Effect of Final Rolling Temperature on Microstructure and Properties of Stainless Steel/Carbon Steel Clad Plate
1. Definition and Fundamental Principles
1.1 Technical Definition
The study of final rolling temperature (FRT) effects on stainless steel/carbon steel clad plate addresses the critical metallurgical window during hot rolling and thermomechanical processing of bimetallic composite materials. Final rolling temperature refers to the temperature at which the last deformation pass is completed in the rolling mill, directly governing the recrystallization behavior, grain morphology, and interfacial bonding quality at the clad interface. For stainless steel/carbon steel clad plate systems—such as 304L/CS, 316L/CS, 904L/CS, or duplex 2205/CS combinations—the FRT determines whether the interface achieves sufficient metallurgical bonding without excessive intermetallic compound formation or grain coarsening.
1.2 Metallurgical Principles
The formation of a sound clad interface depends on several thermodynamic and kinetic factors that are directly influenced by the final rolling temperature:
- Recrystallization behavior: Austenitic stainless steels (e.g., 304L, 316L) exhibit a lower recrystallization temperature range (approximately 700–900°C) compared to carbon steels (typically 600–750°C). The FRT must be selected to ensure complete recrystallization of the stainless layer while avoiding excessive grain growth.
- Interfacial diffusion: At elevated temperatures, carbon migration from the carbon steel substrate into the stainless steel cladding layer occurs, forming a decarburization zone. The FRT controls the width and composition gradient of this transition zone.
- Interface bonding mechanism: Metallurgical bonding at the clad interface is achieved through atomic diffusion, interlocking deformation, and formation of iron-based solid solutions. The FRT determines the extent of atomic interdiffusion and the mechanical interlocking achieved during the final deformation passes.
- Phase stability: In the temperature range of 800–1100°C, sensitization risks arise in chromium-based stainless steels, where chromium carbides (Cr₂₃C₆, Cr₇C₃) precipitate at grain boundaries, depleting local chromium content and reducing corrosion resistance.
2. Category and Business Positioning
2.1 Technical Classification
This technical capability falls within the domain of process metallurgy and thermomechanical processing optimization for clad plate manufacturing. It represents the foundational knowledge base that enables Cladding Technology Shanxi Co., Ltd. to:
- Design and qualify rolling schedules for various stainless steel/carbon steel clad plate combinations
- Provide WPS (Welding Procedure Specification) data for post-cladding welding operations by understanding the base material microstructure
- Support NDT interpretation by predicting expected microstructural features and potential defect signatures
- Deliver certified products meeting ASTM, ASME, and API specifications
2.2 Strategic Value to the Company
Understanding the FRT-microstructure-property relationship positions the company as a technically capable manufacturer rather than a simple processor. This knowledge directly contributes to:
- Product differentiation: Ability to deliver clad plates with tailored properties (toughness, corrosion resistance, formability) for specific end-use applications
- Customer confidence: Technical documentation supporting qualification packages for critical infrastructure projects
- Quality assurance: Predictive capability for detecting and preventing quality deviations before they manifest as non-conformances
3. Technical Purpose and Value
3.1 Primary Objectives
The investigation of final rolling temperature effects serves the following engineering objectives:
- Optimize interface bond strength: Achieve peel test and shear test results exceeding ASTM A491/A491M and ASME Section IX qualification requirements
- Minimize decarburization zone width: Keep the carbon-depleted zone within acceptable limits (typically <0.5 mm) to preserve corrosion resistance at the interface
- Control grain size: Maintain austenitic grain size within ASTM E112 Grade 4–6 for the stainless layer and Grade 6–8 for the carbon steel substrate
- Prevent sensitization: Avoid chromium carbide precipitation at grain boundaries by controlling time-temperature exposure during final rolling passes
- Ensure mechanical property uniformity: Achieve consistent tensile, hardness, and impact properties across the plate thickness and width
3.2 Quantifiable Value Metrics
- Reduction in non-conformance rates by 30–50% through optimized rolling parameters
- Capability to qualify additional material combinations (e.g., 904L/16Mn, 2205/Q345R) with documented FRT windows
- Shortened qualification timelines for new product developments by leveraging established FRT-property correlations
- Enhanced weldability of clad products due to controlled base metal microstructure
4. Key Process and Implementation Points
4.1 Critical FRT Ranges for Common Clad Combinations
| Clad Combination | Recommended FRT Range (°C) | Minimum FRT (°C) | Maximum FRT (°C) | Key Risk Below Min | Key Risk Above Max |
|---|---|---|---|---|---|
| 304L / Q235 | 950–1100 | 900 | 1150 | Insufficient bonding; cold cracks | Excessive grain growth; decarburization |
| 316L / Q345R | 950–1080 | 900 | 1130 | Incomplete recrystallization | Sensitization; intermetallic formation |
| 2205 / Q345R | 1050–1150 | 1000 | 1200 | Phase separation; poor ductility | σ-phase formation; excessive oxidation |
| 904L / 16Mn | 1000–1100 | 950 | 1150 | Insufficient diffusion bonding | Cu enrichment; grain coarsening |
| 309L / 20G (transition) | 950–1050 | 900 | 1100 | Brittle interface | Excessive softening of carbon steel |
4.2 Implementation Parameters and Control Points
| Control Parameter | Target Value | Measurement Method | Acceptance Criteria |
|---|---|---|---|
| Final rolling temperature | Per material-specific schedule | Thermocouple at plate surface + model calculation | ±25°C of target |
| Total reduction in final stand | 15–30% | Roller diameter measurement | ≥15% for bonding assurance |
| Cooling rate after final pass | 50–100°C/min (air cooling) | Surface thermocouple monitoring | Controlled to prevent martensite in stainless |
| Interface bond strength (peel test) | ≥85% of cladding layer tensile strength | ASTM A491/A491M Section 8 | No separation at interface |
| Decarburization zone width | ≤0.5 mm | Macro-etching with 5% NaCl/H₂O₂ | Per ASTM A491/A491M |
| Grain size (stainless layer) | ASTM E112 Grade 4–6 | ASTM E112 comparison charts | Uniform across thickness |
4.3 Process Optimization Sequence
- Step 1 – Thermodynamic modeling: Calculate the austenitization temperature (Ac₃) for the carbon steel substrate and the recrystallization temperature (Tᵣ) for the stainless steel cladding layer. The FRT must exceed both thresholds.
- Step 2 – Rolling schedule design: Determine the number of passes, reduction per pass, and inter-pass temperature to maintain the plate within the target FRT window. Typically, a 3–5 pass final stand configuration is employed.
- Step 3 – Interface temperature monitoring: Since the interface temperature differs from surface temperatures due to thermal gradients through the plate thickness, thermal modeling (using software such as DEFORM or Abaqus) is essential to ensure the interface reaches the bonding temperature.
- Step 4 – Post-rolling cooling control: Implement controlled cooling (air cooling, accelerated cooling, or water quenching depending on the grade) to achieve the desired microstructure without introducing residual stresses or phase transformations detrimental to service performance.
- Step 5 – Verification testing: Conduct peel/shear testing, metallographic examination, hardness profiling, and mechanical testing per ASTM A491/A491M requirements.
5. Applicable Standards and Acceptance Criteria
5.1 Product Standards
- ASTM A491/A491M: Standard Specification for Steel-Clad Steel Plate, Sheet, and Strip — Primary product standard governing clad plate requirements including interface bonding, decarburization limits, and mechanical properties
- ASME SA-466/SA-466M: Specification for Steel Clad Plate, Sheet, and Strip for Pressure Vessel Applications
- GB/T 12770: Chinese national standard for steel-clad steel plates
- API 510/530: Requirements for clad piping components in pressure piping applications
- ISO 4989: Steel clad plates — Technical delivery conditions
5.2 Testing and Acceptance Standards
- ASTM A491/A491M Section 8: Peel test and shear test methods for interface bond verification
- ASTM E112: Standard Test Method for Determining Average Grain Size — Grain size assessment
- ASTM E10/E92: Rockwell and Brinell hardness testing for hardness profiling across the clad thickness
- ASTM A370: Mechanical testing methods (tensile, impact) for both layers
- ASME Section V: Non-destructive examination methods (MT, PT, UT) for surface and internal quality
- ASME Section IX: Welding qualification requirements for subsequent welding operations on clad materials
- GB/T 24511: Chinese standard for ultrasonic testing of clad plates
- NACE SP0437: When clad products are used in cathodic protection applications, interface integrity requirements
5.3 Key Acceptance Criteria Summary
| Test Parameter | Stainless Steel Layer | Carbon Steel Layer | Interface |
|---|---|---|---|
| Tensile strength (MPa) | ≥515 (304L) | Per base material spec | — |
| Yield strength (MPa) | ≥205 (304L) | Per base material spec | — |
| Elongation (%) | ≥40 | Per base material spec | — |
| Hardness (HB) | ≤200 | Per base material spec | Gradual transition, no abrupt change |
| Peel test | — | No separation; failure in base metal or weld | |
| Decarburization zone | — | — | ≤0.5 mm width |
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause (FRT Related) | Detection Method | Control Measure |
|---|---|---|---|
| Interfacial cracking | FRT below minimum bonding temperature; insufficient deformation | MT (ASTM E709), Peel test | Ensure FRT ≥900°C; minimum 15% reduction in final pass |
| Excessive decarburization | FRT above 1150°C; prolonged exposure at high temperature | Macro-etching, microhardness profile | Limit FRT to ≤1100°C; minimize inter-pass time |
| Sensitization (chromium depletion) | FRT in 800–1050°C range with slow cooling | ASTM A262 (salt spray test), grain boundary etching | Avoid FRT in sensitization range; implement rapid cooling if unavoidable |
| Grain coarsening | FRT >1150°C; excessive holding time | ASTM E112 grain size measurement | Cap FRT at 1100°C; optimize rolling speed |
| σ-phase formation (duplex steels) | FRT >1200°C or slow cooling through 600–900°C | Optical microscopy with specific etchants | Limit FRT to ≤1150°C for 2205; control cooling rate |
| Intermetallic compound formation | FRT >1150°C with extended interface exposure | SEM/EDS analysis of interface | Minimize time above 1100°C; control rolling schedule |
6.2 Process Risks
- Thermal gradient risk: The interface temperature may be 50–100°C lower than surface temperature for thick plates (>50 mm). Control: Use thermal modeling to calculate interface temperature; increase deformation strain in final passes to compensate.
- Edge effects: Plate edges cool faster than the center, creating non-uniform FRT across the width. Control: Implement edge temperature compensation in rolling mill design; perform testing at multiple locations across plate width.
- Scale formation: Excessive oxidation at high FRT degrades surface quality and interface cleanliness. Control: Implement controlled atmosphere rolling or pickling after rolling; limit FRT to minimize oxidation kinetics.
- Residual stress accumulation: Non-uniform cooling after rolling creates residual stresses that may cause distortion during subsequent fabrication. Control: Implement stress relief annealing (for carbon steel layer at 550–650°C) or controlled cooling.
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Applications
The FRT optimization knowledge directly supports the company's TIG and MIG weld overlay capabilities in the following ways:
- Base material understanding: Knowledge of the clad plate microstructure resulting from specific FRT conditions enables proper selection of filler metals and welding parameters. For example, a clad plate produced at FRT of 1050°C with fine austenitic grain structure will have different weldability characteristics than one produced at 1150°C with coarser grains.
- WPS qualification: The base material heat input sensitivity, determined by FRT-controlled microstructure, directly influences the qualified heat input range for welding procedures. Lower FRT (finer grain) generally permits higher heat input before issues arise.
- Transition layer design: For dissimilar metal weld overlays on clad plates, the FRT-determined interface condition guides the selection of transition layers (e.g., 309L, 309Cb) and their thickness to prevent cracking.
- Post-weld heat treatment planning: The existing microstructure from FRT processing determines whether PWHT is required and at what temperature to avoid sensitization or over-tempering of the carbon steel substrate.
7.2 Hydraulic Explosive Bonding Applications
While hydraulic explosive bonding (hydroforming/press bonding) is primarily a cold or warm working process, the FRT knowledge of the parent plates is essential:
- Material selection for bonding: The parent stainless steel and carbon steel plates must be in a condition (determined by their own FRT during production) that permits successful bonding under hydraulic pressure. Annealed condition (FRT followed by full anneal) is typically required for maximum formability.
- Post-bonding processing: If the bonded product requires subsequent hot working or heat treatment, the FRT knowledge ensures that the bonding interface will not be compromised during thermal processing.
- Property matching: The mechanical properties of the bonded product must be compatible with the base materials' properties, which are ultimately determined by their rolling and FRT history.
- Quality verification: Understanding the expected microstructure at the interface (from FRT-controlled parent plate conditions) aids in interpreting NDT results and metallographic examinations of the bonded product.
7.3 Explosion Welding Applications
Explosion welding (explosive cladding) creates the clad interface through high-velocity collision and plastic deformation. FRT knowledge contributes as follows:
- Parent plate preparation: The cladding sheet and base plate must be in appropriate conditions (hot rolled, annealed, or normalized) before explosion welding. FRT knowledge ensures the parent plates have adequate ductility and deformation capability for successful bonding.
- Post-explosion welding rolling: Explosion-welded clad plates often require subsequent hot rolling to achieve final dimensions. The FRT during this post-explosion rolling pass is critical for maintaining the explosion-welded interface quality while achieving dimensional accuracy.
- Interface metallurgy prediction: The solid-state bonding in explosion welding creates a wave-patterned interface. Subsequent thermal processing (including the final rolling temperature) affects the stability of this interface and potential interdiffusion.
- Combination processes: For thick clad plates, explosion welding may be followed by hot rolling. The FRT during the hot rolling stage must be carefully controlled to maintain the explosion-welded bond while achieving the required final properties.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Package Enhancement
The technical knowledge documented in this study directly strengthens the company's qualification packages submitted to customers and third-party inspection agencies:
- Process capability documentation: Demonstrates understanding of the metallurgical basis for product quality, supporting ASME "U" stamp or "S" stamp qualification applications
- WPS/PQR support: Provides metallurgical justification for welding procedure selections, particularly regarding base material condition and its effect on weld performance
- Material traceability: Enables the company to document FRT parameters for each production lot, supporting traceability requirements under ASME Section VIII and API standards
- Customer audit readiness: Provides technical documentation demonstrating systematic process control, essential for passing customer quality audits (ISO 9001, ISO 3834, ASME NQA-1)
8.2 Product Delivery Excellence
- Consistent quality: FRT-controlled production ensures lot-to-lot consistency in mechanical properties, corrosion resistance, and formability
- Reduced rework: Prevention of metallurgical defects through FRT optimization reduces non-conformance rates and associated costs
- Faster delivery: Established FRT windows for common material combinations eliminate the need for trial-and-error processing, accelerating production schedules
- Custom solutions: Ability to tailor FRT parameters for specific customer requirements (e.g., higher toughness, better corrosion resistance, improved formability)
8.3 Customer Value Proposition
"Our expertise in final rolling temperature optimization enables us to deliver clad plate products with precisely controlled microstructures that meet or exceed ASTM A491/A491M requirements. This translates to longer service life, reduced maintenance costs, and lower total cost of ownership for our customers' pressure vessels, heat exchangers, and corrosion-resistant piping systems."
9. Conclusion and Recommendations
9.1 Key Takeaways
- Final rolling temperature is the single most influential parameter in determining the microstructure and properties of stainless steel/carbon steel clad plate interfaces
- Each clad combination requires a specific FRT window that balances bonding requirements against sensitization and grain growth risks
- The FRT knowledge base directly supports all three of the company's technology routes (weld overlay, hydraulic bonding, explosion welding) by ensuring parent material quality and post-processing compatibility
- Systematic FRT control is essential for qualification building, regulatory compliance, and customer confidence
9.2 Recommendations for Continued Development
- Establish a comprehensive FRT database for all material combinations produced by the company, with corresponding microstructure and property data
- Implement online thermal monitoring systems (infrared thermography) for real-time FRT measurement and feedback control
- Develop predictive models correlating FRT parameters with final product properties for rapid qualification of new material combinations
- Conduct periodic validation testing (peel, shear, metallographic) to verify ongoing process capability
- Train production personnel on the metallurgical significance of FRT to ensure awareness and compliance during routine operations