Post-Rolling Cooling Rate Effects on Microstructure and Properties of Vacuum Hot-Rolled N08367/Q345R Clad Plate
1. Definition and Technical Principles
1.1 Material System Overview
The N08367/Q345R clad plate system combines UNS N08367 (Alloy 625), a nickel-chromium-molybdenum austenitic alloy renowned for exceptional corrosion resistance in aggressive chemical environments, with Q345R, a Chinese national standard pressure vessel steel compliant with GB/T 18984. The vacuum hot-rolling process produces a metallurgical bond between these dissimilar materials through plastic deformation under elevated temperatures in an inert or vacuum atmosphere, eliminating the need for weld filler metal and producing a homogeneous, defect-free interface.
Post-rolling cooling rate is the critical thermal parameter governing the final microstructural evolution of both the Alloy 625 overlay layer and the Q345R base metal. The cooling regime determines phase transformations, grain morphology, precipitate formation, residual stress distribution, and ultimately the mechanical and corrosion performance of the composite product.
1.2 Thermodynamic and Kinetic Principles
Upon completion of the hot-rolling pass, the clad plate exits at temperatures typically between 950°C and 1150°C. The subsequent cooling trajectory follows a thermodynamic path dictated by the selected cooling medium, cooling velocity, and plate thickness. The fundamental governing equations include:
- Cooling rate (CR): CR = (Troll − Tambient) / tcool, where Troll is the exit temperature and tcool is the time to reach ambient.
- Phase transformation kinetics: Governed by the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation for the Alloy 625 side and the Bainite/Ferrite transformation curves for Q345R.
- Thermal stress development: σthermal = E · α · ΔT / (1 − ν), where differential coefficients of thermal expansion (CTE) between N08367 (~13.1 × 10−6/K) and Q345R (~12.0 × 10−6/K) generate interfacial residual stresses.
1.3 Microstructural Response Mechanisms
In the N08367 layer, cooling rate controls:
- γ-Ni solid solution grain size: Rapid cooling (water quench, CR > 10°C/s) suppresses grain growth, yielding fine equiaxed grains (5–15 μm); slow cooling (air cool, CR < 1°C/s) permits grain coarsening to 30–80 μm.
- δ-phase (Ni3(Al,Ti)) precipitation: Intermediate cooling rates through the 950–1050°C range promote δ-phase formation at grain boundaries, which can be detrimental to hot corrosion resistance.
- MC and M6C carbide distribution: Carbon segregation during slow cooling leads to TiC and NbC precipitation along grain boundaries.
In the Q345R base layer, cooling rate governs:
- Ferrite-bainite microstructure ratio: Fast cooling increases bainite proportion; slow cooling favors coarse pearlite-ferrite.
- Impact toughness: Transition temperature shifts significantly with cooling rate due to grain boundary carbide distribution and ferrite grain size.
- Hardness profile: Vickers hardness in the heat-affected zone (HAZ) varies from 180 HV (slow cool) to 280 HV (fast cool).
2. Category and Business Positioning
2.1 Technology Route Classification
This technical entry falls squarely within the explosion welding and hydraulic explosive bonding technology route, specifically addressing the post-process thermal treatment of vacuum hot-rolled clad plates. The vacuum hot-rolling process is a derivative of explosion welding principles, where the initial explosive bonding is followed by hot-rolling consolidation to achieve the required thickness ratio and metallurgical bond quality.
2.2 Positioning Within the Capability Matrix
| Technology Route | Role of Cooling Rate Control | Typical Product Form |
|---|---|---|
| Explosion Welding / Vacuum Hot Rolling | Primary domain — cooling rate is the decisive parameter for final microstructure | Clad plates (flat, curved), clad pipes, forged clad components |
| TIG/MIG Weld Overlay | Secondary — interpass temperature and post-weld cooling are analogous parameters | Weld overlay cladding on pipes, valves, heat exchanger tubes |
| Hydraulic Explosive Bonding | Directly applicable — post-bonding rolling consolidation shares identical cooling kinetics | Large-format clad plates, tank lining panels |
2.3 Value Proposition
Understanding and controlling post-rolling cooling rate enables the company to deliver clad plates with tailored mechanical properties, optimized corrosion resistance, and predictable long-term service performance. This knowledge transforms the vacuum hot-rolling process from a purely empirical operation into a scientifically controlled manufacturing process, directly supporting:
- WPS/PQR qualification with documented thermal cycles
- Customer-specific property requirements (e.g., impact energy at −46°C)
- Reduced rejection rates through predictive quality control
3. Technical Purpose and Engineering Value
3.1 Primary Technical Objectives
- Microstructural optimization: Achieve fine-grained, δ-phase-free Alloy 625 with controlled grain boundary character.
- Mechanical property balancing: Maintain adequate ductility and impact toughness in both layers without exceeding acceptable hardness limits.
- Interface integrity preservation: Prevent interfacial cracking, delamination, or bond degradation due to thermal mismatch stresses.
- Corrosion resistance maximization: Ensure the Alloy 625 layer retains its full pitting and crevice corrosion resistance as specified by ASTM B626.
3.2 Quantitative Performance Targets
| Performance Parameter | Target Value | Preferred Cooling Regime |
|---|---|---|
| N08367 Grain Size | 5–20 μm (ASTM No. 6–8) | Controlled air cool, CR = 2–5°C/s |
| N08367 δ-Phase Content | < 5% (ASTM B626 Grade 1) | Avoid 900–1050°C dwell; fast cool or slow cool |
| Q345R Hardness | ≤ 220 HV (per GB/T 18984) | Controlled air cool, CR = 1–3°C/s |
| Impact Energy (Q345R, −20°C) | ≥ 47 J (ASTM A516 equivalent) | Avoid quench; moderate cool |
| Interface Bond Strength | ≥ base metal shear strength | Uniform cooling across thickness |
| Residual Stress (interfacial) | ≤ 150 MPa (compressive preferred) | Gradual cool, CR < 5°C/s |
4. Key Process Implementation Points
4.1 Cooling Method Selection
| Cooling Method | Typical CR (°C/s) | Applicable Plate Thickness | Advantages | Limitations |
|---|---|---|---|---|
| Water Quench (direct spray) | 10–50 | ≤ 20 mm total | Fine grain, high strength | High residual stress, cracking risk |
| Forced Air Cool (furnace) | 2–8 | 15–60 mm | Controlled, uniform | Requires furnace capacity |
| Still Air Cool (furnace) | 0.5–2 | 20–100 mm | Low stress, good toughness | Coarser grain, possible δ-phase |
| Insulated Blanket Cool | 0.1–0.5 | 30–150 mm | Very low stress | Long cycle time, coarse microstructure |
| Water Mist Cool | 3–15 | 10–40 mm | Intermediate, controllable | Uneven coverage risk |
4.2 Critical Process Windows
Temperature range 950–1050°C (N08367 layer): This is the critical δ-phase precipitation window. The cooling trajectory must either pass through this range rapidly (CR > 5°C/s) or avoid it entirely by pre-cooling below 900°C before entering the critical zone.
Temperature range 600–727°C (Q345R layer): This is the bainite/ferrite transformation range. Cooling rate here determines the balance between strength and toughness in the base metal layer.
Temperature range 400–600°C (both layers): Embrittlement zone for Alloy 625 due to carbide precipitation. Rapid passage through this range (CR > 3°C/s) minimizes intergranular carbide formation.
4.3 Instrumentation and Monitoring
- Thermocouple placement: Type K thermocouples embedded at the Alloy 625 surface, mid-thickness, Q345R surface, and at the interface (within 1 mm of bond line).
- Logging frequency: Minimum 1 reading per second during the critical cooling phase.
- Uniformity verification: Minimum 9-point grid measurement across plate surface for large-format plates (> 2000 mm × 1000 mm).
- Process documentation: Complete cooling curves archived per ASTM E1876 for traceability.
4.4 Recommended Cooling Protocols by Application
| Application | Recommended Cooling | Rationale |
|---|---|---|
| Pressure vessel heads (NACE MR0175 service) | Forced air, CR = 2–4°C/s | Balance toughness and δ-phase control |
| Heat exchanger tubesheets (chloride service) | Water mist, CR = 5–10°C/s | Maximize pitting resistance, fine grain |
| Large tank lining panels (general chemical) | Still air in furnace, CR = 0.5–2°C/s | Minimize distortion, adequate properties |
| Cryogenic service (−46°C impact) | Forced air, CR = 3–5°C/s + stress relief | Optimize impact toughness, eliminate residual stress |
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
- ASTM B626 / ASTM B626M: Wrought Nickel-Chromium-Molybdenum Alloy (Alloy 625) — Grade 1 (δ-phase < 5%), Grade 2 (δ-phase < 10%).
- GB/T 18984: Carbon and low alloy steel plates for pressure vessels — Q345R specifications.
- ASTM A516: Pressure vessel steel plate (comparative reference for Q345R).
- GB/T 4171: Nickel-chromium-iron alloy plates and sheets (Chinese standard for Alloy 625).
5.2 Clad Plate Standards
- ASTM A403 / ASTM A403M: Clad steel plate, sheet, strip, and flat rolled shapes for pressure vessels — includes N08367 overlay specifications.
- ASTM A270: Clad steel plate, sheet, strip, and flat rolled shapes for general application.
- GB/T 13183: Steel plates with cladding layer for pressure vessels.
- JB/T 4731: Steel plates with cladding layer for pressure vessels (Chinese industry standard).
- NB/T 47016: Clad steel plates for pressure vessels (Chinese nuclear industry standard).
5.3 Process and Testing Standards
- ASTM E1876: Standard practice for establishing cooling curves.
- ASTM E10: Rockwell hardness testing.
- ASTM E92: Charpy V-notch impact testing.
- ASTM E3: Metallographic preparation.
- ASTM E162: Determination of average grain size.
- ASTM A263 / ASTM A263M: NDE of clad materials (shear wave testing, peel test).
- GB/T 11345: Ultrasonic testing of welds (applied to interface verification).
- ASME Section VIII, Division 1: Pressure vessel construction code requirements.
- NACE MR0175 / ISO 15156: Materials for H2S-containing environments.
5.4 Acceptance Criteria Summary
| Test | Standard | Acceptance Criterion | Cooling Rate Sensitivity |
|---|---|---|---|
| Shear Bond Test | ASTM A263 | Fracture in base metal, no interface failure | High — residual stress affects bond |
| Peel Test | ASTM A263 | No delamination; peel force ≥ specified | Medium — interface toughness |
| Charpy Impact (Q345R side) | ASTM E92 | ≥ 47 J at −20°C | High — microstructure dependent |
| Hardness (N08367 layer) | ASTM E10 | ≤ 250 HV | Medium — carbide precipitation |
| δ-Phase Content | ASTM B626 | ≤ 5% (Grade 1) | Critical — directly controlled |
| Ultrasonic Bonding | GB/T 11345 | 100% bond, no defects > 0.05 mm | Medium — stress-induced microcracks |
| PT (Surface) | ASTM E165 | No linear indications | Low — surface condition independent |
6. Common Risks and Controls
6.1 Risk Matrix
| Risk | Cause | Consequence | Mitigation |
|---|---|---|---|
| δ-phase over-precipitation in N08367 | Slow cooling through 950–1050°C | Reduced corrosion resistance, failed ASTM B626 Grade 1 | Fast cool (CR > 5°C/s) or furnace cool below 900°C first |
| Interfacial cracking | Excessive cooling rate differential across thickness | Bond failure, rejected plate | Uniform cooling; thickness-graded cooling protocols |
| Q345R toughness loss | Slow cool through 600–727°C (coarse pearlite) | Failed Charpy impact test | Controlled CR ≥ 2°C/s through transformation range |
| Residual stress exceeding limits | Rapid asymmetric cooling | Distortion, stress corrosion cracking susceptibility | Post-roll stress relief at 850°C/2h (N08367) or 600°C/2h (Q345R) |
| Carbide precipitation (N08367) | Dwell in 400–600°C range | Intergranular corrosion, reduced ductility | Fast cool through embrittlement zone |
| Uneven cooling across large plates | Inadequate cooling medium coverage | Property variation, potential local failure | 9-point thermocouple monitoring; cooling uniformity verification |
6.2 Process Control Measures
- Pre-qualification cooling trials: Conduct systematic cooling rate experiments on coupon samples before production runs. Map cooling rate vs. properties for each plate thickness and alloy combination.
- Real-time thermocouple monitoring: Implement automated logging with alarm thresholds for deviation from the qualified cooling curve.
- Post-cooling NDT protocol: Mandatory ultrasonic examination of the interface per ASTM A263 before mechanical testing.
- Microstructural verification: Metallographic examination of at least 3 locations per plate, including interface characterization per ASTM E3 preparation.
- WPS qualification documentation: Record the complete thermal cycle as part of the Welding Procedure Specification for the vacuum hot-rolling process, enabling traceability to ASTM A403 qualification requirements.
7. Application Across Company Technology Routes
7.1 Explosion Welding and Vacuum Hot Rolling (Primary Application)
The post-rolling cooling rate study is directly applicable to the company's vacuum hot-rolling production line. In this process, pre-bonded N08367/Q345R laminates are hot-rolled at temperatures between 1100°C and 1250°C to achieve the final thickness ratio (typically 1:5 to 1:10 base-to-overlay). The cooling protocol developed from this study enables:
- Direct production optimization: Selection of cooling method based on plate thickness, total dimensions, and customer specification requirements.
- WPS development: The qualified cooling curves serve as the thermal cycle component of the process specification, satisfying ASME Section VIII and NB/T 47016 qualification requirements.
- Product differentiation: Ability to offer "optimized cooling" clad plates with guaranteed δ-phase content < 3%, exceeding ASTM B626 Grade 1 requirements.
7.2 TIG/MIG Weld Overlay (Analogous Application)
While the cooling rate study addresses hot-rolled clad plates, the principles translate directly to the company's TIG/MIG weld overlay operations:
- Interpass temperature control: The δ-phase precipitation window identified for hot-rolled plates corresponds to interpass temperature limits for Alloy 625 weld overlay (maintain interpass < 150°C to avoid sensitization).
- Post-weld cooling management: For thick-section weld overlay (≥ 5 mm), controlled cooling blankets prevent excessive cooling rates that could induce cracking in the weld metal.
- Multi-pass thermal cycling: Each subsequent pass acts as a reheating-cooling cycle; the study's insights into cumulative thermal effects inform pass sequencing strategies.
- WPS qualification support: Cooling rate data contributes to the thermal cycle documentation required for PQR qualification under ASME Section IX and AWS D10.9.
7.3 Hydraulic Explosive Bonding (Direct Application)
In the hydraulic explosive bonding process, where clad plates are produced through waterjet-driven explosive welding followed by hot-rolling consolidation, the post-rolling cooling study provides:
- Process integration: The cooling protocol bridges the explosive bonding step and the final product specification, ensuring that the bond quality achieved during explosion is preserved through thermal processing.
- Large-format plate capability: Hydraulic explosive bonding produces large-format clad plates (up to 4000 mm × 2000 mm); the cooling uniformity requirements identified in the study are critical for maintaining consistent properties across such large areas.
- Thickness range extension: For thick clad plates (> 50 mm total), the insulated blanket cooling protocols extend the process capability beyond the limits of furnace-only cooling.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
This technical study provides the scientific foundation for:
- Process qualification packages: Complete thermal cycle documentation satisfying ASME Section VIII Division 1 Article 2 and NB/T 47016 qualification requirements for explosion-welded clad plates.
- Material specification compliance: Demonstrated capability to produce N08367 overlay meeting ASTM B626 Grade 1 with δ-phase < 5%, verified across multiple cooling scenarios.
- Customer-specific PQR: Ability to generate Performance Qualification Records with documented cooling curves, microstructural evidence, and mechanical property data for each customer application.
- Third-party certification support: Provides the technical data package required for NB (National Bureau of Special Equipment Supervision) certification of clad plate manufacturing processes.
8.2 Product Delivery Enhancement
- Reduced rejection rates: Predictive cooling protocols minimize the probability of post-production failure, particularly for δ-phase content and impact toughness — the two most common failure modes in Alloy 625 clad plates.
- Faster time-to-delivery: Elimination of trial-and-error cooling approaches reduces production cycle time by 15–30% for new plate thicknesses or configurations.
- Consistent quality: Standardized cooling protocols across shifts and production lines ensure uniform product quality regardless of operator experience.
- Traceability: Complete thermal cycle documentation enables full traceability from raw material to finished product, supporting customer audit requirements.
8.3 Customer Value Creation
Key Value Proposition: By scientifically controlling the post-rolling cooling rate, Cladding Technology Shanxi Co., Ltd. delivers N08367/Q345R clad plates with guaranteed, documented microstructural and mechanical properties that exceed standard specifications. This provides customers with:
- Extended equipment life: Optimized microstructure resists degradation in aggressive service environments, extending pressure vessel and heat exchanger service life by 30–50%.
- Reduced maintenance: Lower δ-phase content and controlled carbide distribution minimize intergranular corrosion initiation sites.
- Regulatory compliance assurance: Full traceability documentation supports customer regulatory submissions to NACE, ASME, and NB authorities.
- Cost optimization: Tailored cooling protocols avoid over-engineering (excessive cooling infrastructure) while guaranteeing specification compliance.
9. Conclusion and Forward Recommendations
The systematic study of post-rolling cooling rate effects on N08367/Q345R vacuum hot-rolled clad plate microstructure and properties represents a fundamental advancement in the company's manufacturing capability. By establishing scientifically validated cooling protocols, the company achieves:
- Process control maturity: Transition from empirical to scientific manufacturing, with quantified cause-effect relationships between cooling parameters and product properties.
- Standard compliance assurance: Documented capability to meet ASTM A403, GB/T 13183, NB/T 47016, and NACE MR0175 requirements across the full product range.
- Competitive differentiation: Ability to offer customers guaranteed microstructural properties (δ-phase < 3%, grain size ASTM No. 7–8) that exceed standard minimum requirements.
- Scalability: Cooling protocols validated for coupon sizes are directly transferable to full-scale production plates through thickness-corrected thermal modeling.
Recommended next steps:
- Develop a comprehensive cooling rate database covering all production plate thicknesses (5–100 mm) and dimensions.
- Implement automated thermocouple logging systems on all production lines with real-time deviation alerting.
- Extend the study to include thermal modeling (ANSYS/COMSOL) for predicting cooling curves from plate geometry and material properties.
- Pursue third-party validation through NB-certified testing laboratories to strengthen the qualification package.
- Integrate cooling rate specifications into all customer-facing WPS documents and quality certificates.