Effects of Heat Treatment on Microstructure and Properties of 316L/Q235B Explosion-Welded Clad Plate
1. Definition and Fundamental Principles
1.1 Explosion Welding Process Overview
Explosion welding (also termed explosive bonding) is a solid-state joining technique in which two or more dissimilar metals are brought into high-velocity collision, producing a metallurgical bond through plastic deformation, jetting, and interfacial mixing. In the case of a 316L/Q235B clad plate, a thin austenitic stainless steel overlay (316L) is explosively bonded onto a thicker carbon steel base plate (Q235B), creating a composite material that combines the corrosion resistance of 316L with the structural strength and economic efficiency of Q235B.
1.2 Role of Heat Treatment in Explosion-Welded Clad Plates
Following explosion welding, the interface region and adjacent base/overlay materials experience severe plastic deformation, strain hardening, and thermal cycling. These transient conditions can result in residual stresses, microstructural heterogeneity, and localized phase transformations that may compromise long-term mechanical and corrosion performance. Post-weld heat treatment (PWHT) is therefore applied to:
- Relieve residual stresses generated during the explosive collision event
- Homogenize the microstructure in the deformation zone (typically 1–3 mm from the interface)
- Promote recovery and recrystallization in the strain-hardened regions
- Restore ductility and toughness to the overlay and base material near the bond line
- Prevent delayed cracking during subsequent cold forming or service loading
1.3 Microstructural Evolution Under Heat Treatment
In the as-welded condition of 316L/Q235B explosion-welded clad plates, the interface displays a characteristic wavy bond line with jetting features. The 316L overlay near the interface exhibits elongated grain structures with elevated dislocation density due to severe plastic deformation. The Q235B base metal shows localized work hardening and potential ferrite-pearlite refinement in the deformation zone. Upon controlled heat treatment:
- 316L side: Recovery and partial recrystallization occur at temperatures above 600°C, restoring equiaxed austenite grains and reducing dislocation density. Sensitization (chromium carbide precipitation at grain boundaries) becomes a concern above 800°C.
- Q235B side: Ferrite-pearlite morphology is refined; pearlite lamellae may spheroidize at elevated temperatures, improving ductility. Grain growth is limited in low-carbon steel but must be monitored.
- Interface zone: Diffusion-driven changes are minimal due to the immiscibility of iron-chromium-nickel and iron-carbon systems at practical heat treatment temperatures. No intermetallic compound formation is expected at typical PWHT temperatures (≤850°C).
2. Category and Business Positioning
2.1 Classification Within the Company's Capability Framework
This technical entry falls under the Explosion Welding technology route of Cladding Technology Shanxi Co., Ltd. It represents a critical knowledge asset in the post-process optimization domain—specifically, the heat treatment qualification and process development for explosion-welded clad products. While explosion welding creates the initial bond, heat treatment is the essential finishing step that unlocks full mechanical and corrosion performance, making this study foundational to product qualification.
2.2 Strategic Value in the Value Chain
Heat treatment optimization for explosion-welded clad plates occupies a pivotal position in the manufacturing value chain:
- Upstream: Validates explosion welding parameters by confirming that the as-welded microstructure can be successfully restored through standard heat treatment protocols
- Midstream: Enables compliance with customer specifications requiring post-bond heat treatment (e.g., ASME Section IX PWHT requirements for pressure vessel applications)
- Downstream: Ensures that downstream fabrication operations (cold bending, machining, stress relief) do not induce interfacial cracking or degradation
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Determine optimal heat treatment temperature, time, and cooling rate for 316L/Q235B explosion-welded clad plates
- Quantify the effect of heat treatment on tensile strength, hardness, and ductility across the overlay, interface, and base material
- Assess corrosion resistance retention (pitting, crevice, intergranular) after heat treatment
- Establish a validated heat treatment window that balances stress relief with sensitization avoidance
- Provide empirical data for WPS/PQR qualification packages
3.2 Quantifiable Performance Benefits
| Property | As-Welded Condition | After Optimal Heat Treatment | Improvement |
|---|---|---|---|
| Overlay hardness (HV) | 280–320 | 180–210 | 35–40% reduction (stress relief) |
| Base metal hardness (HV) | 160–180 | 130–150 | 15–20% reduction |
| Interface shear strength (MPa) | 280–320 | 260–300 | Minimal loss (<10%) |
| Overlay elongation (%) | 8–12 | 35–45 | 3–4× improvement |
| Pitting resistance (PREN) | 24–25 | 23–24 | Minimal degradation (<5%) |
4. Key Process and Implementation Points
4.1 Recommended Heat Treatment Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Treatment temperature | 650–800°C | Above recrystallization temperature of deformed 316L; below sensitization threshold (800°C) |
| Soak time | 1.0–2.0 hours (per 25 mm thickness) | Adequate diffusion time for stress relief; limited to avoid grain growth |
| Heating rate | ≤150°C/hour | Minimize thermal gradient across clad plate to prevent delamination |
| Cooling method | Furnace cool or controlled rate ≤100°C/hour | Prevent thermal shock; avoid quench-induced residual stresses |
| Atmosphere | Protective (N₂, Ar, or vacuum) | Prevent oxidation of 316L surface; avoid decarburization of Q235B |
| Maximum thickness ratio | Overlay:Base ≤ 1:5 | Thermal mass difference creates differential contraction during cooling |
4.2 Critical Process Control Points
- Thermocouple placement: Minimum three thermocouples—on overlay surface, at interface (embedded during welding if possible), and on base metal surface—to verify uniform temperature distribution.
- Distortion monitoring: Clad plates exceeding 1:3 overlay-to-base thickness ratio require support fixtures during heat treatment to prevent bowing or curling.
- Post-treatment inspection: Visual examination for surface oxidation, dimensional check for distortion, and ultrasonic testing for bond integrity confirmation.
- Hardness survey: Traverse hardness testing across the full thickness (overlay → interface → base) at multiple points to verify uniform stress relief.
- Corrosion coupon testing: Retain witness coupons subjected to identical heat treatment for intergranular corrosion testing per ASTM A262 Practice E or ASTM G48.
4.3 Microstructural Assessment Protocol
- Optical microscopy: Grain size measurement per ASTM E112 in overlay, interface, and base zones
- SEM/EDS: Interface characterization for bond quality, jetting patterns, and elemental distribution
- XRD: Phase identification to confirm single-phase austenite in 316L and ferrite-pearlite in Q235B (no intermetallics)
- EBSD: Misorientation mapping to quantify recrystallization fraction in the deformation zone
5. Applicable Standards and Acceptance Criteria
5.1 Standards Governing Explosion-Welded Clad Plate Heat Treatment
| Standard | Scope | Key Requirements |
|---|---|---|
| GB/T 12548-2016 | Explosion-welded clad plates—general technical conditions | Post-weld heat treatment requirements, mechanical testing, bond strength criteria |
| GB/T 17748-2020 | Explosion-welded clad plate for pressure vessels | PWHT parameters, acceptance criteria for pressure vessel applications |
| NB/T 47013.2-2015 | Ultrasonic testing of welded joints in pressure equipment | Bond integrity verification after heat treatment |
| ASTM A490/A490M | Standard specification for clad steel plate | General clad plate requirements including heat treatment provisions |
| ASME Section IX, QW-401 | Qualification of post-weld heat treatment | Temperature range, time, and cooling rate qualification limits |
| ASTM A262 Practice E | Intergranular corrosion of austenitic stainless steels | Sensitization assessment of 316L after heat treatment |
| ASTM G48 Practice A | Pitting and crevice corrosion of stainless steels | Corrosion resistance verification |
| ISO 16520-1:2009 | Explosive welding of metals—general specifications | International standard for explosion welding process qualification |
| NACE MR0175/ISO 15156 | Materials for H₂S-containing environments | Hardness limits for 316L overlay in sour service |
5.2 Acceptance Criteria Summary
- Bond strength: Shear test specimens must fracture in the base metal (Q235B), not at the interface. Minimum bond strength ≥ 0.95 × minimum tensile strength of base material (GB/T 12548).
- Overlay hardness: ≤ 220 HV for general service; ≤ 220 HV (NACE MR0175) or ≤ 250 HV (ASME VIII Div. 1) for sour service applications.
- Grain size: 316L overlay grain size ≤ ASTM E112 No. 4 after heat treatment.
- Intergranular corrosion: 316L overlay must pass ASTM A262 Practice E, 5% citric acid solution test (180°C, 6 hours) with no intergranular attack.
- Ultrasonic inspection: No bond defects exceeding 25 mm in length or 1 mm in width (per GB/T 17748 and NB/T 47013.2).
6. Common Risks and Controls
6.1 Risk Identification and Mitigation
| Risk | Cause | Mitigation Control |
|---|---|---|
| Interfacial delamination during heat treatment | Excessive thermal gradient; differential thermal expansion between 316L and Q235B | Controlled heating rate ≤150°C/h; support fixtures; thickness ratio limitation |
| Sensitization of 316L overlay | Temperature exceeding 800°C or prolonged soak in 450–850°C range | Strict temperature control; maximum treatment temperature 800°C; limit soak time |
| Plate distortion/warping | Asymmetric thermal mass; unsupported edges | Use of backing plates; controlled cooling; dimensional monitoring |
| Surface oxidation | Atmospheric exposure at elevated temperatures | Protective atmosphere (N₂/Ar); vacuum furnace; passivation after treatment |
| Excessive grain growth in 316L | Overheating or excessive soak time | Temperature limit enforcement; time-temperature instrumentation with alarms |
| Hardness exceedance (NACE non-compliance) | Insufficient stress relief; incomplete recovery | Adequate soak time; post-treatment hardness survey; re-treatment if required |
6.2 Quality Assurance Measures
- Implement documented heat treatment procedures with temperature-time curves recorded and retained for traceability
- Conduct periodic calibration of furnace thermocouples and temperature control systems (minimum annual)
- Maintain witness coupon programs for each heat treatment batch
- Perform non-destructive examination (UT/MT) before and after heat treatment to detect any new defects
- Establish corrective action protocols for out-of-specification heat treatment events
7. Application Scenarios Across Company Technology Routes
7.1 Explosion Welding Route (Primary Application)
This technical entry is most directly applicable to the company's explosion welding operations. The heat treatment knowledge developed through this study directly informs:
- Process qualification for explosion-welded clad plate products destined for pressure vessel manufacturing (GB/T 150, NB/T 47013 series)
- Product specifications for oil and gas industry applications requiring 316L corrosion resistance over Q235B structural substrates
- WPS/PQR packages for explosion welding followed by post-weld heat treatment per ASME Section IX requirements
- Customer technical submissions demonstrating full compliance with heat treatment and performance requirements
7.2 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding produces similar interface characteristics to traditional explosion welding, with the added benefit of reduced environmental impact. The heat treatment principles established in this study apply directly:
- The strain-hardened deformation zone in hydraulic explosive bonding requires identical PWHT parameters for stress relief
- Microstructural evolution during heat treatment is governed by the same metallurgical mechanisms regardless of the explosive energy source
- Product qualification packages for hydraulic explosive bonded clad plates can reference the heat treatment data developed in this study
7.3 TIG/MIG Weld Overlay Route
While heat treatment is less critical for weld overlay applications (due to lower residual stresses compared to explosion welding), the knowledge contributes to:
- Stress relief requirements for thick-section weld overlay deposits on Q235B substrates
- Post-overlay heat treatment for maintaining 316L corrosion resistance in welded cladding layers
- Understanding of thermal effects on previously explosion-welded clad plates that undergo additional weld overlay repairs
- Hybrid process development: explosion-welded base with TIG/MIG overlay finish layers requiring integrated heat treatment planning
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
- WPS/PQR Development: Provides the technical basis for qualifying heat treatment procedures within welding procedure specifications. The temperature-time parameters, cooling rates, and acceptance criteria established here form the backbone of qualification packages.
- ISO 9001/ISO 3834 Compliance: Demonstrates systematic approach to process development, validation, and control required for quality management system certification.
- ASME/NB Pressure Vessel Qualification: Enables the company to supply explosion-welded clad plates for pressure equipment manufacturing, requiring documented PWHT procedures per ASME Section IX and NB/T 47013.
- NACE MR0175/ISO 15156 Compliance: Hardness control data supports qualification for sour service applications, a high-value market segment.
8.2 Product Delivery Enhancement
- Reduced Rework: Optimized heat treatment parameters minimize the risk of sensitization, delamination, and distortion, reducing scrap rates and improving first-pass yield.
- Faster Turnaround: Validated heat treatment windows allow for confident scheduling without iterative trial-and-error for each new product specification.
- Wider Product Range: Understanding of heat treatment effects enables the company to confidently offer 316L/Q235B clad plates for applications requiring post-fabrication heat treatment (e.g., cold forming, stress relief of fabricated assemblies).
- Traceability: Documented heat treatment procedures with full parameter records support complete product traceability from raw material through final delivery.
8.3 Customer Value Creation
- Risk Reduction: Customers receive clad plates with verified post-heat-treatment performance, reducing the risk of in-service failure due to residual stresses or sensitization.
- Design Flexibility: Customers can incorporate explosion-welded clad plates into fabrication sequences that include their own heat treatment operations, knowing the material will perform predictably.
- Certification Support: The company can provide complete technical dossiers including heat treatment data, microstructural evidence, and mechanical property reports, accelerating customer approval and project timelines.
- Cost Optimization: By demonstrating that controlled heat treatment preserves or enhances performance, the company can offer value-engineered solutions that minimize unnecessary conservatism in design margins.
9. Conclusion and Recommendations
The study of heat treatment effects on 316L/Q235B explosion-welded clad plates represents a fundamental knowledge asset for Cladding Technology Shanxi Co., Ltd. The technical understanding developed through this work directly enables:
- Process qualification and certification for explosion-welded clad products in regulated industries
- Consistent, repeatable heat treatment procedures that ensure product quality and performance
- Technical authority in customer interactions regarding post-weld heat treatment requirements
- Foundation for developing more complex clad plate specifications (e.g., duplex stainless steel overlays, nickel-based alloys)
It is recommended that this technical knowledge be formally incorporated into the company's WPS library, quality procedures, and training programs. Continued research into alternative heat treatment cycles (e.g., multi-step treatments, subcritical annealing) and their effects on long-term creep resistance and fatigue performance would further strengthen the company's technical positioning in the high-performance clad plate market.