Explosion Weld Overlay Repair Technology for Defects in Titanium-Steel Clad Plates Used in Pressure Vessels
1. Definition and Fundamental Principles
Explosion weld overlay repair technology for titanium-steel clad plates is an advanced solid-state bonding and repair methodology used to remediate surface defects—such as inclusions, porosity, laps, and bonding discontinuities—found in titanium-lined steel pressure vessels. Unlike conventional fusion welding repair, which introduces dilution and metallurgical incompatibility between the titanium overlay and the carbon steel backing, explosion welding leverages controlled detonation of primary explosives to achieve high-velocity impact bonding between the titanium repair plate and the underlying substrate.
The fundamental principle relies on the formation of a metal-to-metal jet at the collision interface. When a titanium repair sheet is accelerated to velocities typically exceeding 50 m/s and directed toward the steel base plate, the resulting oblique collision generates intense shear stresses and localized adiabatic heating. This produces a turbulent interface with characteristic sinusoidal lobe-and-wave morphology, ensuring mechanical interlock and metallurgical continuity without melting. The process is governed by the critical velocity condition:
V₁ + V₂ ≥ V_c
where V₁ and V₂ are the velocity components of the flyer and base plate at impact, and V_c is the critical velocity determined by material properties (typically 60–80 m/s for titanium on carbon steel). When this threshold is met, metallurgical bonding occurs; below it, the plates merely bounce apart.
2. Category and Business Positioning
This repair technology falls under the company's explosion welding technology route and serves a distinct operational niche within the three principal cladding methodologies:
- TIG/MIG Weld Overlay: Suitable for new fabrication and minor repair of weld overlay layers where dilution is tolerable or transition layers can be applied.
- Hydraulic Explosive Bonding: Used for large-format clad plate production with water as the reaction medium, offering environmental and safety advantages.
- Explosion Welding (Dry Powder):strong> Employed for high-performance cladding and, critically, for the repair of existing titanium-steel clad components where solid-state integrity is paramount.
The repair application positions this technology as a value-added service that extends the operational life of pressure vessels, avoids costly scrapping, and addresses field failures that cannot be resolved through conventional welding due to the Ti-Fe intermetallic compound formation (Fe₂Ti, TiFe) that embrittles fusion weld zones.
3. Technical Purpose and Engineering Value
The primary technical objectives of explosion weld overlay repair for titanium-steel clad plates include:
- Defect Remediation: Elimination of bonding defects (unbonded areas exceeding 1% per ASTM A283), inclusions, porosity, and surface damage in the titanium overlay layer.
- Metallurgical Integrity: Restoration of the cladding interface without introducing dilution, cracking, or residual intermetallic phases that compromise corrosion resistance.
- Compliance Restoration: Bringing non-conforming clad plates or in-service vessels back into compliance with design codes (GB 150, NB/T 47014, ASME VIII Div. 2).
- Economic Value: Avoidance of full plate replacement or vessel scrapping, reducing lifecycle costs by 40–70% compared to new fabrication.
The engineering value extends to enabling continued operation of pressure vessels in aggressive service environments—hydrochloric acid, sulfuric acid, and chloride-containing media—where titanium cladding is essential but defect repair through fusion welding would compromise long-term integrity.
4. Key Process and Implementation Points
4.1 Defect Assessment and Classification
Before initiating repair, a comprehensive non-destructive examination (NDE) program must be conducted to characterize the defect:
| Defect Type | Detection Method | Acceptable Limit (Pre-Repair) | Repair Applicability |
|---|---|---|---|
| Unbonded area | Magnetic flux leakage (MFL) per ASTM A283 | ≤1% of surface area | Explosion repair if area < 500 mm² |
| Inclusion/porosity | Ultrasonic testing (UT) per ASTM A376 | ≤5% of area, ≤50 mm² individual | Explosion repair for isolated defects |
| Surface lap/delamination | Visual + MFL | Not permitted in service | Full explosion overlay repair |
| Corrosion pitting | Visual + thickness measurement | >10% wall loss | Machining + explosion overlay |
4.2 Surface Preparation
The base surface must be prepared to ensure proper collision dynamics:
- Removal of defective titanium layer: CNC machining or grinding to remove all compromised material to a minimum depth of 2 mm beyond the defect boundary (typically 15–30 mm overlap zone).
- Base plate conditioning: The exposed carbon steel surface must be ground to a matte finish (Ra 3.2–6.3 μm) with no oxide scale, oil, or moisture.
- Repair plate preparation: Titanium sheet (Grade 2 or Grade 5, per design requirement) is cut to size with a 25–50 mm overlap beyond the defect boundary on all sides, with edges deburred to R0.5–1.0 mm.
4.3 Charge Assembly and Detonation Parameters
| Parameter | Typical Value (Ti on CS) | Notes |
|---|---|---|
| Standoff distance | 40–60 mm | Calibrated per material thickness combination |
| Impact angle | 15°–25° | Oblique angle for shear wave generation |
| Primary explosive | Ammonium nitrate fuel oil (ANFO) or equivalent | Energy density 3.5–4.0 MJ/kg |
| Flyer velocity | 60–90 m/s | Monitored via high-speed photonic velocimetry |
| Base plate velocity | 10–20 m/s | Passive response to detonation shock |
| Titanium flyer thickness | 3–10 mm | Dependent on repair overlay specification |
| Carbon steel base thickness | ≥20 mm | Minimum for effective shock transmission |
4.4 Post-Bonding Treatment
- Visual inspection: Confirmation of the characteristic explosion weld interface pattern (ripples, jetting marks, no bounce-back zones).
- Magnetic flux leakage testing: Verification that bonding area exceeds 99% per ASTM A283 Section 7.
- Ultrasonic testing: Confirmation of no internal voids or disbonds per ASTM A376.
- Macrographic examination: Cross-section analysis of the interface morphology to confirm metallurgical continuity (optional, for qualification purposes).
- Surface finishing: Machining of the titanium overlay to final dimensional tolerance (±0.1 mm) and surface roughness (Ra ≤ 3.2 μm).
5. Applicable Standards and Acceptance Criteria
5.1 Design and Fabrication Standards
- GB 150.1–150.4: Pressure Vessels – General rules, materials, design, fabrication, inspection, and acceptance.
- NB/T 47014: Qualification rules for welding procedures and welders of pressure vessels.
- ASTM A283: Standard specification for explosion-bonded titanium-clad steel plate.
- ASTM A376: Standard specification for explosion-bonded nickel-clad steel plate (methodology applicable to titanium).
- ASME VIII Div. 2: Rules for construction of pressure vessels – Alternative rules (Section 5, Clad Vessels).
- ISO 16199: Explosion welding – General principles and guidelines.
- NACE SP0432: Repair of corrosion-damaged carbon steel and low alloy steel in service.
5.2 Acceptance Criteria for Repaired Areas
| Criterion | Acceptance Requirement | Standard Reference |
|---|---|---|
| Bonding area | ≥99% of total cladding area | ASTM A283 §7.2 |
| Maximum unbonded area | ≤1% of total area; individual ≤50 mm² | ASTM A283 §7.2 |
| Overlay thickness | ≥90% of nominal after machining | GB 150.4 §7.5 |
| Interface integrity | No cracks, voids, or delamination | ASME VIII Div. 2 §5-7 |
| Corrosion resistance | Equivalent to original cladding (salt spray ≥1000 h) | NACE SP0432 |
| WPS qualification | Valid WPS per NB/T 47014 | NB/T 47014 §4 |
6. Common Risks and Control Measures
6.1 Technical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Insufficient bonding (bounce-back) | Velocity below critical threshold | Pre-calibrated charge geometry; velocity monitoring; pilot coupons |
| Excessive jetting/material loss | Velocity too high; angle too steep | Optimized standoff and angle; high-speed imaging verification |
| Cracking in titanium overlay | Residual stress from detonation; material brittleness | Post-weld stress relief at 350°C/2h (Grade 2 Ti); controlled cooling |
| Contamination at interface | Moisture, oxide, or oil on base plate | Strict surface preparation protocol; witness coupon testing |
| Damage to adjacent bonded area | Shock wave propagation into sound cladding | Strategic charge placement; isolation grooves; UT verification post-repair |
6.2 Safety and Regulatory Risks
- Explosive handling: All operations must comply with local explosive storage and usage regulations; licensed personnel only.
- Shock and noise: Personnel exclusion zone of minimum 50 m; hearing protection mandatory within 100 m.
- Structural vibration: Assessment of nearby piping, instrumentation, and vessel supports for shock-induced damage.
- Permit-to-work: Full compliance with plant safety management systems; hot work and explosive work permits.
7. Application Scenarios Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route – Complementary Role
In the weld overlay technology route, titanium repair through fusion welding is inherently limited by:
- Dilution: Carbon steel base dilutes into titanium weld, forming brittle Fe-Ti intermetallics (Fe₂Ti, TiFe) with hardness exceeding 800 HV.
- Cracking susceptibility: Hydrogen-induced cracking in titanium weld zones, particularly in Grade 2 titanium.
- Transition layer requirement: Mandatory 309L/310L stainless steel transition layer (1–2 mm) to buffer dilution, adding complexity and cost.
Explosion weld repair complements the weld overlay route by addressing defects that cannot be economically or metallurgically repaired through fusion welding. The two routes are applied in sequence: weld overlay for initial fabrication and minor touch-up, explosion welding for major defect repair requiring solid-state integrity.
7.2 Hydraulic Explosive Bonding Route – Production Parallel
Hydraulic explosive bonding (water-medium detonation) is primarily employed for new clad plate production. The repair application of explosion welding uses dry-powder detonation, but shares the same fundamental physics. The knowledge transfer is bidirectional:
- Process parameters developed for hydraulic bonding (charge geometry, standoff optimization) inform repair charge design.
- Repair experience with field conditions (variable substrate geometry, residual stresses) feeds back into production parameter refinement.
- Both routes require identical NDE qualification and interface characterization protocols.
7.3 Explosion Welding Route – Core Application
The explosion welding route is the primary delivery vehicle for this repair technology. Key application scenarios include:
- Field repair of in-service pressure vessels: On-site explosion welding of titanium overlay repair plates onto vessel shells, heads, and nozzles where defects are discovered during inspection.
- Manufacturing repair: Remediation of clad plate defects discovered during factory acceptance testing, avoiding full plate rejection.
- Emergency restoration: Rapid repair of titanium-lined equipment following corrosion damage or mechanical impact in petrochemical plants.
- Thick overlay restoration: Rebuilding titanium cladding thickness where original overlay has been consumed by corrosion (e.g., after 5–10 years in HCl service).
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Mastering explosion weld repair technology enables the company to:
- Qualify under NB/T 47014 for repair welding procedures specific to titanium-clad pressure vessels, expanding the WPS/WPS qualification matrix.
- Achieve certification for on-site repair of in-service pressure vessels under TSG 21 (Chinese pressure vessel inspection regulations), a high-value service qualification.
- Build a proprietary database of process parameters for various titanium grades (Grade 1, 2, 3, 5, 7) on different steel substrates (SA350LF2, 16MnR, Q345R, 304/316L), creating intellectual property barriers.
- Obtain ASME "U" stamp authorization for clad vessel repair, opening international market access.
8.2 Product Delivery Enhancement
The repair capability directly improves delivery economics:
- Yield improvement: Reduces clad plate rejection rates from typical 3–5% to <1%, directly improving fabrication margins.
- Schedule protection: Enables same-shift repair of discovered defects without ordering replacement plates (lead time: 4–8 weeks for titanium clad plates).
- Customer satisfaction: Single-source delivery of fabrication plus repair capability eliminates subcontracting delays and interface management issues.
8.3 Customer Value Proposition
"Explosion weld repair of titanium-clad pressure vessels transforms a potential vessel replacement (cost: ¥2–5 million, downtime: 6–12 weeks) into a targeted repair (cost: ¥150,000–500,000, downtime: 1–2 weeks), delivering a 90%+ cost saving and 85%+ downtime reduction while maintaining full metallurgical and regulatory compliance."
9. Learning Reflection and Continuous Improvement
The learning reflection document (学习心得) associated with this technology captures critical experiential knowledge that transcends written procedures:
- Material sensitivity: Grade 2 titanium exhibits greater tolerance to process variation than Grade 5 (Ti-6Al-4V), which requires tighter velocity control due to its lower ductility at collision temperatures.
- Temperature effects: Winter operations require standoff distance reduction of 5–10% to compensate for increased material brittleness and reduced explosive sensitivity.
- Substrate condition: Previously welded areas (e.g., nozzle welds) exhibit altered shock wave propagation; charge geometry must be locally adjusted to account for weld metal impedance mismatch.
- Sequencing: When multiple defects exist on a single plate, repair should proceed from the center outward to prevent cumulative stress concentration at previously bonded interfaces.
10. Conclusion
Explosion weld overlay repair technology for titanium-steel clad plates represents a critical capability that bridges the gap between new fabrication and in-service maintenance. It provides a metallurgically superior alternative to fusion welding repair, eliminates dilution concerns inherent in titanium welding, and enables economical restoration of pressure vessel integrity in aggressive chemical environments. By integrating this technology across the company's three cladding routes and maintaining rigorous qualification under NB/T 47014, ASTM A283, and ASME VIII Div. 2, Cladding Technology Shanxi Co., Ltd. delivers a differentiated service that maximizes asset life, minimizes customer downtime, and establishes a defensible technical position in the pressure vessel repair market.