Heat Treatment Process and Analysis for Explosion-Welded 304L/Q345R Clad Plate
1. Introduction and Technical Definition
Explosion welding (also known as explosive bonding or explosion bonding) is a solid-state joining process that produces a metallurgical bond between two dissimilar metals through the controlled detonation of an explosive charge. The resulting high-velocity impact causes a fluidized metal jet along the interface, creating a mechanically and metallurgically sound bond without melting the bulk material. The specific composite plate configuration of 304L stainless steel facing bonded to Q345R low-alloy structural steel backing is one of the most widely demanded clad plate products in pressure vessel, chemical processing, and energy industries.
The heat treatment process applied to explosion-welded 304L/Q345R clad plate is a critical post-bonding operation designed to relieve residual stresses introduced during the explosive welding event, stabilize the microstructure of both the base metal and the weld interface, and ensure long-term mechanical and corrosion performance. This technical entry documents the systematic study and optimization of heat treatment parameters, including temperature selection, ramp rates, soaking times, cooling methods, and the resulting metallurgical and mechanical outcomes.
2. Principles of Explosion Welding and the Necessity of Post-Weld Heat Treatment
2.1 Explosion Welding Mechanism
In the explosion welding process, a flyer plate (typically the 304L stainless steel facing, usually 1.5–3.0 mm thick) is accelerated by a shaped explosive charge to impact velocities in the range of 2,500–4,500 m/s. At impact, the metal surfaces are fluidized and ejected as a jet, which is then compressed against the backing plate (Q345R, typically 8–50 mm thick). The rapid compression and subsequent cooling produce a characteristic wavy bonding interface with interlocking solid-state bonds. The process is completed in microseconds, meaning no heat-affected zone (HAZ) extends significantly into the bulk material — however, the explosive event imparts substantial residual stresses into the composite structure.
2.2 Sources of Residual Stress
Residual stresses in explosion-welded clad plates originate from multiple sources:
- Impact loading: The dynamic shock wave propagates through both plates, inducing compressive and tensile residual stress fields.
- Plastic deformation: Localized plastic flow at the bonding interface and in the near-surface region of both plates creates locked-in stresses.
- Thermal gradients: Although the process is nominally "cold," frictional heating at the interface and shock-induced temperature elevation create transient thermal gradients that contribute to residual stress.
- Constraint effects: The geometric constraint of the flyer plate bonded to the backing plate prevents free contraction, generating additional stress.
2.3 Why Heat Treatment Is Essential
Post-explosion-welding heat treatment serves the following critical functions:
- Residual stress relief: Reduces locked-in stresses to levels that comply with applicable codes (typically below 40% of yield strength per ASME Section VIII Div. 1, UG-116).
- Microstructural stabilization: Recovers any work-hardened microstructure in the Q345R backing plate near the interface and ensures the 304L facing retains its austenitic structure.
- Interface integrity: Prevents stress-corrosion cracking (SCC) susceptibility at the bonding interface, particularly in chloride-containing or high-temperature service environments.
- Dimensional stability: Minimizes distortion during subsequent fabrication operations (cutting, forming, welding of attachments).
- Code compliance: Satisfies mandatory heat treatment requirements per GB/T 13904, NB/T 20003, ASME SA-491, and API 675.
3. Technical Purpose and Business Value
3.1 Technical Purpose
The documented heat treatment study for 304L/Q345R explosion-welded clad plate establishes a validated, repeatable process window that ensures:
- Full metallurgical bonding integrity is preserved post-heat-treatment.
- Mechanical properties of both the 304L facing and Q345R backing meet or exceed minimum requirements.
- Residual stress levels are reduced to acceptable limits verified by X-ray diffraction or hole-drilling methods.
- The process is qualified for inclusion in WPS/PQR packages for customer and third-party inspection approval.
3.2 Business Value
For Cladding Technology Shanxi Co., Ltd., this technical capability delivers significant commercial value:
- Product qualification: Enables delivery of explosion-welded clad plates that meet the strictest customer specifications and code requirements for pressure vessels, heat exchangers, and storage tanks.
- Competitive differentiation: A well-documented and optimized heat treatment process demonstrates engineering maturity and reduces customer qualification lead time.
- Quality assurance: Provides a defined process control basis that minimizes rework, scrap, and field failures.
- Market expansion: Opens access to high-value applications in nuclear, petrochemical, pharmaceutical, and food processing industries where certified clad plate performance is mandatory.
4. Key Process Parameters and Implementation
4.1 Heat Treatment Temperature Selection
The selection of heat treatment temperature for 304L/Q345R explosion-welded clad plate requires careful consideration of both material systems. The 304L austenitic stainless steel facing must not be heated above the sensitization range (450–850°C) for extended periods to avoid chromium carbide precipitation at grain boundaries. The Q345R low-alloy steel backing benefits from stress relief in the 550–650°C range. The optimal compromise is typically a single-stage stress relief treatment at 620–650°C for a controlled duration.
| Parameter | Recommended Value | Rationale |
|---|---|---|
| Heating Rate | ≤ 140°C/h (based on thickest section) | Minimize thermal gradient and distortion |
| Treatment Temperature | 620–650°C | Above Q345R stress relief range; below 304L sensitization threshold |
| Soaking Time | 1 hour per 25 mm of thickness (min. 2 hours) | Ensure uniform temperature and adequate stress relief |
| Cooling Method | Furnace cool to ≤ 300°C, then air cool | Avoid quench cracking in Q345R; prevent sensitization in 304L |
| Maximum Plate Thickness | Up to 80 mm total (3 mm 304L + 77 mm Q345R) | Beyond this, differential thermal expansion requires segmented treatment |
| Atmosphere | Neutral (air) or protective (N₂) for high-purity applications | Prevent surface oxidation or carburization |
4.2 Process Implementation Steps
- Pre-heat treatment inspection: Verify bonding quality via bend test (GB/T 13904), ultrasonic testing (NB/T 47013), or peel test. Confirm that the explosion-welded interface meets minimum bond area requirements (≥90% per ASME SA-491).
- Surface preparation: Clean the plate surfaces to remove mill scale, coatings, and contaminants that could cause localized overheating or uneven heat absorption.
- Furnace loading: Position plates on ceramic or refractory supports to ensure uniform heat distribution. Thermocouples should be placed at the 304L surface, mid-thickness of Q345R, and near the interface (if accessible via embedded thermocouple during bonding or through a witness coupon).
- Heating: Ramp temperature at the controlled rate. Monitor for distortion via laser displacement sensors or dial indicators at plate corners and edges.
- Soaking: Maintain at target temperature for the calculated duration. Verify uniformity across the plate surface (±15°C tolerance).
- Cooling: Controlled furnace cool to 300°C, followed by air cool in the furnace or to ambient. For thick sections, extended slow cooling may be necessary.
- Post-treatment verification: Conduct mechanical testing, residual stress measurement, and bonding integrity verification.
4.3 Microstructural Considerations
The heat treatment at 620–650°C produces the following microstructural effects:
- 304L facing: At 650°C for limited duration (typically 2–4 hours for standard thicknesses), the austenitic structure is stable. No significant sensitization occurs if the total time in the 450–850°C range is minimized. The carbon content of 304L (≤0.03% C) further reduces sensitization risk compared to 304.
- Q345R backing: The ferrite-pearlite microstructure undergoes stress relief without phase transformation. Dislocations are recovered, reducing yield strength by approximately 5–10% while improving ductility and toughness.
- Interface region: The wavy bonding interface remains intact. No intermetallic compounds form at this temperature range. Any work-hardened material at the interface is partially recovered, improving fatigue resistance.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| GB/T 13904-2017 | Explosion-welded steel clad plates — Technical conditions | Bonding quality, mechanical properties, heat treatment requirements |
| GB/T 9866.1-2008 | Explosion-welded steel clad plates — Testing methods (Part 1: Bonding tests) | Bend test, peel test, ultrasonic testing methods |
| NB/T 20003-2010 | Steel clad plates for pressure vessels — Technical conditions | Material requirements, heat treatment, inspection for nuclear-grade applications |
| ASME SA-491 | Specification for steel clad plates, seamless steel clad pipe, and forged steel clad fittings | Material specs, bond testing, heat treatment per Section VIII |
| ASME BPV Code Section VIII Div. 1 | Pressure vessel code — UG-116 (stress relief) | Residual stress limits, heat treatment temperature/time, verification |
| ASTM A491/A491M | Standard Specification for steel clad plates, seamless steel clad pipe, and forged steel clad fittings | Material requirements, testing, heat treatment |
| API 675 | Flanged and threaded steel valves for petroleum and natural gas industries | Material certification, heat treatment documentation |
| NACE MR0175/ISO 15156 | Materials for use in H₂S-containing environments | Hardness limits, heat treatment requirements for sour service |
5.2 Acceptance Criteria
- Bonding integrity (post-heat-treatment): Bend test per GB/T 9866.1 — no delamination, cracking, or debonding at the interface for 180° bend (304L side up) and 90° bend (304L side down).
- Ultrasonic testing: Per NB/T 47013.2 or ASTM E164 — continuous bonding across the entire plate surface. No indications of separation exceeding 10% of plate width in any linear dimension.
- Mechanical properties — Q345R backing: Tensile strength ≥ 470 MPa, yield strength ≥ 315 MPa, elongation ≥ 21% (per GB/T 1591).
- Mechanical properties — 304L facing: Tensile strength ≥ 520 MPa, yield strength ≥ 205 MPa, elongation ≥ 40% (per GB/T 4237).
- Residual stress: Measured by X-ray diffraction or incremental hole-drilling. Maximum principal stress ≤ 40% of material yield strength (per ASME UG-116).
- Hardness (for sour service): Q345R backing hardness ≤ 22 HRC (per NACE MR0175/ISO 15156).
- Corrosion resistance (304L facing): No intergranular corrosion per GB/T 4334 or ASTM A262 Practice A (acid solution test).
6. Common Risks and Controls
6.1 Technical Risks
| Risk | Cause | Control Measure |
|---|---|---|
| Sensitization of 304L facing | Excessive time at 450–850°C causing Cr₂₃C₆ precipitation | Limit total time in sensitization range; use 304L (low carbon) rather than 304; minimize soaking time |
| Plate distortion/warpage | Uneven heating, differential thermal expansion between 304L and Q345R | Controlled heating rate; uniform furnace temperature; support plates on refractory saddles; post-treatment flatness check |
| Debonding at interface | Excessive temperature causing interface weakening; thermal shock during cooling | Cap temperature at 650°C; controlled cooling rate; verify bonding integrity post-treatment |
| Over-aging of Q345R | Prolonged exposure at high temperature reducing strength | Strict time-temperature control; limit soaking per thickness-based formula |
| Surface oxidation/decarburization | Prolonged air exposure at elevated temperature | Use protective atmosphere (N₂) for critical applications; apply anti-oxidation coating for non-critical surfaces |
| Incomplete stress relief | Insufficient temperature or time; uneven temperature distribution | Adequate thermocouple coverage; verify temperature uniformity; post-treatment residual stress measurement |
6.2 Quality Management Controls
- Process documentation: Maintain a complete heat treatment record including temperature-time profile, thermocouple locations, furnace calibration certificate, and operator identification. This record must be traceable to the individual plate heat number and heat treatment certificate (HTC).
- Furnace calibration: Calibrate heat treatment furnaces at least annually per ASTM E2208 or equivalent. Use calibrated reference thermocouples for verification during each heat treatment cycle.
- Witness coupon testing: Include witness coupons of the same thickness and composition in the same heat treatment cycle. Test these coupons for bonding integrity, mechanical properties, and corrosion resistance as representative of the production plates.
- Third-party inspection (TPI): For critical applications (nuclear, pressure vessels), engage a recognized TPI to witness heat treatment and testing per the applicable code.
7. Application Scenarios Across Three Technology Routes
7.1 Explosion Welding Route (Primary Application)
The 304L/Q345R explosion-welded clad plate is the flagship product of the explosion welding route. Heat treatment is mandatory for this route because:
- The explosive bonding process inherently introduces residual stresses that must be relieved before the plate can be used in pressure-containing or structural applications.
- Explosion welding produces a cold bond with no HAZ, meaning the only opportunity for stress relief is post-bonding heat treatment.
- The heat treatment process documented in this study directly supports the explosion welding product line's qualification for use in pressure vessels (ASME Section VIII), storage tanks (API 650), and heat exchangers (TEMA).
Typical explosion welding configurations include:
| Configuration | 304L Facing | Q345R Backing | Typical Application |
|---|---|---|---|
| Standard clad plate | 2–3 mm | 10–50 mm | Pressure vessel shells, heads, and flanges |
| Heavy-duty clad plate | 3–5 mm | 20–80 mm | Large storage tanks, reactor vessels |
| Lightweight clad plate | 1.5–2 mm | 6–12 mm | Heat exchanger tubesheets, pipe flanges |
7.2 TIG/MIG Weld Overlay Route
While the heat treatment study is specifically for explosion-welded plate, the principles and process knowledge directly transfer to the TIG/MIG weld overlay route in the following ways:
- Post-weld heat treatment (PWHT): Weld overlay clad plates (304L overlay on Q345R backing via TIG or MIG) also require PWHT to relieve welding residual stresses. The temperature selection logic (620–650°C) and time-temperature control principles are identical.
- Process qualification synergy: The heat treatment parameters established for explosion-welded plate can serve as the baseline for developing PWHT procedures for weld overlay, accelerating WPS/PQR qualification.
- Transition layer considerations: When weld overlay is used on Q345R backing, a 309L transition layer is typically applied before the 304L capping layer. The heat treatment study confirms that the 620–650°C range is compatible with 309L austenitic microstructure, ensuring no sensitization of the transition layer.
- Distortion control: The distortion management techniques developed for explosion-welded plate heat treatment (support fixtures, controlled cooling) are equally applicable to weld overlay PWHT, where distortion is a more significant concern due to the directional nature of welding.
7.3 Hydraulic Explosive Bonding Route
Hydraulic explosive bonding (also known as hydraulic explosion welding or hydrodynamic bonding) is a variant of explosion welding that uses water as the transmission medium. The heat treatment requirements and process are fundamentally the same as conventional explosion welding:
- Identical heat treatment parameters: The residual stress levels introduced by hydraulic explosive bonding are comparable to conventional explosion welding, requiring the same 620–650°C stress relief treatment.
- Interface quality: Hydraulic explosive bonding produces a wavy interface similar to conventional explosion welding. Post-heat-treatment bonding integrity testing follows the same acceptance criteria (GB/T 9866.1, ASME SA-491).
- Advantage for complex geometries: Hydraulic explosive bonding can be applied to curved surfaces (pipes, cylinders) where conventional explosion welding is impractical. The heat treatment process documented here can be adapted for pipe and cylinder heat treatment using tube furnaces or induction heating with controlled cooling.
- Process validation: The heat treatment study provides a metallurgical foundation that validates the hydraulic explosive bonding route for 304L/Q345R combinations, supporting the development of qualified procedures for this technology route.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
This heat treatment study is a critical component of the company's qualification infrastructure:
- WPS/PQR development: The documented heat treatment parameters form the basis for welding procedure specifications (WPS) and performance qualification records (PQR) for clad plate products. Customers and inspectors require evidence that heat treatment is a controlled, qualified process.
- Material certification: The study enables the issuance of material certificates (EN 10204 3.1 or 3.2) that include heat treatment data, mechanical test results, and bonding test results — essential for customer acceptance.
- Code compliance: The study demonstrates compliance with ASME Section VIII, NB/T 20003, and GB/T 13904 requirements for heat treatment of clad plate, enabling the company to supply products to code-regulated applications.
- Third-party approval: The documented process and test data support third-party inspection and approval (e.g., by TUV, Lloyd's, DNV, or NRC for nuclear applications).
8.2 Product Delivery Enhancement
- Reduced lead time: A qualified, optimized heat treatment process eliminates the need for trial-and-error during production, reducing qualification time for new products by an estimated 30–50%.
- Higher yield rate: Controlled heat treatment reduces the risk of distortion, debonding, and property degradation, improving first-pass yield and reducing rework costs.
- Consistent quality: Standardized heat treatment procedures ensure batch-to-batch consistency, which is critical for customer confidence and long-term supply relationships.
8.3 Customer Value
- Risk mitigation: Customers receive clad plate products with verified residual stress levels and bonding integrity, reducing the risk of in-service failures (SCC, fatigue cracking, debonding).
- Design flexibility: The qualified heat treatment process enables customers to use 304L/Q345R clad plate in a wider range of applications, including higher-temperature service and more demanding corrosion environments.
- Cost optimization: By providing fully qualified clad plate, the company eliminates the need for customers to perform their own heat treatment, reducing overall project cost and schedule.
- Regulatory compliance: Customers in regulated industries (nuclear, pharmaceutical, food) receive documentation that satisfies regulatory audit requirements, reducing their compliance burden.
9. Conclusion
The heat treatment process and analysis for explosion-welded 304L/Q345R clad plate represents a foundational technical capability for Cladding Technology Shanxi Co., Ltd. The systematic study of temperature selection, time-temperature profiles, cooling methods, and post-treatment verification establishes a validated process that ensures the mechanical integrity, corrosion resistance, and dimensional stability of the composite product. This capability directly supports the company's explosion welding product line, provides transferable knowledge to the TIG/MIG weld overlay and hydraulic explosive bonding routes, and delivers measurable value to customers through reduced risk, accelerated qualification, and consistent product quality. The adherence to international standards (GB/T 13904, ASME SA-491, NB/T 20003, ASTM A491, API 675, NACE MR0175/ISO 15156) ensures that the heat treated clad plate products are accepted in the most demanding industrial applications worldwide.