Hot Rolling Process Effects on Weld Microstructure and Residual Stress in Stainless Steel Clad Plates
1. Definition and Fundamental Principles
The interaction between hot rolling processes and the weld zone of stainless steel clad plates represents a critical metallurgical phenomenon that directly governs the mechanical performance, fatigue resistance, and service life of composite materials. When a stainless steel clad plate—fabricated through weld overlay, explosion welding, or hydraulic explosive bonding—undergoes subsequent hot rolling operations (such as pipe forming, ring rolling, or plate re-rolling), the weld interface and heat-affected zone (HAZ) are subjected to severe thermomechanical cycling that fundamentally alters the microstructural evolution and residual stress state established during the initial cladding process.
The fundamental metallurgical principles at play include:
- Dynamic Recrystallization: During hot rolling, stored strain energy in the deformed microstructure drives nucleation and growth of new, strain-free grains. The critical temperature for dynamic recrystallization in austenitic stainless steels (e.g., 304, 316L, 321) typically ranges from 950°C to 1100°C, while ferritic and duplex grades exhibit lower thresholds.
- Precipitate Dissolution and Re-precipitation: Carbides (M₂₃C₆, Cr₇C₃), intermetallic phases (σ, χ), and nitrides that formed during welding or prior heat treatment may dissolve at elevated rolling temperatures and re-precipitate during subsequent cooling, altering local composition and phase stability.
- Thermal Residual Stress Relaxation: The high temperatures achieved during hot rolling (typically 800–1150°C depending on alloy grade) promote stress relaxation through creep mechanisms, dislocation climb, and grain boundary sliding, partially or fully relieving the residual stresses locked in during cladding.
- Mechanical Residual Stress Redistribution: Non-uniform deformation across the clad thickness—driven by differences in flow stress between the base steel, transition layer, and cladding layer—generates new mechanical residual stresses that superimpose on any remaining thermal residual stresses.
- Grain Boundary Migration and Texture Development: Hot rolling imposes a preferred crystallographic orientation (texture) that can influence anisotropic mechanical properties, crack propagation paths, and corrosion resistance at the clad interface.
2. Category and Business Positioning
This technical competency falls squarely within the post-cladding thermomechanical processing domain, serving as a critical knowledge bridge between the initial cladding fabrication process and the final product geometry. Within Cladding Technology Shanxi Co., Ltd.'s capability matrix, this understanding is essential for:
- Ensuring that subsequent forming operations (pipe rolling, ring rolling, plate leveling) do not compromise the metallurgical integrity of the weld overlay or explosion bond interface.
- Optimizing the interaction between hot rolling parameters and the pre-existing weld microstructure to achieve target mechanical properties in the final product.
- Providing metallurgical justification for product qualification under pressure vessel, piping, and heat exchanger codes.
- Reducing post-manufacture failure modes—particularly stress corrosion cracking (SCC), fatigue cracking, and hydrogen-induced cracking (HIC)—that originate at or near the clad interface.
From a business perspective, mastery of this subject area enables the company to deliver fully rolled and formed clad products (pipes, rings, rolled sections) with confidence in interface integrity, thereby expanding the addressable market beyond flat clad plate into high-value pipe and ring applications.
3. Technical Purpose and Value
The systematic study and application of hot rolling effects on clad plate weld microstructure and residual stress serves multiple strategic purposes:
3.1 Technical Purposes
- Microstructural Control: Predict and control the grain size, phase distribution, and precipitate state at the clad interface after hot rolling to ensure compliance with code requirements for toughness, ductility, and corrosion resistance.
- Residual Stress Management: Quantify the net residual stress state (thermal + mechanical) after hot rolling to assess risk of SCC, fatigue failure, and dimensional instability during downstream welding or machining.
- Process Window Definition: Establish the allowable hot rolling temperature, reduction ratio, cooling rate, and interpass temperature ranges that maintain clad interface integrity without inducing detrimental microstructural changes.
- WPS/PQR Qualification Support: Provide metallurgical data to support welding procedure qualifications where clad products are subsequently joined by welding in the field or in fabrication shops.
3.2 Business Value
- Enables acceptance of orders for hot-rolled clad pipes and rings that require demonstrated metallurgical control through the full processing chain.
- Reduces warranty claims and field failures by proactively addressing residual stress and microstructural concerns at the design and manufacturing stage.
- Strengthens the company's technical credibility with end-users in the oil & gas, power generation, and chemical processing industries.
- Supports the development of proprietary process specifications that differentiate the company's products in competitive bidding.
4. Key Process and Implementation Points
4.1 Critical Hot Rolling Parameters
| Parameter | Typical Range | Effect on Weld Microstructure | Effect on Residual Stress |
|---|---|---|---|
| Starting Rolling Temperature | 1000–1150°C (austenitic SS); 1050–1200°C (duplex SS) | Above recrystallization temperature: full grain refinement; below: partial recrystallization with retained elongated grains | Higher temperatures promote greater thermal stress relaxation via creep |
| Total Reduction Ratio | 10–30% per pass; cumulative 40–70% | Higher reductions increase stored energy, driving more complete recrystallization; excessive reduction may cause edge cracking | Higher reductions introduce greater mechanical residual stress due to differential strain |
| Rolling Speed | 0.5–3.0 m/s | Higher speeds increase adiabatic heating, potentially causing local overheating and grain coarsening in the weld zone | Higher speeds reduce time for stress relaxation during deformation |
| Interpass Temperature | 800–1050°C (maintained above recrystallization onset) | Too low: strain accumulation without recovery leads to brittle microstructure; too high: grain coarsening | Lower interpass temperatures preserve more mechanical residual stress |
| Cooling Rate After Rolling | Controlled (furnace cool) or air cool; avoid water quench | Slow cool: allows precipitate coarsening and possible σ-phase formation in susceptible alloys; rapid cool: retains fine grains but may increase residual stress | Faster cooling rates increase thermal residual stress gradients |
| Number of Rolling Passes | 2–6 passes depending on final geometry | Multiple passes with adequate interpass recovery produce more uniform grain structure | Multi-pass rolling with high interpass temperatures reduces net residual stress |
4.2 Microstructural Assessment Protocol
- Sample Preparation: Extract longitudinal and transverse specimens from the weld zone, HAZ, and base metal of both clad and base layers. Metallographic preparation following ASTM E3 with final polish to 0.05 µm alumina.
- Optical Microscopy (OM): Examine grain size, phase distribution, inclusion morphology, and any signs of intergranular attack or cracking at the clad interface. Etch with appropriate reagents (e.g., Beraha's reagent for austenitic SS, Vilella's reagent for duplex SS).
- Scanning Electron Microscopy (SEM) with EDS: Characterize precipitate phases, segregation at grain boundaries, and elemental distribution across the weld interface. Perform line scans across the clad/base boundary.
- X-Ray Diffraction (XRD): Identify phase fractions (austenite, ferrite, martensite, precipitates) and quantify lattice strain as an indicator of localized residual stress.
- Hardness Mapping: Perform Vickers hardness traverses (HV0.5) across the clad thickness at 0.5 mm intervals to identify soft zones, hard zones, and microstructural transitions.
4.3 Residual Stress Measurement Methods
| Method | Standard | Spatial Resolution | Depth Sensitivity | Applicability to Clad Plates |
|---|---|---|---|---|
| X-ray Diffraction (sin²ψ method) | ASTM E975 | ~0.1–0.5 mm | Surface to ~100 µm | Excellent for surface residual stress mapping across clad thickness |
| Hole Drilling | ASTM E837 | ~1–3 mm | Surface to ~1 mm | Good for bulk residual stress; requires sufficient material thickness |
| Neutron Diffraction | ASTM E1482 | ~0.5–1 mm | Up to 15–30 mm (material dependent) | Best for through-thickness residual stress profiles in thick clad plates |
| Slitting/Sectioning | ASTM E2228 | ~1 mm | Full thickness | Destructive; provides complete through-thickness residual stress profile |
| Contour Cracking | ASTM E3975 | ~0.5 mm | Full thickness | Useful for stress distribution mapping; semi-destructive |
4.4 Process Optimization Strategy
The following decision logic should guide hot rolling parameter selection for clad plates:
- Assess the initial clad interface condition: Determine the as-welded (or as-bonded) microstructure, residual stress state, and any pre-existing defects through NDT and metallurgical examination.
- Define the target post-rolling condition: Based on the applicable product standard and end-use requirements, specify target grain size, phase fraction, hardness range, and maximum allowable residual stress.
- Select the rolling temperature window: Ensure the rolling temperature is above the recrystallization temperature of both the base and cladding materials to achieve complete microstructural homogenization. Avoid temperatures that promote σ-phase or other brittle intermetallic precipitation.
- Control the reduction schedule: Distribute total reduction across multiple passes with adequate interpass recovery to minimize mechanical residual stress buildup and ensure uniform deformation.
- Specify the cooling protocol: Match cooling rate to the alloy's precipitation kinetics to avoid detrimental phase formation while achieving target hardness and toughness.
- Validate through post-rolling inspection: Perform full metallurgical examination and residual stress measurement to confirm compliance with target specifications.
5. Applicable Standards and Acceptance Criteria
5.1 Product Standards
- GB/T 24511-2017 — Steel and nickel alloys—Steel/overlay weld stainless steel clad plates and strips
- GB/T 17748-2017 — Steel and nickel alloys—Steel/stainless steel explosion-welded clad plates and strips
- ASTM A490 — Standard Specification for Steel/Overlay Weld Stainless Steel Clad Plates and Strips
- ASTM A270 — Standard Specification for Steel/Overlay Weld Stainless Steel Clad Plates and Strips (for pressure vessels)
- ASME SA-467 — Specification for Steel/Overlay Weld Stainless Steel Clad Plates and Strips for Pressure Vessels
- ASME SA-466 — Specification for Steel/Overlay Weld Carbon Steel/Stainless Steel Clad Plates and Strips for Pressure Vessels
- ASTM A240 — Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessel Applications
- NACE MR0175/ISO 15156 — Materials for Use in H₂S Environments in Oil and Gas Production
5.2 Residual Stress Acceptance Criteria
| Application Category | Standard Reference | Maximum Allowable Residual Stress | Measurement Location |
|---|---|---|---|
| Pressure vessels (general) | ASME BPVC Section VIII Div. 1, UW-44 | Not explicitly specified; stress relief recommended per UW-44 | Weld zone and HAZ |
| Pressure vessels (SS clad) | ASME BPVC Section VIII Div. 1, UCS-66 | Stress relief required when specified; residual stress <0.5 σ_y recommended | Clad interface and surface |
| Oil & gas pipelines | API 5L / ASME B31.4 | Longitudinal residual stress <0.4 σ_y for SCC-sensitive applications | Full circumference and longitudinal |
| Nuclear components | NB/T 20310 / RCC-E | Strict limits per specific component design; typically <0.3 σ_y | Full thickness profile |
| General industrial equipment | GB/T 24511 / GB/T 17748 | No explicit residual stress limit; stress relief recommended for SCC-prone environments | Surface and clad interface |
5.3 Metallurgical Acceptance Criteria
- Grain size: ASTM E112 grain size rating ≥ Grade 3 for the cladding layer (unless otherwise specified); no coarse-grained bands exceeding 3× the nominal grain size.
- Phase composition: For duplex stainless steel cladding, ferrite/austenite ratio within 35–65% (ASTM A240, UNS S31803/S32205); for austenitic grades, no martensite formation in the weld zone.
- Precipitate control: No continuous grain boundary σ-phase or χ-phase in the HAZ or weld metal (per NACE MR0175 sensitization requirements).
- Hardness: Cladding layer hardness within the range specified by the applicable product standard (typically 150–250 HV for austenitic grades; 250–350 HV for duplex grades).
- Interface integrity: No cracking, delamination, or lack of bond at the clad/base interface, verified by full-penetration radiography (ASME BPVC Section V, Article 2) or ultrasonic testing.
6. Common Risks and Controls
6.1 Microstructural Risks
| Risk | Mechanism | Consequence | Control Measures |
|---|---|---|---|
| σ-Phase Precipitation | Rolling in the 600–850°C range promotes σ-phase nucleation in Cr-rich austenitic and duplex grades | Severe embrittlement, loss of ductility, SCC susceptibility | Maintain rolling temperature >900°C; avoid prolonged exposure in the σ-phase precipitation window; apply post-rolling solution heat treatment if necessary |
| Grain Coarsening | Excessive rolling temperature or slow cooling allows grain growth beyond target specification | Reduced toughness, lower fatigue strength | Limit rolling temperature to minimum required for recrystallization; control cooling rate; verify grain size per ASTM E112 |
| Partial Recrystallization | Rolling temperature below recrystallization onset or insufficient strain per pass | Retained elongated grains, anisotropic properties, incomplete stress relief | Ensure rolling temperature ≥ T_rx + 50°C; maintain adequate reduction per pass (>15%); monitor interpass temperature |
| Carbon Segregation | Non-equilibrium cooling after rolling allows carbon to segregate to grain boundaries | Intergranular corrosion susceptibility, sensitization | Control cooling rate; apply stabilizing heat treatment (620–650°C for Ti-stabilized grades); verify via ASTM A262 Practice 1E or 5A |
| Phase Transformation (Duplex SS) | Excessive temperature or slow cooling shifts ferrite/austenite balance outside acceptable range | Loss of duplex characteristics, reduced pitting resistance | Monitor phase fraction via ASTM A240 requirements; control cooling rate; consider post-rolling solution treatment |
6.2 Residual Stress Risks
| Risk | Mechanism | Consequence | Control Measures |
|---|---|---|---|
| Incomplete Stress Relief | Rolling temperature insufficient for full stress relaxation; rapid cooling re-introduces thermal stresses | SCC initiation at clad interface; fatigue cracking | Verify residual stress via XRD or neutron diffraction; apply supplementary stress relief anneal (ASME UW-44) if required | Over-Stress Relief (Softening) | Excessive temperature or prolonged hold time causes grain growth and hardness reduction | Failure to meet minimum hardness or strength requirements | Limit stress relief temperature and duration; verify hardness after treatment |
| Non-Uniform Stress Distribution | Differential deformation between base and clad layers due to mismatch in flow stress | Localized high-stress concentrations at the clad interface; distortion | Use multi-pass rolling with controlled reduction per pass; model stress distribution using FEA; measure through-thickness stress profile |
| Hydrogen-Induced Cracking (HIC) | Residual tensile stress combined with hydrogen pickup during rolling (from moisture, lubricants) | Sub-surface cracking, particularly in high-strength base steels | Control rolling atmosphere; use dry lubricants; apply post-rolling bake-out per NACE MR0175; monitor residual stress |
6.3 Process Control Checklist
- Confirm hot rolling temperature is within the validated range for the specific alloy combination (base steel + cladding material).
- Verify that rolling lubricants are compatible with the clad material and will not introduce hydrogen contamination.
- Monitor and record rolling temperature, reduction ratio, and speed at each pass.
- Ensure interpass temperature remains above the minimum recrystallization temperature throughout the rolling sequence.
- Implement controlled cooling (furnace cool or air cool) to prevent re-introduction of high thermal residual stresses.
- Conduct post-rolling metallurgical examination: grain size, phase fraction, hardness profile, and precipitate assessment.
- Perform residual stress measurement at defined locations (surface, clad interface, mid-thickness) using validated methods.
- Apply supplementary stress relief treatment if measured residual stresses exceed acceptance criteria.
- Document all process parameters and inspection results in the product traceability record.
7. Application Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
In the TIG (Gas Tungsten Arc Welding) and MIG (Gas Metal Arc Welding) weld overlay process, the clad plate is built up by depositing successive layers of stainless steel filler metal onto a carbon or low-alloy steel base. The resulting weld structure exhibits a characteristic multi-pass weld with a fusion line at the base/weld interface and multiple interpass boundaries within the overlay.
Interaction with Hot Rolling:
- The multi-pass weld structure provides multiple grain refinement sites during hot rolling, as each pass boundary acts as a potential recrystallization nucleation site.
- The dilution zone at the base/weld interface (where base metal and weld metal mix) is particularly sensitive to hot rolling temperature, as the local composition determines the phase stability during deformation.
- Residual stresses from the multi-pass welding (typically high tensile stresses in the weld metal and compressive stresses in the base) are partially relieved during hot rolling, but the relief is rarely complete, especially in thicker sections.
- Post-rolling, the weld overlay clad plate may require a supplementary stress relief anneal per ASME BPVC Section VIII Div. 1 UW-44 to achieve residual stress levels acceptable for SCC-sensitive service.
Typical Application: Clad plates subsequently hot-rolled into pipe (per ASTM A335 or ASME SA-234) or formed into pressure vessel components. The weld overlay route offers flexibility in clad thickness (3–25 mm) and alloy selection, making it suitable for a wide range of hot-rolled product geometries.
7.2 Hydraulic Explosive Bonding (Hydrodynamic Explosive Welding) Route
Hydraulic explosive bonding (also known as hydrodynamic welding or hydraulic explosion welding) uses a shaped charge to generate a controlled detonation that propels the cladding material onto the base plate at high velocity (typically 300–700 m/s), achieving metallurgical bonding through jetting and plastic deformation at the interface.
Interaction with Hot Rolling:
- The explosive bond interface is characterized by a wavy or lobed morphology with high-strain jetting zones and low-strain regions. During hot rolling, the high-strain zones are preferential sites for recrystallization, potentially flattening the interface morphology.
- The extreme plastic deformation at the explosive bond interface creates a zone of refined grains and high dislocation density that is thermodynamically unstable. Hot rolling temperatures above the recrystallization temperature will fully homogenize this zone, potentially eliminating the characteristic wavy interface.
- Residual stresses from the explosive bonding process are complex, with high compressive stresses in the cladding layer and tensile stresses in the base. Hot rolling provides significant stress relief but may introduce new mechanical stresses due to differential deformation.
- The interface microstructure after hot rolling must be carefully evaluated to ensure that the bond strength is maintained (typically >60 MPa shear strength per ASTM A720 or equivalent).
Typical Application: Clad plates for hot-rolled pipe production where a thin, uniform cladding layer (0.5–3 mm) is required with minimal dilution. The hydraulic explosive bonding route is particularly suited for long-length products where weld overlay would be impractical.
7.3 Explosion Welding Route
Traditional explosion welding uses a high explosive charge to propel a cladding plate onto a base plate at supersonic velocities (typically 500–1000 m/s), achieving metallurgical bonding through intense plastic deformation, adiabatic shear, and jetting at the collision interface.
Interaction with Hot Rolling:
- The explosion weld interface exhibits the most extreme microstructural refinement of the three routes, with grain sizes as small as 100–500 nm in the jetting zones. Hot rolling will completely recrystallize these ultrafine grains, resulting in a more conventional grain structure.
- The high energy input from explosion welding creates a wide HAZ with significant microstructural changes in both the base and cladding materials. Hot rolling provides an opportunity to homogenize this HAZ through dynamic recrystallization.
- Residual stresses from explosion welding are typically very high (approaching yield strength in some regions) due to the rapid deceleration of the cladding plate upon impact. Hot rolling is highly effective at relieving these stresses, provided temperatures exceed the recrystallization range.
- Post-hot-rolling, the explosion weld interface must be evaluated for bond integrity, as the recrystallized microstructure may differ significantly from the as-welded condition. Bond strength testing per ASTM A720 or GB/T 24511 is essential.
Typical Application: Thick clad plates (cladding thickness 1–10 mm) for pressure vessel and heat exchanger applications where the product is subsequently hot-rolled into pipe, ring, or formed shapes. The explosion welding route offers superior bond strength and minimal dilution, making it ideal for applications requiring high-performance cladding interfaces.
7.4 Comparative Summary
| Characteristic | TIG/MIG Weld Overlay | Hydraulic Explosive Bonding | Explosion Welding |
|---|---|---|---|
| Interface Morphology (As-Built) | Flat fusion line with dilution zone | Wavy/lobed with jetting zones | Wavy with intense jetting and deformation |
| Interface Grain Size (As-Built) | 10–100 µm (weld microstructure) | 1–10 µm (deformation refined) | 0.1–1 µm (ultrafine in jetting zones) |
| Post-Hot Rolling Interface | Recrystallized weld grain structure; possible dilution zone homogenization | Recrystallized; wavy morphology may be reduced | Fully recrystallized; conventional grain size achieved |
| Residual Stress Relief by Hot Rolling | Partial to significant relief (depends on temperature and cooling) | Significant relief; new mechanical stresses possible | Complete relief of welding residual stresses; new mechanical stresses introduced |
| Key Risk After Hot Rolling | Incomplete stress relief; sensitization of dilution zone | Interface flattening; potential bond weakening | Loss of ultrafine grain benefits; interface homogenization |
| Supplementary Treatment Often Required | Stress relief anneal (ASME UW-44) | Usually not required if rolling parameters are optimized | Usually not required; stress relief if code-specified |
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR Development: Understanding hot rolling effects enables the development of welding procedure qualifications that account for post-weld thermomechanical processing. This is essential for qualifying welding procedures for clad products that will undergo hot rolling before final welding.
- Material Qualification: Metallurgical data from hot rolling studies supports material qualification for specific service conditions (e.g., high-temperature creep, low-temperature impact, cyclic loading). This data is required for ASME Section VIII Division 2, Section III, and NB/T nuclear component qualifications.
- Process Certification: Demonstrated control over hot rolling parameters and post-rolling metallurgical outcomes supports ISO 9001, ISO 3834, and ASME "N" or "R" stamp certification requirements for welding and fabrication processes.
8.2 Product Delivery
- Reduced Rework: By predicting and controlling microstructural and residual stress outcomes of hot rolling, the company minimizes the need for post-manufacture corrective actions (stress relief re-treatment, machining allowances, re-inspection).
- Accelerated Delivery: Pre-validated hot rolling process windows allow for parallel processing of multiple products without individual trial-and-error optimization for each order.
- Consistent Quality: Standardized hot rolling procedures based on metallurgical understanding ensure batch-to-batch consistency, reducing quality variation and customer rejection rates.
- Expanded Product Range: Technical confidence in hot rolling outcomes enables the company to accept orders for complex geometries (small-diameter pipes, thick rings, multi-curve sections) that require aggressive hot rolling parameters.
8.3 Customer Value
- Performance Assurance: Customers receive clad products with documented metallurgical control through the entire manufacturing chain, providing confidence in long-term service performance.
- Compliance Documentation: Comprehensive metallurgical and residual stress data packages support customer compliance with their own code and regulatory requirements (ASME, NB/T, API, NACE).
- Design Optimization: Metallurgical insights enable customers to optimize their design for material utilization, weight reduction, and performance enhancement, knowing the clad interface will maintain integrity through subsequent processing.
- Risk Mitigation: Proactive identification and control of SCC, fatigue, and HIC risks at the manufacturing stage reduces the customer's operational risk and potential for unplanned shutdowns.
- Technical Partnership: Demonstrated expertise in hot rolling metallurgy positions the company as a technical partner rather than a simple supplier, fostering long-term customer relationships and premium pricing.
9. Conclusion
The systematic understanding of hot rolling process effects on stainless steel clad plate weld microstructure and residual stress represents a foundational technical competency for Cladding Technology Shanxi Co., Ltd. This knowledge bridges the gap between initial clad fabrication and final product performance, enabling the company to deliver metallurgically sound products across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—regardless of subsequent hot rolling requirements.
By establishing validated process windows, comprehensive inspection protocols, and rigorous acceptance criteria aligned with international standards (GB, ASTM, ASME, API, NACE, ISO, NB/T), the company ensures that every clad product meets the highest standards of metallurgical integrity and residual stress control. This technical depth not only supports qualification building and code compliance but also delivers measurable value to customers through reduced risk, enhanced performance, and reliable long-term service in demanding industrial applications.
Continued investment in this technical area—through research, process optimization, and personnel training—will further strengthen the company's competitive position in the global clad materials market and ensure sustained growth in high-value applications across the oil & gas, power generation, chemical processing, and nuclear industries.