Explosive Surfacing Composite Repair Technology for Titanium Steel Clad Plate Defects

Literature Overview

The paper by Shi Changgen, You Jun, and Feng Jian (2008), published in Pressure Vessel (Vol. 25, No. 7, pp. 29-31), addresses a critical engineering challenge in the fabrication of titanium-steel clad plates used in pressure vessels. The authors, affiliated with the Engineering Institute of the PLA University of Science and Technology and Nanjing Runbang Metal Clad Materials Co., Ltd., present a hybrid explosive welding plus surfacing repair technique that achieves a 100% bonding rate for titanium-steel clad plates. The research was funded by the Jiangsu Applied Basic Research Fund (Grant No. BJ97096). The work is classified under TG456.6 (explosive welding) and represents a significant contribution to improving the yield rate of titanium-steel clad materials.

Core Technical Problem and Background

Titanium and titanium alloys offer exceptional corrosion resistance, particularly in marine and chemical processing environments, while carbon steel or low-alloy steel provides the necessary structural strength and economic viability. Explosive welding (EW) is the primary method for joining these dissimilar metals, yet the process is inherently difficult due to several metallurgical and physical constraints.

Key Challenges in Titanium-Steel Explosive Welding

Low bonding rate: The explosive welding of titanium to steel typically yields bonding rates well below 100% due to the formation of unbonded zones, often referred to as "dead zones" or "non-bond areas."
Intermetallic compound formation: The reaction between titanium and iron at the bonding interface can produce brittle intermetallic phases such as TiFe, Ti₂Fe, and Ti₃Fe, which degrade joint integrity.
Poor weldability: Titanium and steel have fundamentally different weldability characteristics. Titanium has a high chemical affinity for oxygen, nitrogen, and hydrogen, making it extremely sensitive to contamination during welding. Steel, while more tolerant, introduces iron diffusion into the titanium side, embrittling the titanium zone.
Low yield rate: The combination of low bonding rate and poor repairability results in an overall low yield rate for titanium-steel clad plates, increasing material costs and production cycle times.

Technical Solution: Explosive Plus Surfacing Composite Repair

The authors propose a two-stage repair methodology that combines explosive welding with overlay surfacing to achieve full bonding integrity. The process involves:

Explosive bonding of a thin titanium transition layer: A thin titanium plate is explosively bonded to the defective area of the steel substrate, creating a metallurgically sound titanium-steel interface at a reduced thickness that improves bonding probability.
Special-process surfacing: After the explosive bonding step, a specialized surfacing process is applied to build up the titanium layer to the required final thickness while maintaining metallurgical compatibility and mechanical integrity.

Process Parameters and Control Points

Parameter	Description	Critical Control Point
Explosive charge configuration	Shape charge or plate charge geometry	Charge-to-plate ratio optimized for thin titanium plate bonding
Standoff distance	Gap between explosive charge and target plate	Must ensure optimal collision velocity for titanium-steel interface
Collision velocity	Interface collision speed during explosive welding	Typically 200-300 m/s for titanium-steel systems
Surfacing process	Overlay welding technique applied after explosive bonding	Special flux or shielding to prevent titanium oxidation
Surfacing layer composition	Filler metal composition for titanium overlay	Must match base titanium grade to prevent cracking
Preheating and interpass temperature	Thermal control during surfacing	Prevents excessive cooling rate in titanium HAZ
Post-weld heat treatment	Stress relief or solution treatment	Eliminates residual stresses from surfacing

Metallurgical Analysis and Defect Mechanism

The formation of unbonded zones in titanium-steel explosive welding can be attributed to several factors. During the explosive welding process, the titanium plate and steel plate are accelerated to high velocities and collide at the interface. The bonding mechanism relies on the formation of a turbulent jet at the collision point, which cleans oxide layers and promotes intimate metal-to-metal contact. However, titanium's low density and high specific energy absorption can lead to incomplete turbulence development, resulting in regions where oxide films are not fully disrupted.

The surfacing repair step addresses these unbonded zones by locally removing the defective area and rebuilding the titanium layer with a process that provides better control over the bonding interface. The "special process" mentioned in the abstract likely involves:

Vacuum or high-purity inert gas shielding: To prevent oxidation of the titanium surface during surfacing.
Low-heat-input welding parameters: To minimize intermetallic compound formation at the titanium-steel interface.
Multi-pass surfacing with controlled interpass temperatures: To manage thermal cycling and reduce residual stresses.

Engineering Practice Implications

The practical significance of this technology extends to several industrial sectors:

Nuclear power industry: Titanium-steel clad vessels are used for containment of radioactive fluids where corrosion resistance is paramount.
Shipbuilding and marine engineering: Clad plates are used for ballast tanks and seawater piping systems.
Chemical processing: Reactors and heat exchangers handling aggressive media require titanium cladding on structural steel.

The ability to repair defective clad plates rather than scrap them represents a substantial economic benefit. For a typical pressure vessel project, the material cost of titanium-steel clad plates can constitute a significant portion of the total fabrication cost. A repair technology that achieves 100% bonding rate directly translates to reduced material waste and shorter production schedules.

Key Questions and Reflections

The paper raises several important questions for further investigation:

Long-term durability: Does the repaired area exhibit the same long-term corrosion resistance and mechanical properties as the original explosive weld? Fatigue and stress corrosion cracking behavior in the repair zone warrants long-term testing.
Scalability: The technology is demonstrated for laboratory or small-scale production. How does it perform for large-format clad plates used in industrial pressure vessels?
Code acceptance: Would ASME, NB/T, or other pressure vessel codes accept this repair methodology? The qualification and acceptance criteria for dissimilar metal repairs in pressure vessels are stringent, and the hybrid explosive-surfacing approach would require extensive qualification testing.
Standardization: The "special process" for surfacing is not fully detailed in the abstract. Standardization of the process parameters is essential for industrial adoption.

Study Insights and Implications

This research represents a pragmatic engineering approach to a persistent problem in dissimilar metal joining. The concept of using a thin titanium transition layer to bridge the explosive welding gap is elegant in its simplicity. By reducing the thickness of the titanium layer subjected to explosive bonding, the authors effectively increase the probability of achieving complete metallurgical bonding. The subsequent surfacing step then builds up the required thickness with a process that offers better control over metallurgical quality.

The key insight for practicing engineers is that dissimilar metal repair does not need to be limited to conventional welding methods. Hybrid approaches that combine multiple joining technologies can overcome the limitations of any single process. This philosophy is applicable to other challenging dissimilar metal systems, such as nickel-based alloys on carbon steel or copper on steel.

From a quality assurance perspective, the repair process requires rigorous non-destructive testing at each stage. Ultrasonic testing (UT) or radiographic testing (RT) should be performed after the explosive bonding step to verify the bonding quality of the transition layer. Additional NDT should be conducted after surfacing to detect any lack of fusion, porosity, or cracking in the overlay welds.