Effect of Annealing Temperature on Interface and Microstructure of Explosion-Welded Copper-Steel Clad Plate

1. Definition and Fundamental Principles

Explosion welding (also termed explosive bonding) is a solid-state joining process in which two dissimilar metal sheets—typically a copper facing sheet and a steel backing plate—are propelled toward each other at supersonic velocities (typically 200–1000 m/s) using a controlled detonation of a shaped explosive charge. The resulting high-strain-rate impact produces a characteristic sinusoidal wave pattern at the interface, with localized plastic deformation, jetting of surface oxides, and intimate atomic-level bonding under pressures exceeding 10 GPa. The resulting copper-steel clad plate combines the excellent electrical and thermal conductivity of copper with the structural strength and formability of steel.

However, the explosion welding process inherently introduces significant residual stresses, work hardening, and microstructural heterogeneity in the copper facing sheet and near-interface regions. The severe plastic deformation during impact causes dislocation density in the copper to increase by orders of magnitude, leading to elevated hardness and reduced ductility. Annealing—a controlled post-explosion thermal treatment—serves as the critical process step to relieve residual stresses, restore ductility, and stabilize the microstructure while preserving the metallurgical integrity of the explosion-welded bond.

2. Category and Business Positioning

This technical capability falls squarely within the company's Explosion Welding technology route, one of three core manufacturing pathways alongside TIG/MIG weld overlay and hydraulic explosive bonding. The study of annealing temperature effects on copper-steel explosion-welded interfaces represents a fundamental metallurgical competency that underpins the company's ability to deliver certified clad plate products for demanding electrical, thermal, and structural applications.

Within the qualification framework, mastery of annealing parameters directly contributes to:

• WPS/PQR development: Establishing qualified Welding Procedure Specifications for post-explosion thermal treatments
• Material certification: Ensuring delivered products meet mechanical property requirements per ASTM, ASME, and API specifications
• Customer value: Providing engineering confidence that the clad plate will perform reliably in service, particularly where cyclic loading, thermal fatigue, or formability are concerns

3. Technical Purpose and Value

The primary technical purpose of optimizing annealing temperature for explosion-welded copper-steel clad plates is threefold:

1. Residual stress relief: Explosion welding introduces tensile and shear residual stresses in the copper sheet and near-interface zone of the steel backing. Unrelieved stresses can lead to distortion during subsequent fabrication (cutting, bending, machining), cracking during service, or reduced fatigue life.
2. Microstructural stabilization: The heavily deformed copper microstructure near the interface contains high dislocation density and sub-grain structures. Controlled annealing promotes recovery and partial recrystallization, restoring ductility without compromising bond strength.
3. Dimensional stability: Properly annealed clad plates exhibit minimal distortion during downstream processing, reducing scrap rates and improving manufacturing efficiency for end customers.

4. Microstructural Evolution with Annealing Temperature

4.1 As-Welded Condition (Post-Explosion, Pre-Annealing)

In the as-explosion-welded state, the copper-steel interface exhibits a highly deformed microstructure. The copper side shows:

• Severe grain elongation and fragmentation, particularly in the first 0.5–2 mm from the interface
• Dislocation density exceeding 10¹⁵ m^-2 in the heavily deformed zone
• Hardness values of 100–130 HV (compared to ~40–50 HV for annealed pure copper)
• Intermetallic compound formation (FeCu, FeCu₂, Fe₂Cu₇) along the wavy interface, typically 1–10 μm thick
• Entrapped oxide films and jetting products at the wave troughs

The steel backing plate shows work hardening in a narrow band (typically 0.2–0.5 mm) adjacent to the interface, with increased dislocation density but no phase transformation (ferrite-pearlite microstructure remains stable).

4.2 Low-Temperature Annealing (300–450°C)

At annealing temperatures between 300°C and 450°C, the primary mechanism is recovery:

• Dislocation rearrangement and annihilation reduces hardness by 10–20% in the deformed copper zone
• Sub-grain boundaries become more defined through polygonization
• Residual stresses are partially relieved (estimated 30–50% reduction)
• Intermetallic compounds at the interface remain stable—no significant growth or dissolution
• Bond strength is maintained or slightly improved due to stress relief without interface degradation

4.3 Intermediate-Temperature Annealing (500–650°C)

This range represents the optimal annealing window for most copper-steel clad plate applications:

• Partial recrystallization begins in the heavily deformed copper zone
• Hardness reduction of 30–50% from as-welded condition, approaching 60–80 HV
• Residual stress relief of 70–90%
• Intermetallic layer may show slight thickening (to 5–15 μm) but remains below critical threshold
• Ductility (elongation) improves significantly, enabling downstream forming operations
• Bond strength (shear, peel, and fracture) remains above minimum requirements

4.4 High-Temperature Annealing (700–850°C and Above)

Excessive annealing temperatures introduce significant risks:

• Full recrystallization of copper occurs, eliminating the beneficial work-hardening contribution to bond strength
• Intermetallic compounds grow substantially (Fe₂Cu₇ can exceed 20–50 μm), becoming brittle and prone to cracking
• Diffusion of iron into copper and copper into iron becomes significant, altering composition profiles
• Bond strength degrades—shear and peel strength can drop below acceptance thresholds
• Potential for grain boundary segregation and embrittlement
• Steel backing plate may approach or exceed the A_c1 transformation temperature, risking microstructural changes in the base metal

5. Key Process Parameters and Implementation

5.1 Recommended Annealing Parameter Matrix

Parameter	Recovery (Low)	Optimal (Recommended)	Over-Annealing (Avoid)
Temperature (°C)	300–450	500–650	>700
Soak Time (hours)	1–2	2–4	4–8
Cooling Rate	Furnace cool or air cool	Furnace cool preferred	N/A
Atmosphere	Protective (N2, vacuum, or controlled H2/N2)	Protective (mandatory)	N/A
Cu Hardness After (HV)	80–110	50–80	<45 (over-softened)
Intermetallic Thickness (μm)	1–5 (stable)	5–15 (acceptable)	>20 (risk of cracking)
Residual Stress Relief	30–50%	70–90%	>95% (bond risk)
Bond Strength Retention	>95%	>90%	<80% (failure risk)

5.2 Critical Implementation Points

1. Protective atmosphere is mandatory: Copper oxidizes readily above 400°C. Use high-purity nitrogen (dew point <-40°C), vacuum (<10^-2 Pa), or a controlled reducing atmosphere (5% H₂/95% N₂). Oxidation of the copper surface during annealing degrades both surface finish and corrosion resistance.
2. Uniform heating is essential: Temperature gradients across the clad plate thickness can induce differential expansion and new residual stresses. Use furnace designs with high temperature uniformity (<±10°C across the load zone) and proper thermocouple placement.
3. Thickness-dependent soaking: Thicker clad plates (e.g., 50+ mm total thickness) require extended soaking times to achieve thermal equilibrium through the section. A general rule is 30 minutes per 25 mm of total thickness, with a minimum of 2 hours.
4. Slow cooling preferred: Furnace cooling (or controlled cooling rates of 50–100°C/hour) prevents the introduction of new thermal stresses. Rapid air cooling may be acceptable for thinner plates (<20 mm) but can cause distortion in thicker sections.
5. Post-annealing inspection: Verify hardness profiles (Vickers hardness across the interface), bond strength (shear/peel tests), and microstructural integrity (metallographic examination of the interface) before releasing product.

5.3 Material-Specific Considerations

Copper Alloy	Recommended Anneal Temp (°C)	Notes
C11000 (ETP Copper)	500–650	Standard reference material; well-documented behavior
C10100 (OFHC Copper)	500–650	Sensitive to oxygen; vacuum or high-purity N2 mandatory
C15000 (Copper-Beryllium)	500–550 (stress relief only)	Do NOT full anneal; precipitation hardening must be preserved. Solution treat separately if required.
C18200 (Cu-Ni 90/10)	550–650	Higher strength alloy; anneal temperature can be slightly elevated
C18000 (Cu-Ni 70/30)	600–700	Corrosion-resistant alloy; higher anneal temp acceptable
C19400 (Cu-Al)	550–650	Watch for intermetallic growth at interface; limit soak time

6. Applicable Standards and Acceptance Criteria

6.1 Material and Product Standards

• ASTM A333 – Standard Specification for Steel, Clad, for Low-Temperature Service (when steel backing is for cryogenic applications)
• ASTM A240 – Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip (when stainless steel backing is used)
• ASTM B151 – Standard Specification for Seamless Copper Tube
• ASTM B169 – Standard Specification for Copper or Copper Alloy Pipe and Tube
• ASTM B296 – Standard Specification for Copper, Copper Alloy, and Composite Plate, Sheet, Strip, and Rolled Bar
• GB/T 17748 – Composite Plates and Sheets of Carbon Steel and Stainless Steel (Chinese national standard for clad plate)
• GB/T 22894 – Metallic Materials – Impact Testing – Instrumented Charpy Pendulum Test Method
• NB/T 47014 – Qualification Rules for Welding Procedure of Pressure Vessel (Chinese NB standard for WPS qualification)

6.2 Testing and Acceptance Criteria

• Bond strength: Shear test per ASTM E8 or GB/T 228 – minimum 80 MPa for Cu/Steel (or per customer specification)
• Peel test: Peel strength per ASTM E8 or ISO 9525 – minimum 60 MPa for Cu/Steel
• Fracture test: Fracture surface examination – failure must occur in the base metal (not at the interface) for a qualified bond
• Hardness profile: Vickers hardness (HV0.5 or HV1) across the interface – copper side should be <100 HV, steel side should maintain base metal properties
• Visual inspection: Interface wave pattern should be continuous and free of unbonded areas, cracks, or excessive intermetallic
• NDT: Ultrasonic testing (UT) per ASTM E164 or GB/T 11345 for internal bond defects; visual inspection (VT) per ASTM E94 or GB/T 1955 for surface defects
• Chemical composition: Verify copper and steel compositions per ASTM B151/B169 and ASTM A333/A240 respectively

6.3 Code and Certification Requirements

• ASME Section VIII, Division 1, UCS-64 – Clad Materials for Pressure Vessels (when clad plate is used in pressure vessels)
• ASME Section IX – Qualification of Welders, Welding Operators, and Welding and Brazing Procedures (for WPS/PQR qualification of post-explosion welding treatments)
• API 510/570/580 – Inspection Code for Pressure Vessels / Piping / Storage Tanks (for in-service evaluation of clad components)
• NACE SP0437 – Recommended Practice for Cathodic Protection of Underground or Submerged Metallic Piping Systems (when clad components are used in cathodically protected systems)

7. Common Risks and Controls

Risk	Cause	Consequence	Control Measure
Over-annealing (interface degradation)	Temperature >700°C or excessive soak time	Brittle intermetallic growth; bond strength below specification	Strict temperature control; thermocouple calibration; maximum temperature interlocks; metallographic verification
Copper oxidation	Insufficient protective atmosphere; air ingress during cooling	Surface discoloration; reduced corrosion resistance; potential debonding	Use high-purity N2 or vacuum; monitor atmosphere purity; controlled cooling rate to avoid air contact above 400°C
Distortion during annealing	Non-uniform heating; rapid cooling; unsupported loading	Flatness deviation exceeding tolerance; downstream fabrication difficulty	Use proper supports/jigs; ensure furnace temperature uniformity; furnace cool slowly
Incomplete stress relief	Temperature too low or soak time too short	Residual stresses cause distortion during downstream machining/bending	Verify with X-ray diffraction or hole-drilling method; adjust parameters based on thickness and as-welded condition
Intermetallic embrittlement	High temperature annealing; prolonged soak; high-temperature service	Cracking at interface during service or forming	Limit anneal temperature to ≤650°C; monitor intermetallic thickness via metallography; avoid service temperatures >250°C for Cu/Steel clad
Steel microstructural change	Anneal temperature approaching Ac1 of steel backing	Unintended phase transformation; altered mechanical properties of base metal	Keep anneal temperature below 650°C for most carbon steels (Ac1 ~727°C); verify steel hardness after anneal

8. Application Scenarios Across Company Technology Routes

8.1 Explosion Welding (Primary Application)

This annealing knowledge is most directly applied in the explosion welding route, where copper-steel clad plates are manufactured for:

• Electrical contact components: Busbars, switchgear contacts, and current collectors requiring high conductivity (copper) with structural support (steel). Annealing ensures the copper maintains ductility for forming and that residual stresses do not cause cracking during assembly.
• Thermal management: Heat exchanger plates, condenser tubes, and radiator components where copper's thermal conductivity is exploited with steel's corrosion resistance and strength.
• Electromagnetic shielding: Shielding enclosures and housings where copper provides EMI/RFI protection while steel provides structural integrity.
• Wear-resistant linings: Copper-clad steel for mining and handling equipment where copper's anti-galling properties are needed.

8.2 TIG/MIG Weld Overlay (Cross-Reference Application)

While this entry specifically addresses explosion-welded clad plates, the annealing principles have cross-cutting relevance to the TIG/MIG weld overlay route:

• Post-weld heat treatment (PWHT): Weld overlay deposits on steel substrates often require stress relief annealing. The understanding of temperature-dependent microstructural evolution in copper-steel systems informs PWHT parameter selection for copper alloy overlay welds.
• Interface integrity: In weld overlay applications, the dilution zone between deposited copper alloy and steel substrate is analogous to the explosion-welded interface. Annealing temperature control prevents excessive intermetallic formation in both cases.
• WPS qualification: The annealing parameters established for explosion-welded clad plates can be referenced in WPS development for weld overlay procedures, ensuring consistency across technology routes.

8.3 Hydraulic Explosive Bonding (Adjacent Process)

Hydraulic explosive bonding, while using a different energy delivery mechanism (hydraulic pressure-driven explosive charges), produces similar microstructural conditions in the bonded interface:

• Similar annealing requirements: Hydraulic explosive bonded copper-steel plates require comparable post-bond annealing to relieve residual stresses and stabilize the microstructure. The annealing parameter matrix developed for explosion welding is directly transferable.
• Process qualification: Understanding the annealing effects provides a metallurgical baseline for comparing bond quality across different joining processes (explosion welding vs. hydraulic explosive bonding), supporting process selection decisions for specific applications.
• Customer qualification support: When customers require qualification of hydraulic explosive bonded products, the metallurgical data from explosion welding annealing studies provides comparative evidence of interface quality and long-term stability.

9. Contribution to Qualification Building and Customer Value

9.1 Qualification Building

Mastery of annealing temperature effects on explosion-welded copper-steel interfaces directly contributes to the company's qualification infrastructure:

• PQR documentation: Qualified Performance Records must include post-explosion thermal treatment parameters. Documented annealing studies provide the technical basis for PQR submissions to ASME, API, or NB certifying bodies.
• WPS development: Welding Procedure Specifications for explosion welding must specify the post-bond thermal treatment. The annealing parameter matrix serves as the reference for WPS qualification.
• Material certification: Product certificates must demonstrate that mechanical properties (hardness, tensile strength, elongation) meet specification requirements. Annealing optimization ensures these properties are achieved consistently.
• Third-party inspection readiness: Understanding the expected microstructural evolution enables the company to respond confidently to third-party inspector queries regarding interface quality and thermal treatment adequacy.

9.2 Product Delivery and Quality Assurance

• Process control: Defined annealing windows (500–650°C, 2–4 hours, protective atmosphere) enable repeatable, consistent production with minimal batch-to-batch variation.
• Non-destructive testing correlation: Knowledge of microstructural changes at different annealing temperatures allows correlation of UT signals with expected bond quality, improving NDT acceptance/rejection decisions.
• Traceability: Each batch of clad plate should have documented annealing parameters (temperature profile, soak time, atmosphere purity, cooling rate) enabling full traceability from raw material to finished product.

9.3 Customer Value

• Reduced downstream processing costs: Properly annealed clad plates require less corrective processing (straightening, stress-relief annealing) during customer fabrication, reducing overall project costs.
• Extended service life: Residual stress relief and microstructural stabilization improve fatigue resistance and reduce the risk of stress-corrosion cracking in service.
• Formability assurance: Customers forming clad components (bending, rolling, spinning) benefit from the restored ductility of annealed copper, reducing crack risk during forming operations.
• Technical support: The company can provide customers with metallurgical reports documenting the annealing treatment, interface microstructure, and bond quality—supporting customer qualification and regulatory submissions.

10. Summary and Recommendations

The study of annealing temperature effects on explosion-welded copper-steel clad plate interfaces represents a critical metallurgical competency for the company's explosion welding technology route. The optimal annealing window of 500–650°C for 2–4 hours in a protective atmosphere provides the best balance between residual stress relief, microstructural stabilization, and bond strength preservation.

Key recommendations for implementation:

1. Establish and document qualified annealing procedures (WPS) for each copper alloy/steel backing combination in the product portfolio.
2. Implement rigorous furnace temperature control and calibration programs to ensure annealing temperature accuracy within ±10°C.
3. Mandate metallographic examination of the interface for each production batch to verify intermetallic thickness remains below 15 μm.
4. Conduct periodic bond strength testing (shear and peel) on annealed samples to confirm ongoing process capability.
5. Maintain cross-reference between annealing parameters for explosion welding, TIG/MIG weld overlay, and hydraulic explosive bonding to ensure consistent metallurgical outcomes across all three technology routes.
6. Document all annealing studies in the company's technical knowledge base to support future WPS/PQR development, customer qualification support, and continuous process improvement.

By maintaining deep technical understanding of the annealing temperature-microstructure-bond strength relationship in explosion-welded copper-steel systems, Cladding Technology Shanxi Co., Ltd. positions itself as a qualified, reliable supplier of clad plate products for the most demanding electrical, thermal, and structural applications in the energy, transportation, and industrial sectors.