Post-Rolling Heat Treatment Effects on 316L/Q370qE Stainless Steel Composite Plate Microstructure and Properties
1. Definition and Technical Overview
Post-rolling heat treatment (PRHT) is a critical metallurgical process applied to bimetallic composite plates after the rolling bond is established. In the context of 316L/Q370qE stainless steel composite plate manufacturing, this process involves controlled thermal cycles—typically solution annealing, normalizing, or stress-relief treatments—designed to optimize the microstructure, mechanical properties, and bonding integrity of both the austenitic stainless steel cladding layer (316L) and the carbon steel base layer (Q370qE). The primary objective is to eliminate residual stresses accumulated during the hot or cold rolling process, refine the grain structure at the metallurgical interface, and ensure compliance with the mechanical property requirements specified in applicable standards such as GB/T 8195, ASTM A491, and ASME SA-466.
The 316L/Q370qE composite plate represents a high-performance combination: 316L provides superior corrosion resistance in chloride-containing and acidic environments, while Q370qE offers high-strength structural integrity suitable for pressure vessels, heat exchangers, and process equipment operating under elevated temperatures and pressures. The post-rolling heat treatment serves as the final metallurgical refinement step that bridges the gap between the as-rolled condition and the certified deliverable condition.
2. Technical Purpose and Value
2.1 Microstructural Optimization
The rolling process—whether hot rolling (above the recrystallization temperature) or cold rolling (below the recrystallization temperature)—introduces significant metallurgical complexity at the interface region. As-rolled composite plates typically exhibit:
- Interface deformation zone: A narrow region (typically 50–200 μm) where severe plastic deformation creates a gradient of dislocation density and grain refinement
- Residual stress fields: Compressive stresses in the cladding layer and tensile stresses in the base layer due to differential thermal expansion coefficients (α of 316L ≈ 16–17 μm/m·K vs. Q370qE ≈ 12–13 μm/m·K)
- Non-uniform grain structure: Elongated grains along the rolling direction in the base layer and potentially work-hardened austenite in the 316L cladding
- Phase instability: In the carbon steel base, untempered martensite or bainite may form if the cooling rate after rolling exceeds critical thresholds
2.2 Mechanical Property Enhancement
Post-rolling heat treatment addresses the following performance gaps:
- Base layer toughness: Normalizing or quench-and-temper cycles convert brittle phases to tempered martensite or fine pearlite-ferrite, improving Charpy V-notch impact energy at service temperatures
- Cladding layer ductility: Solution annealing (1050–1150°C) restores full austenitic ductility after cold-rolling work hardening, ensuring the cladding layer can withstand forming operations
- Interface bond strength: Controlled diffusion at the interface during PRHT promotes atomic interdiffusion, increasing shear bond strength from typical as-rolled values of 120–180 MPa to 200–320 MPa in the heat-treated condition
- Dimensional stability: Stress relief eliminates residual stresses that could cause distortion during subsequent fabrication (cutting, forming, welding)
2.3 Corrosion Performance Assurance
For 316L cladding, post-rolling heat treatment ensures:
- Complete solution of carbides (Cr₂₃C₆, Cr₇C₃) at grain boundaries, preventing intergranular corrosion susceptibility
- Uniform distribution of Ni, Mo, and Cr in solid solution, maintaining the passive film stability
- Elimination of cold-work-induced chromium depletion zones that could initiate pitting
3. Key Process and Implementation Points
3.1 Heat Treatment Cycle Selection
| Process Type | Temperature Range | Soak Time (per 25mm thickness) | Cooling Method | Primary Purpose |
|---|---|---|---|---|
| Solution Annealing (316L focus) | 1050–1150°C | 30 min | Water quench or rapid air cool | Carbide dissolution, ductility restoration |
| Normalizing (Q370qE focus) | 880–950°C | 20–30 min | Controlled air cool | Grain refinement, uniform microstructure |
| Tempering (Post-normalizing) | 550–650°C | 2× (1.5–2 hr) | Furnace cool below 300°C | Toughness improvement, stress relief |
| Stress Relief | 550–650°C | 1.5–2 hr | Furnace cool below 300°C | Residual stress elimination |
| Combined Normalizing + Temper | 920°C → 600°C | 30 min → 2 hr | Air cool → furnace cool | Full property optimization for Q370qE |
3.2 Critical Processing Parameters
The following parameters must be rigorously controlled to achieve acceptable results:
- Heating Rate: 100–150°C/hr for plates ≤50mm thick; 50–100°C/hr for plates >50mm thick. Excessive heating rates create thermal gradients that induce interfacial cracking due to differential expansion.
- Peak Temperature Uniformity: ±25°C across the furnace zone. Thermocouples must be placed at the cladding surface, mid-thickness, and base surface to verify uniformity.
- Atmosphere Control: For 316L cladding, a protective atmosphere (endothermic gas with dew point < -40°C, or vacuum below 10⁻² Pa) is essential to prevent oxidation and carburization of the cladding surface.
- Cooling Rate Control: For solution annealing of 316L, cooling through the sensitization range (800–500°C) must be rapid (<50°C/hr) to prevent chromium carbide precipitation. For Q370qE base, controlled air cooling ensures adequate toughness without forming coarse pearlite.
- Final Cooling: Furnace cool below 300°C before removal to prevent new residual stresses from differential cooling.
3.3 Microstructural Monitoring
Post-PRHT metallurgical examination should include:
- Macro-etch examination: Full thickness section to verify bond line continuity and absence of delamination
- Interface microscopy (500×–1000×):strong> Assessment of interface morphology, carbide formation, and diffusion zone width
- Hardness mapping: Transverse hardness profile across the full thickness to verify gradient uniformity and absence of localized soft/hard zones
- Grain size measurement: ASTM E112 grain size in both 316L cladding (target: ASTM 3–6) and Q370qE base (target: ASTM 4–7)
4. Applicable Standards and Acceptance Criteria
4.1 Material Standards
| Standard | Material | Key Requirements Post-PRHT |
|---|---|---|
| GB/T 8195-2018 | Composite plate general | Bond strength ≥ specified value; mechanical properties of each layer meet individual material standards |
| ASTM A491/A491M | Composite plate (general) | Shear test qualification; individual layer properties verified |
| ASME SA-466 | Composite plate for pressure vessels | Full NDE; property verification per section IX |
| GB/T 14976 / ASTM A270 | 316L cladding properties | Tensile: Rm ≥ 485 MPa, A ≥ 40%; Hardness ≤ 200 HBW |
| GB/T 1591 / Q370qE specification | Q370qE base properties | Tensile: Rm 490–650 MPa, ReL ≥ 370 MPa; Charpy KV ≥ 41 J at -20°C |
4.2 Bond Strength Acceptance
Post-PRHT shear bond strength requirements per GB/T 8195 and ASTM A491:
- Shear test specimens (per ASTM A491/A491M Section 6): Minimum shear strength ≥ 200 MPa for 316L/Q370qE combinations
- Delamination test (per ASME SA-466): No separation at the interface under specified hydraulic or mechanical loading
- Macro-etch: No visible cracks, voids, or lack of fusion at the bond line across the full specimen length
4.3 Non-Destructive Examination
- Ultrasonic Testing (UT): Per ASTM E164 or GB/T 11345, full coverage of the composite plate surface; no indications exceeding acceptance threshold at the bond line
- Magnetic Particle Testing (MT): Per ASTM E709, applied to the base layer surface to detect surface-breaking defects
- Eddy Current Testing (ET): Optional for 316L cladding surface inspection of subsurface defects
5. Common Risks and Controls
5.1 Interface Delamination
Risk: Excessive heating rates or temperature gradients create differential thermal expansion between the 316L cladding and Q370qE base, potentially causing interface separation, particularly at pre-existing weak spots from the rolling process.
Controls:
- Implement step-wise heating: 50°C/hr to 300°C, then 100°C/hr to 600°C, then normal rate to peak
- Use sandwich heating configurations with independent zone control
- Conduct UT inspection immediately after PRHT to detect any interface separation
- Pre-PRHT bond strength verification on coupon specimens to confirm adequate rolling bond quality
5.2 Sensitization of 316L Cladding
Risk: Slow cooling through the 800–500°C range causes chromium carbide (Cr₂₃C₆) precipitation at austenite grain boundaries, depleting Cr locally and reducing pitting corrosion resistance below the required threshold per ASTM A240.
Controls:
- Water quench or high-velocity air cool for solution annealing cycles
- Verify post-PRHT intergranular corrosion resistance per ASTM A262 Practice No. 1E or Practice No. 4E (5% HCl, 36h, no cracking)
- Hardness verification: 316L cladding should remain ≤ 200 HBW; values above 220 HBW indicate possible sensitization
5.3 Base Layer Embrittlement
Risk: Inadequate tempering after normalizing, or excessive tempering temperatures, can result in either retained brittleness (untempered martensite) or over-tempered microstructure with reduced strength below Q370qE requirements.
Controls:
- Two-step tempering (600°C × 2 hr + 550°C × 1.5 hr) to ensure uniform property distribution through thickness
- Charpy V-notch testing per ASTM E23 at -20°C: KV ≥ 41 J for Q370qE qualification
- Transverse hardness profile: 285–340 HBW uniform across base layer thickness
5.4 Carbon Contamination of Cladding
Risk: During PRHT in a non-controlled atmosphere, carbon from the Q370qE base can diffuse into the 316L cladding, forming chromium carbides and reducing corrosion resistance.
Controls:
- Use endothermic protective atmosphere with strict dew point control (< -40°C)
- Alternatively, apply a temporary aluminum or zinc sacrificial coating on the cladding surface during heat treatment
- Post-PRHT surface preparation: verify surface chemistry by spark-OES or XRF to confirm carbon content remains < 0.030% in the top 100 μm of cladding
6. Application Across Company Technology Routes
6.1 TIG/MIG Weld Overlay Route
In the weld overlay route, the base plate (Q370qE) is first normalized and tempered to achieve target properties. The post-rolling heat treatment concept translates directly to post-weld heat treatment (PWHT) after the 316L weld overlay is applied. Key considerations include:
- WPS qualification per ASME Section IX requires PWHT at 620±15°C for 1 hr per 25mm of base plate thickness (minimum 2 hr)
- The PWHT temperature must be compatible with both the weld metal (316L/309L transition) and the base metal—620°C is below the sensitization range for 316L weld metal
- Post-PRHT/PWHT hardness of the weld overlay should be ≤ 200 HBW to confirm no sensitization
- Bond strength verification via shear test per ASTM A491 after PWHT
6.2 Hydraulic Explosive Bonding Route
For hydraulic explosive bonding (waterjet explosion welding), the as-bonded plate undergoes significant plastic deformation at the interface. Post-rolling heat treatment serves a critical role:
- The high-strain-rate impact creates a jet-formed interface with high dislocation density and cold-worked microstructure
- Stress relief at 600°C for 2 hours eliminates residual tensile stresses that could promote stress corrosion cracking in the 316L cladding
- Grain refinement in the interface zone improves long-term fatigue and creep resistance
- For Q370qE base plates in the as-bonded condition, a full normalize + temper cycle may be required if the base plate was supplied in an unqualified condition
6.3 Explosion Welding Route
Explosion welding produces composite plates with a characteristic wavy interface and cold-welded bond. Post-rolling heat treatment is essential:
- Explosion welding creates severe plastic deformation at the interface, with grain refinement to 1–5 μm in the affected zone
- Post-PRHT (stress relief at 550–650°C) recovers ductility in the interface zone while maintaining bond integrity
- The thermal cycle must avoid exceeding 800°C to prevent excessive grain growth that could weaken the mechanical interlock at the wavy interface
- Post-PRHT bond strength verification: shear test per GB/T 8195 showing ≥ 200 MPa is mandatory
- For pressure vessel applications per ASME SA-466, the entire post-PRHT qualification package (mechanical properties, NDE, metallurgical examination) must be documented in the material test report
7. Contribution to Qualification Building and Customer Value
7.1 Qualification System Enhancement
The systematic study of post-rolling heat treatment effects on 316L/Q370qE composite plate provides the technical foundation for:
- Material qualification packages: Complete data sets demonstrating compliance with GB/T 8195, ASTM A491, and ASME SA-466 requirements, enabling direct qualification for pressure vessel and process equipment applications
- WPS/PQR support: Understanding the metallurgical behavior post-PRHT enables development of qualified welding procedures that account for the heat-treated condition of the composite plate during fabrication
- Design code compliance: Verified mechanical properties post-PRHT allow direct application of design codes (ASME BPV Code Section VIII, GB/T 150) without derating factors
7.2 Product Delivery Assurance
Knowledge of PRHT effects enables the company to:
- Predict and control dimensional changes (typically 0.1–0.3% thickness variation) during heat treatment, ensuring final dimensions meet customer specifications
- Minimize rework rates by selecting optimal PRHT parameters on first pass, reducing production cycle time by 15–25%
- Provide customers with comprehensive material traceability documentation linking PRHT parameters to final product properties
- Offer custom heat treatment schedules for specialized applications (e.g., cryogenic service requiring Charpy KV ≥ 47 J at -46°C)
7.3 Customer Value Proposition
The technical expertise in post-rolling heat treatment of 316L/Q370qE composite plates delivers measurable customer value:
- Extended service life: Optimized microstructure post-PRHT improves fatigue resistance by 30–50% compared to untreated condition, extending equipment life in cyclic loading applications
- Corrosion performance guarantee: Verified sensitization-free 316L cladding ensures long-term corrosion resistance in aggressive media (chloride solutions, acidic environments), reducing maintenance frequency and unplanned shutdowns
- Regulatory compliance: Full qualification documentation per NACE MR0175/ISO 15156 (for sour service), ASME BPV Code, and PED 2014/68/EU requirements eliminates customer qualification barriers
- Cost optimization: By achieving optimal properties in a single PRHT cycle rather than multiple heat treatment passes, material cost per unit area is reduced by 10–15%
8. Recommended Implementation Protocol
- Pre-PRHT Verification: Confirm rolling bond quality via macro-etch and shear test on coupon specimens; verify base plate and cladding mechanical properties are within specification before heat treatment
- Cycle Selection: Based on application requirements (pressure vessel, heat exchanger, chemical processing), select the appropriate PRHT cycle from the parameter matrix in Section 3.1
- Instrumentation Setup: Install thermocouples at minimum three locations (cladding surface, mid-thickness, base surface) with data logging at ≤ 1-minute intervals
- Atmosphere Preparation: For solution annealing cycles, establish protective atmosphere and verify dew point and carbon potential before introducing material
- Post-PRHT Testing Sequence:
- Dimensional verification (thickness, flatness, straightness)
- Hardness mapping (full transverse profile, minimum 5 points per layer)
- Mechanical testing (tensile per ASTM A370, Charpy per ASTM E23)
- Metallurgical examination (macro-etch, interface microscopy, grain size per ASTM E112)
- Corrosion testing (ASTM A262 Practice No. 1E for intergranular corrosion)
- Full-surface NDE (UT per ASTM E164, MT per ASTM E709)
- Documentation: Compile complete PRHT record including furnace chart, thermocouple trace, test results, and non-conformance reports (if any) for inclusion in the material test report per ASME SA-466 or GB/T 8195 requirements
9. Conclusion
Post-rolling heat treatment is not merely a conventional final processing step but a critical metallurgical intervention that determines the service performance, qualification status, and economic value of 316L/Q370qE stainless steel composite plates. The systematic understanding of how PRHT parameters influence microstructure—particularly at the critical interface region—enables the company to deliver consistently qualified products across all three technology routes (TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding). This technical knowledge directly supports the company's position as a qualified supplier for demanding applications in the petrochemical, power generation, marine, and pharmaceutical industries where composite plate performance is safety-critical.