Zhuojin Pipe Fitting Co., LtdExplosive Cladding Defect Repair for Titanium-Steel Composite Plates in Pressure Vessels
Literature Overview and Background
The paper by Shi Changgen, You Jun, and Feng Jian, published in the journal Pressure Vessel in 2008 (Vol. 25, No. 7, pp. 29-31), addresses a critical engineering challenge in the fabrication of titanium-steel composite plates used in pressure vessels. Titanium and its alloys offer exceptional corrosion resistance, particularly in chloride-containing environments, while carbon and low-alloy steels provide the structural strength required for pressure-containing equipment. However, the explosive welding process used to join these dissimilar materials frequently produces bonding defects such as unbonded areas, interfacial cracks, and intermetallic compound formation. The authors propose a composite repair technique combining explosive cladding of a thin titanium transition layer followed by specialized surfacing, achieving a 100% composite rate and significantly improving the material yield of titanium-steel composite plates.
Core Technical Content and Process Description
The fundamental difficulty in titanium-steel explosive welding lies in the large difference in physical properties between the two materials. Titanium has a significantly lower melting point (approximately 1668 °C) compared to common steels, and the two metals have incompatible crystal structures and thermal expansion coefficients. During explosive welding, the high-velocity impact (typically 2,000-4,000 m/s) generates interfacial turbulence and plastic deformation, but the formation of stable metallurgical bonds is not guaranteed across the entire cladding interface. Unbonded regions, often appearing as dark patches under ultrasonic testing, represent critical defects because they create pathways for corrosion media to penetrate between the titanium cladding and the steel backing plate.
The proposed repair methodology involves two sequential stages. In the first stage, a thin titanium sheet is explosively clad onto the defect area of the composite plate, serving as a transition layer that bridges the gap between the existing defective titanium layer and the steel substrate. In the second stage, specialized surfacing welding is performed under controlled conditions to build up the titanium layer to the required thickness. The key innovation is the selection of welding parameters and consumables that prevent excessive intermetallic compound formation at the titanium-steel interface while ensuring adequate metallurgical bonding.
Key Technical Parameters and Process Windows
The following table summarizes the critical process parameters identified or implied in the study:
| Parameter | Typical Range | Rationale |
|---|---|---|
| Explosive cladding velocity | 2,500-4,000 m/s | Sufficient for plastic deformation and interfacial bonding |
| Titanium transition layer thickness | 0.5-2.0 mm | Thin enough to control intermetallic formation, thick enough for bonding |
| Surfacing welding process | TIG (GTAW) preferred | Low heat input, inert gas protection prevents oxidation |
| Preheating temperature | 150-250 °C | Reduce thermal stress in steel substrate |
| Interpass temperature | Below 300 °C | Prevent excessive diffusion at Ti-steel interface |
| Shielding gas | Pure argon or argon-helium mixture | Prevent titanium oxidation at elevated temperatures |
| Welding current density | 10-25 A/mm² | Balance penetration and heat input |
Engineering Practice Implications and Defect Analysis
From a practical standpoint, this technology has direct relevance to the repair of pressure vessels in the chemical, petrochemical, and nuclear industries where titanium-steel composite materials are employed for corrosion resistance. The most common defects encountered in titanium-steel explosive cladding include:
- Unbonded areas: These appear as dark or gray regions under ultrasonic C-scan testing and represent the most critical defect type because they compromise the corrosion barrier function of the titanium layer.
- Interfacial cracks: Caused by residual stresses from differential thermal expansion during cooling, particularly in thicker composite plates.
- Intermetallic compound formation: Compounds such as TiFe, TiFe₂, and Ti₂Fe form at the interface during high-temperature exposure, which are brittle and can lead to intergranular corrosion.
The proposed explosive-plus-surfacing repair technique effectively addresses the unbonded area problem by introducing a fresh explosive bonding interface. The subsequent surfacing builds up the titanium layer to specification while the controlled welding parameters limit intermetallic compound growth. However, engineers must recognize that this repair method is most effective for localized defects rather than extensive unbonded areas exceeding 10% of the cladding surface.
Study Insights and Reflections
This work represents a pragmatic engineering solution to a persistent problem in composite material fabrication. The authors' approach of combining two established technologies—explosive welding and arc surfacing—into a sequential repair process demonstrates the value of process integration in solving complex material problems. The achievement of 100% composite rate is particularly noteworthy, as it suggests that the transition layer concept effectively eliminates the root cause of bonding failure. From a quality assurance perspective, the repaired areas should undergo thorough ultrasonic testing and, if possible, microstructural examination to confirm the absence of excessive intermetallic compounds. The technology has clear applicability to the repair of titanium-clad heat exchanger tubes, reactor internals, and pressure vessel components in the nuclear and chemical processing industries.