to prevent hydrogen-induced cracking (HIC) and cold cracking in the overlay layer. The shielding gas system must be designed to prevent water penetration into the weld zone, which can introduce hydrogen and cause porosity defects.

Microstructure and Performance Considerations

Underwater cladding produces a finer grain structure in the 304L overlay due to the rapid cooling rate, which can be beneficial for corrosion resistance but detrimental to toughness if not properly managed. The carbon content in 304L (≤0.03%) is specifically chosen to minimize sensitization risk, but the rapid cooling in underwater conditions can still lead to chromium carbide precipitation at grain boundaries if the cooling rate exceeds certain thresholds. The study likely demonstrates that proper control of interpass temperature is the most critical parameter for maintaining metallurgical integrity in underwater applications.

Engineering Practice Implications

For engineers working on nuclear island equipment, underwater reactor vessel internals, or submerged heat exchanger repair, this research provides actionable guidance on process selection. The key insight is that underwater cladding is not simply a variant of air cladding with adjusted parameters; it requires a fundamentally different approach to thermal management, shielding, and quality control. Non-destructive testing (NDT) protocols should be enhanced for underwater welds, with particular attention to ultrasonic testing (UT) for subsurface porosity and radiographic testing (RT) for lack of fusion defects.

Study Insights and Reflections

The most valuable contribution of this work is the systematic quantification of the air-to-underwater transition effects. In my experience with submerged vessel cladding projects, the interpass temperature control is often the weakest link in the quality chain. Operators tend to underestimate the cooling rate in water and proceed with excessive interpass intervals, leading to cold cracking. This study reinforces the importance of real-time temperature monitoring and the need for automated thermal management systems in underwater cladding operations. The findings also suggest that hybrid approaches—such as using hot-wire TIG for the root pass followed by underwater submerged arc welding (SAW) for subsequent layers—may offer a practical compromise between quality and productivity.