Experimental Study of Underwater Explosive Welding of Steel Pipes

Literature Overview and Research Context

This experimental study investigates the application of underwater explosive welding technology to steel pipe fabrication, specifically addressing the formation of clad or composite steel pipes with enhanced surface properties. Underwater explosive welding of pipes presents unique challenges compared to flat plate welding due to the cylindrical geometry, which introduces curvature effects on shock wave propagation, collision dynamics, and bond quality uniformity around the pipe circumference.

The research is motivated by the need for corrosion-resistant and wear-resistant steel pipes in oil and gas production, chemical processing, and marine applications. Conventional methods for producing clad pipes — such as explosive welding followed by cold expansion, or weld overlay — have limitations in terms of bond quality uniformity, cost, and scalability. Underwater explosive welding offers the potential for improved bond quality and process control.

Experimental Setup and Process Parameters

The experimental configuration involves a cylindrical pipe assembly where the inner pipe (base material) and outer sleeve (cladding material) are detonated together in an underwater environment. The key process parameters and their ranges are documented as follows:

The cylindrical geometry introduces non-uniformity in the collision process. The shock wave propagates differently along the axial and circumferential directions, and the curvature of the pipe surface affects the collision angle and velocity distribution.

Bond Quality and Defect Analysis

The bond quality of underwater explosively welded steel pipes is evaluated through multiple inspection methods:

Common defects identified in the experimental study include:

1. Bonding defects (lack of bond): Occur when the local collision velocity falls below the critical bonding velocity, resulting in unbonded regions. These are more prevalent at the pipe ends and at the seam weld of the pipe.
2. Excessive interdiffusion: In regions where the collision velocity exceeds the upper critical limit, excessive plastic deformation and heat generation lead to thick diffusion zones and possible embrittlement.
3. Circumferential non-uniformity: The bond quality varies around the pipe circumference due to the non-uniform shock wave loading caused by the cylindrical geometry.
4. Axial variation: The bond strength varies along the pipe length, with typically weaker bonding at the pipe ends where the shock wave reflects differently.

The study reveals that the critical velocity window for bonding is narrower for pipe geometries compared to flat plates, requiring more precise control of the detonation process.

Process Optimization and Parameter Sensitivity

The study employs a systematic approach to optimize the process parameters for achieving uniform bonding around the pipe circumference. Key findings include:

• The optimal gap distance for a given pipe diameter follows a scaling law related to the pipe radius, with larger diameters requiring proportionally larger gaps
• The charge thickness must be sufficient to maintain detonation stability around the full circumference, with a minimum charge-to-pipe diameter ratio of approximately 0.3
• Water depth affects the shock wave intensity and uniformity; depths between 1.0 and 2.0 m provide the best balance of shock intensity and uniformity
• The collision angle is inherently determined by the gap geometry and pipe diameter, and must fall within the 10-15 degree range for reliable bonding

A parametric study using the method of fractional factorial design reveals that the gap distance and charge thickness are the most influential parameters, followed by water depth and pipe diameter. The interaction between gap distance and water depth is significant, requiring coupled optimization.

Engineering Applications and Standards Considerations

Underwater explosively welded steel pipes find applications in:

1. Oil and gas production: Corrosion-resistant lined pipes for sour service (H2S-containing environments)
2. Chemical processing: Acid-resistant lined pipes for aggressive chemical transport
3. Marine applications: Wear-resistant pipes for slurry transport and seawater systems
4. Hydrogenation reactors: 9% Ni steel lined pipes for high-pressure hydrogen service

The fabrication and inspection of these pipes must comply with relevant standards, including:

The study notes that current standards for explosively clad pipes primarily address flat plate and conventional pipe geometries, and that the underwater variant requires additional qualification testing and possibly supplementary standard provisions.

Summary and Practical Recommendations

The experimental study of underwater explosive welding for steel pipe fabrication demonstrates that the technology is viable for producing high-quality clad pipes with uniform bonding. The cylindrical geometry introduces additional complexity compared to flat plate welding, requiring careful optimization of process parameters to achieve uniform bond quality around the circumference and along the pipe length. The underwater environment provides advantages in terms of shock wave uniformity and oxide suppression, but also introduces challenges related to process setup and safety. For engineering implementation, the study recommends rigorous qualification testing, including full-scale bond strength verification, non-destructive examination, and metallographic inspection of representative samples. The technology holds promise for producing corrosion and wear resistant pipes for demanding industrial applications, but further standardization and qualification data are needed before widespread industrial adoption.