Effect of Final Rolling Temperature on Interface Microstructure and Mechanical Properties of 2205 Duplex/Q345C Roll-Bonded Clad Plate
1. Technical Definition and Metallurgical Principles
The roll-bonded (roll-clad) composite plate is a solid-state bonded product in which a corrosion-resistant alloy layer—in this case, 2205 duplex stainless steel—is metallurgically bonded to a structural carbon steel base plate—Q345C—through a hot rolling process. The final rolling temperature (FRT), defined as the temperature at which the last pass of rolling is completed before the slab exits the rolling mill, is the single most critical process variable governing the quality of the metallurgical bond at the interface.
The bonding mechanism operates through a combination of oxide film rupture, plastic deformation, and atomic diffusion. During the final rolling passes, the two layers undergo severe plastic deformation at temperatures typically between 950°C and 1150°C. This deformation causes the native oxide layers (primarily Fe₂O₃, Fe₃O₄, and Cr₂O₃) at the interface to fracture and spread laterally, exposing clean metallic surfaces. Subsequent atomic diffusion across the interface establishes a continuous metallic bond. The final rolling temperature directly controls:
- Oxide film reformation kinetics: At temperatures above approximately 1050°C, rapid re-oxidation of exposed surfaces can inhibit bonding. Below approximately 900°C, insufficient diffusion prevents adequate atomic intermixing.
- Austenite-to-ferrite phase equilibrium in 2205: The FRT determines the gamma (austenite) to alpha (ferrite) phase ratio upon cooling, which must remain within the duplex range (typically 35-65% ferrite by volume fraction) to maintain the corrosion resistance and mechanical integrity of the cladding layer.
- Interfacial diffusion layer thickness: Higher FRT promotes a wider diffusion zone, which can dilute the high-alloy cladding composition near the interface, potentially reducing local corrosion resistance.
- Interface cleanliness: Inclusion migration and segregation behavior at the interface is temperature-dependent, affecting peel strength and bond quality.
2. Technical Purpose and Engineering Value
Understanding and controlling the FRT is essential for achieving the following engineering objectives in 2205/Q345C clad plate production:
- Ensuring 100% metallurgical bond: The interface must exhibit continuous metallurgical bonding with no delamination, voids, or weak zones across the full plate area, as verified by ultrasonic testing and peel/shear testing.
- Preserving duplex microstructure: The 2205 cladding layer must maintain a balanced duplex microstructure (alpha + gamma) to resist chloride stress corrosion cracking, pitting, and crevice corrosion in aggressive environments.
- Achieving target mechanical properties: The Q345C base plate must retain its structural yield strength (minimum 345 MPa) and impact toughness, while the 2205 layer must maintain adequate hardness (typically 22-32 HRC) and elongation.
- Minimizing interfacial dilution: Excessive diffusion at high FRT can create a transition zone where Cr and Ni concentrations drop below the minimum required for duplex stainless steel corrosion performance.
3. Key Process Parameters and Implementation Points
3.1 Critical Rolling Parameters
| Parameter | Recommended Range | Effect of Deviation |
|---|---|---|
| Final Rolling Temperature (FRT) | 1000°C – 1080°C | Below 1000°C: insufficient bond, high interface resistance. Above 1080°C: excessive diffusion, phase imbalance in 2205. |
| Total Reduction Ratio (per layer) | ≥ 30% (each layer) | Below 30%: inadequate oxide rupture, incomplete bonding. |
| Rolling Speed | 0.5 – 1.5 m/s | Too fast: insufficient contact time for diffusion. Too slow: excessive heat input, grain coarsening. |
| Interpass Temperature | ≥ 1050°C | Below 1050°C: oxide film reformation between passes, bond degradation. |
| Coil Tension | 0.5 – 2.0 kN/mm | Insufficient tension: reduced normal pressure at interface, poor bonding. |
| Surface Condition (pre-roll) | Scale-free, cleaned | Heavy scale or contamination: localized bond failures, inclusion-rich interface. |
3.2 Microstructural Evolution by FRT
| FRT Range | Interface Microstructure | 2205 Phase Composition | Bond Quality |
|---|---|---|---|
| 900–950°C | Thin diffusion zone; persistent oxide inclusions at interface | Ferrite-dominant (>65% alpha); delta-ferrite enrichment risk | Poor: incomplete bonding, high interface resistance |
| 950–1000°C | Moderate diffusion; partial oxide rupture | Near-equilibrium duplex (50-65% alpha) | Adequate: marginal bonding, requires high reduction |
| 1000–1080°C | Continuous metallic bond; clean interface; moderate diffusion | Balanced duplex (40-60% alpha) | Excellent: full metallurgical bond, optimal properties |
| 1080–1150°C | Wide diffusion zone; significant Cr/Ni dilution at interface | Austenite-dominant (<40% alpha); loss of duplex character | Good bond but compromised corrosion resistance |
3.3 Heat Treatment Considerations
Following roll bonding, the clad plate typically undergoes a solution annealing heat treatment to restore the duplex microstructure in the 2205 layer and relieve residual stresses. The annealing temperature (typically 1050–1100°C) must be coordinated with the FRT to ensure:
- Complete dissolution of sigma phase (σ) and chi phase (χ) precipitates that may have formed during rolling.
- Re-establishment of the optimal ferrite/austenite ratio (35-65% ferrite) throughout the cladding layer.
- Avoidance of excessive grain growth in the Q345C base plate, which would reduce impact toughness.
4. Applicable Standards and Acceptance Criteria
4.1 Product Standards
| Standard | Scope | Key Requirements for 2205/Q345C Clad Plate |
|---|---|---|
| GB/T 24511-2017 | Steel and stainless steel roll bonded composite plates—Technical conditions | Composition, mechanical properties, bonding quality, dimensions, surface quality |
| ASTM A490/A490M | Standard specification for clad plate, strip, and sheet | Bonding strength, corrosion resistance, dimensional tolerances |
| ASME SA-467 | Clad plate for pressure vessels and other equipment | Peel test, ultrasonic testing, hardness requirements |
| NB/T 47017.2-2012 | Pressure vessel steel plates—Part 2: Roll-bonded composite plates | Chinese pressure vessel industry-specific requirements for bonded quality |
| GB/T 24512-2017 | Methods for testing bonding of composite steel plates | Peel test procedure, acceptance thresholds |
| ISO 14224 | Steel and stainless steel clad plates—Definitions and nomenclature | Classification and designation system |
4.2 Non-Destructive Testing (NDT) Requirements
- Ultrasonic Testing (UT): Full-area UT scanning per GB/T 24511 or ASTM A490 to detect delamination, voids, and bond defects at the interface. Acceptance: no indication exceeding the specified amplitude threshold (typically 6 dB above reference block).
- Magnetic Particle Testing (MT): Surface and near-surface defect detection on both faces and edges per ASTM E1444.
- Visual Inspection (VT): Surface quality, edge condition, and marking verification per GB/T 24511.
4.3 Destructive Testing and Acceptance
| Test Method | Standard | Acceptance Criteria |
|---|---|---|
| Peel Test | GB/T 24512 / ASTM A490 | No separation at the interface; fracture must occur within the base plate (Q345C) or within the cladding layer (2205), not at the bond line. |
| Shear Test | GB/T 24512 | Minimum shear strength ≥ 200 MPa; fracture within the base plate material. |
| Hardness Mapping | ASTM E18 / ASTM E10 | 2205 layer: 22-32 HRC. Q345C base: 13-22 HRC. No hardness gradient exceeding 5 HRC per mm at the interface. |
| Impact Test (Charpy V-Notch) | GB/T 229 / ASTM E23 | Q345C base: ≥ 34 J at -20°C (or specified temperature). 2205 layer: ≥ 47 J at -20°C. |
| Ferrite Content (Ferritscope) | ASTM E1824 / ISO 8044 | 2205 cladding layer: 35-65% ferrite volume fraction across the full thickness. |
| Intergranular Corrosion Test | ASTM A262 Practice E / NACE TM0172 | No intergranular attack in the 2205 layer after sensitization treatment. |
5. Common Risks and Control Measures
5.1 Interface Bonding Defects
- Risk: Incomplete bonding due to low FRT or insufficient reduction ratio, resulting in delamination zones detectable by UT but potentially missed in spot testing.
- Control: Implement real-time FRT monitoring with thermocouples on the slab surface; maintain FRT within the 1000-1080°C window with a ±15°C tolerance. Conduct 100% UT scanning with calibrated equipment and trained operators.
5.2 Phase Imbalance in 2205 Cladding
- Risk: Excessive austenite formation (ferrite < 35%) at high FRT, leading to susceptibility to stress corrosion cracking in chloride-containing environments.
- Control: Perform ferrite content measurement (Ferritscope) at multiple positions across the cladding thickness. If out of range, apply corrective solution annealing (1050-1100°C, water quench).
5.3 Interfacial Dilution and Corrosion Risk
- Risk: High FRT (>1080°C) combined with prolonged rolling time causes Cr and Ni to diffuse from the 2205 layer into the Q345C base, creating a low-alloy transition zone with reduced pitting resistance.
- Control: Limit the diffusion zone thickness to ≤ 0.5 mm. Verify by transverse metallographic examination with Cr/Ni microprobe (EPMA) analysis. Maintain minimum cladding thickness of 3 mm to provide adequate corrosion-resistant overlay.
5.4 Surface Contamination and Inclusions
- Risk: Residual mill scale, rolling oil, or atmospheric contamination on the slab surfaces prior to final rolling can create localized bond failures and inclusion-rich interfaces.
- Control: Implement strict surface cleaning protocols (shot blasting or pickling) prior to the final rolling pass. Control the rolling mill atmosphere to minimize re-oxidation during the interpass period.
5.5 Residual Stress and Dimensional Distortion
- Risk: Differential thermal contraction between 2205 and Q345C during cooling from FRT creates residual stresses at the interface, potentially leading to warping or micro-cracking.
- Control: Apply controlled cooling rates (≤ 10°C/min) after the final rolling pass. Conduct stress-relief annealing (600-650°C for Q345C compatibility) if residual stresses exceed 50 MPa as measured by XRD or strain gauge methods.
6. Application Across the Company's Technology Routes
6.1 Roll-Bonded Clad Plate Production (Primary Route)
This FRT optimization study is directly applicable to the company's roll-bonded clad plate manufacturing route. The findings provide the metallurgical basis for:
- Establishing the rolling mill operating window for 2205/Q345C clad plate production.
- Developing WPS-equivalent process specifications for the roll bonding operation.
- Training rolling mill operators on critical temperature control parameters.
- Providing the technical justification for heat treatment schedules following roll bonding.
6.2 TIG/MIG Weld Overlay Integration
While roll bonding is the primary bonding method for clad plate, the FRT study has implications for the company's weld overlay operations:
- Pre-overlay substrate preparation: Understanding the interface microstructure helps determine the optimal grinding depth to remove any dilution zone before applying TIG/MIG weld overlay with 309L or 316L transition layers.
- Weld overlay WPS development: The hardness profile across the 2205/Q345C interface informs the selection of filler metals and welding parameters to avoid excessive dilution and cracking during overlay welding.
- Post-overlay inspection: Knowledge of the base clad plate's interface quality helps distinguish between inherited defects and weld-induced defects during NDT of overlaid components.
6.3 Hydraulic Explosive Bonding and Explosion Welding
The metallurgical insights from FRT optimization are transferable to the company's explosive bonding routes:
- Collision velocity correlation: The plastic deformation and oxide rupture mechanisms identified in the FRT study are analogous to the high-strain-rate deformation in explosion welding. The target collision velocity for 2205/Q345C in explosion welding (typically 200-400 m/s) achieves similar oxide rupture and diffusion bonding as the optimized FRT range in roll bonding.
- Post-explosion heat treatment: The solution annealing parameters derived from the FRT study can be adapted for post-explosion bonding heat treatment to restore duplex microstructure in the 2205 layer.
- Quality comparison: The peel strength and diffusion zone characteristics established from roll-bonded plates serve as benchmark values for acceptance criteria in explosion-welded 2205/Q345C composites.
7. Contribution to Qualification Building and Customer Value
7.1 Qualification and Certification Support
This technical study directly supports the company's qualification programs in the following ways:
- NB/T 47017.2 compliance: Provides documented evidence of process control and metallurgical understanding required for Chinese pressure vessel industry certification of roll-bonded composite plates.
- ASME Section VIII qualification: Supports the technical data package for ASME SA-467 clad plate qualification, including peel test results, UT acceptance records, and heat treatment documentation.
- ISO 9001 process capability: Demonstrates statistical process control (SPC) of the FRT parameter, supporting process capability indices (Cpk ≥ 1.33) required for ISO 9001 certification of critical manufacturing processes.
- Customer-specific WPS qualification: Provides the metallurgical basis for customer-specific welding procedure qualification when clad plate is used as a substrate for subsequent welding operations.
7.2 Product Delivery Quality Assurance
- Reduced non-conformance rate: By establishing the optimal FRT window and implementing real-time monitoring, the company can reduce interface bonding defects by an estimated 60-80% compared to uncontrolled rolling.
- Accelerated customer acceptance: Comprehensive metallurgical documentation (microstructure reports, hardness maps, ferrite content data, UT records) accelerates customer inspection and acceptance of clad plate deliveries.
- Traceability: Each production batch is associated with specific FRT records, enabling full traceability from slab heat number to final product certification.
7.3 Customer Value Proposition
"By mastering the final rolling temperature control for 2205/Q345C clad plate, Cladding Technology Shanxi Co., Ltd. delivers composite plates with guaranteed metallurgical bond integrity, optimized duplex microstructure for maximum corrosion resistance, and predictable mechanical properties—reducing customer risk in pressure vessel, heat exchanger, and chemical processing applications where failure at the clad interface would be catastrophic."
8. Implementation Roadmap
- Phase 1 – Process Standardization: Codify the FRT operating window (1000-1080°C) into the company's rolling process specification with defined control limits and alarm thresholds.
- Phase 2 – Instrumentation Upgrade: Install high-precision thermocouples and infrared pyrometers on the rolling mill to achieve ±10°C FRT monitoring accuracy.
- Phase 3 – Statistical Process Control: Implement SPC charts for FRT data collection across all production batches; establish Cpk ≥ 1.33 target for the critical parameter.
- Phase 4 – Metallurgical Verification Program: Conduct periodic (per batch) metallographic examination, hardness mapping, ferrite content measurement, and peel/shear testing to validate process control.
- Phase 5 – Knowledge Transfer and Documentation: Compile all findings into a company technical manual; train quality engineers, process engineers, and customer service representatives on the metallurgical implications of FRT control.
9. Conclusion
The study of final rolling temperature effects on 2205/Q345C clad plate represents a fundamental metallurgical capability that underpins the company's roll-bonded composite plate product line. By establishing the optimal FRT window, defining acceptance criteria aligned with GB/T 24511, ASTM A490, ASME SA-467, and NB/T 47017.2, and implementing rigorous process control, the company can deliver clad plate products with guaranteed interface integrity, superior corrosion resistance, and full traceability. This capability not only strengthens the company's qualification position in the Chinese and international pressure vessel markets but also provides the metallurgical foundation for integrating roll-bonded clad plate with the company's TIG/MIG weld overlay and explosion welding services, creating a comprehensive cladding technology platform for demanding industrial applications.