Effects of Heat Treatment on Microstructure and Properties of 316L/Q345R Explosion-Welded Clad Plate

1. Definition and Fundamental Principles

Explosion welding (also known as explosive bonding or explosive cladding) is a solid-state joining process in which a flyer plate and a base plate are accelerated to supersonic velocities and collide at a precisely controlled angle, producing a permanent metallurgical bond along the interface without melting either substrate. The 316L/Q345R explosion-welded clad plate combines a corrosion-resistant austenitic stainless steel overlay (316L) with a high-strength low-alloy carbon-manganese structural steel substrate (Q345R), creating a composite panel that delivers both structural integrity and resistance to aggressive chemical environments.

Post-weld heat treatment (PWHT) is a critical thermal process applied after explosion welding to relieve residual stresses, refine the microstructure in the heat-affected zone (HAZ), and optimize the mechanical properties at the bond interface. The heat treatment regime—encompassing heating rate, peak temperature, hold time, and cooling rate—directly governs the phase transformations, grain growth behavior, and stress state within the clad plate. Understanding these relationships is essential for ensuring long-term service reliability in pressure vessels, heat exchangers, and chemical processing equipment.

1.1 Metallurgical Background

The explosion welding interface between 316L and Q345R is characterized by a distinctive wavy or cellular bonding pattern formed by aerodynamic instabilities (Kelvin-Helmholtz instability) during the high-velocity collision. The interface region typically exhibits:

Residual stresses from explosion welding can reach 300–600 MPa near the interface due to the extreme plastic deformation and rapid cooling inherent in the process. These stresses, if unaddressed, can compromise fatigue life, promote stress corrosion cracking (SCC), and reduce dimensional stability during subsequent fabrication operations.

2. Category and Business Positioning

This technical entry falls within the company's explosion welding technology route and represents a knowledge management deliverable focused on process optimization and qualification support. Within the three primary technology routes offered by Cladding Technology Shanxi Co., Ltd., this expertise directly supports:

From a business positioning perspective, mastery of heat treatment effects on explosion-welded clad plates enables the company to:

3. Technical Purpose and Value

3.1 Primary Objectives

The investigation of heat treatment effects on 316L/Q345R explosion-welded clad plates serves several critical engineering objectives:

  1. Residual stress relief: Reduce peak residual stresses from 400–600 MPa to below 100 MPa, preventing dimensional distortion and cracking during subsequent machining or forming
  2. Microstructure stabilization: Convert strain-induced martensite in the 316L overlay back to austenite, restoring ductility and corrosion resistance
  3. Grain structure optimization: Control grain growth in the Q345R substrate HAZ to maintain adequate toughness
  4. Interface integrity preservation: Ensure that thermal exposure does not degrade the mechanical bond strength of the explosion-welded interface

3.2 Value Chain Contribution

4. Key Process and Implementation Points

4.1 Recommended Heat Treatment Regimes

Parameter Conservative Regime Standard Regime Aggressive Regime Rationale
Heating Rate ≤ 20 °C/h 20–40 °C/h 40–60 °C/h Slower rates minimize thermal gradients across the clad composite, preventing delamination
Peak Temperature 550 °C 600–650 °C 700–720 °C Below Ac1 (≈727 °C) for Q345R to avoid austenitization; above 500 °C for 316L stress relief
Hold Time 2 h 2–4 h 4–6 h Sufficient for stress diffusion; longer times risk interfacial diffusion and property degradation
Cooling Rate ≤ 20 °C/h (furnace cool) 20–30 °C/h 30–50 °C/h (air cool below 400 °C) Controlled cooling prevents new residual stresses; avoid rapid cooling through 400–550 °C for 316L (σ-phase risk)
Maximum Temperature Must not exceed 720 °C Above 720 °C risks excessive grain growth in Q345R and chromium carbide precipitation at grain boundaries in 316L

4.2 Microstructural Response to Heat Treatment

Microstructural Feature As-Explosion-Welded After PWHT at 600 °C / 2h After PWHT at 700 °C / 4h
316L Overlay Work-hardened austenite; 5–15% strain-induced martensite; elongated grains Martensite fully reverted to austenite; slight grain recovery; dislocation density reduced ~60% Noticeable grain growth; potential Cr-rich carbide precipitation at grain boundaries; slight sensitization risk
Q345R Substrate Deformed ferrite-pearlite; elongated grains; dislocation cells Recovery and partial recrystallization of ferrite; pearlite morphology preserved Significant ferrite grain growth; possible pearlite spheroidization; reduction in yield strength ~10–15%
Interface Zone Thin diffusion layer (0.5–2 μm); high dislocation density Diffusion layer may grow to 2–5 μm; bond strength maintained or improved Diffusion layer growth to 5–15 μm; intermetallic formation possible; bond strength may decrease if excessive
Residual Stress 300–600 MPa (tensile near interface) Reduced to 50–100 MPa Reduced to <50 MPa; however, new stresses from thermal gradients possible

4.3 Critical Process Control Parameters

4.4 Non-Destructive Verification After PWHT

  1. Visual inspection: Check for surface oxidation, discoloration, or signs of overheating on the 316L overlay surface
  2. Ultrasonic testing (UT):strong> Per GB/T 24231 or ASME Sec. V Art. 23—scan for interface delamination or cracking
  3. Magnetic particle testing (MT): Inspect Q345R surface for new cracks (316L is non-magnetic and requires liquid penetrant testing)
  4. Liquid penetrant testing (PT): Inspect 316L surface for surface-breaking defects per GB/T 18851
  5. Hardness mapping: Traverse hardness test across the cross-section to verify uniformity and absence of abnormal softening

5. Applicable Standards and Acceptance Criteria

5.1 Material and Product Standards

Standard Scope Key Requirements
GB/T 150.2 Pressure vessel materials Q345R mechanical properties: YS ≥ 245 MPa, TS ≥ 425–620 MPa, A ≥ 21%
GB/T 24511 Explosion-welded clad plates Interface bonding quality, minimum dimensions, NDE requirements
ASTM A490 / A490M Explosively welded clad plates (pipe and plate) Bond strength ≥ 200 MPa shear; visual inspection for unbonded areas
ASME Sec. VIII Div. 1 UG-70 Pressure vessel clad requirements Qualification of cladding process; post-weld heat treatment per PWHT procedure
NB/T 47002.1 Pressure vessel steel plates (Chinese) Q345R chemical composition and mechanical property acceptance
ASTM A270 / A270M Explosively welded clad tubing Applicable to pipe products; bond integrity verification
GB/T 24231 Ultrasonic testing of clad plates Interface defect detection sensitivity and acceptance levels

5.2 Acceptance Criteria for Post-Heat-Treatment Clad Plate

  • 316L overlay: Hardness 150–250 HV; elongation ≥ 40%; grain size ≤ 0.05 mm average (ASTM E112 equivalent)
  • Q345R substrate: Hardness 120–200 HV; YS ≥ 245 MPa; TS ≥ 425 MPa; Charpy V-notch ≥ 34 J at −20 °C (if required)
  • Interface bond strength: Shear strength ≥ 200 MPa (ASTM A490 Method 2 or GB/T 24511 equivalent)
  • Interfacial diffusion layer: Maximum thickness ≤ 20 μm (to prevent embrittlement); no continuous intermetallic phase formation
  • Residual stress: Maximum residual stress ≤ 100 MPa in all directions at the interface
  • NDE acceptance: No unbonded area exceeding 100 mm² or containing more than 30% of the interface area (per ASTM A490); no cracks longer than 3 mm

6. Common Risks and Controls

Risk Cause Consequence Mitigation Control
Interface delamination Excessive thermal gradient; too rapid heating rate Loss of bond integrity; pressure vessel failure Limit heating rate to ≤ 20 °C/h; ensure furnace uniformity ≤ ±5 °C; use thermocouples on both faces
Chromium carbide precipitation (sensitization) Exposure to 450–850 °C range in 316L Intergranular corrosion susceptibility; reduced pitting resistance Limit peak temperature to ≤ 700 °C; minimize time in 450–850 °C range; consider solution treatment if sensitization detected
Excessive grain growth in Q345R Overheating above 700 °C or excessive hold time Reduced toughness; potential brittle fracture risk Strict temperature control; monitor via thermocouples; limit hold time per thickness
Intermetallic compound formation at interface Prolonged exposure at high temperatures (>650 °C) Brittle interface; reduced bond strength; cracking under load Limit diffusion layer growth; monitor via metallographic examination of coupon samples
σ-phase precipitation in 316L Slow cooling through 550–750 °C range Embrittlement of 316L overlay; reduced ductility Control cooling rate; avoid prolonged hold in σ-phase formation temperature range
Dimensional distortion Uneven thermal expansion during heating/cooling Out-of-flatness; machining difficulties; fit-up problems Uniform heating; support fixtures during PWHT; measure flatness before and after

7. Application Scenarios Across Technology Routes

7.1 Explosion Welding Route (Primary Application)

This heat treatment knowledge is directly applicable to the company's explosion welding production line. Typical products include:

  • Pressure vessel head cladding: 316L/Q345R explosion-welded plates for reactor vessels, separators, and distillation columns in petrochemical plants. PWHT per ASME Sec. VIII Div. 1 UG-115 after forming and welding.
  • Heat exchanger tube sheets: Clad plates for acid service where the Q345R provides mechanical strength and 316L provides corrosion resistance to sulfuric acid, hydrochloric acid, or seawater environments.
  • Storage tank bottom plates: For chemical storage facilities requiring corrosion-resistant inner surfaces with structural-grade substrates.
  • Reactor internals: Supports, baffles, and channel plates in nuclear or chemical reactors where both structural and corrosion performance are critical.

7.2 Hydraulic Explosive Bonding Route

Hydraulic explosive bonding produces similar residual stress profiles to conventional explosion welding, requiring comparable PWHT protocols. Key considerations:

  • Hydraulic explosive bonding typically produces lower peak velocities (subsonic to transonic), resulting in somewhat lower residual stresses (200–400 MPa vs. 300–600 MPa)
  • The PWHT temperature regime remains the same, but hold times may be slightly reduced given the lower initial stress levels
  • Interface diffusion behavior is similar, and the same sensitization and intermetallic risks apply
  • Applicable to smaller-diameter pipe products where hydraulic confinement improves bonding consistency

7.3 TIG/MIG Weld Overlay Route (Indirect Application)

While the primary subject is explosion-welded clad plates, the metallurgical understanding gained from this study directly informs weld overlay process optimization:

  • WPS development: PWHT parameters for explosion-welded clad plates inform the PWHT requirements for weld overlay cladding on the same Q345R substrate
  • HAZ management: Understanding of how thermal exposure affects the Q345R microstructure applies to controlling weld HAZ properties in multi-pass overlay welding
  • 316L overlay layer behavior: Knowledge of sensitization and σ-phase risks in 316L during heat treatment directly applies to TIG/MIG overlay of 316L on Q345R, where similar thermal cycles occur
  • Transition layer design: The understanding of interfacial diffusion and intermetallic formation supports the design of 309L transition layers in weld overlay sequences

8. Contribution to Qualification Building and Customer Value

8.1 Qualification Package Support

This technical study directly contributes to building robust qualification packages for:

  • WPS/PQR qualification: Provides metallurgical justification for PWHT parameters specified in welding procedure specifications for clad plate fabrication
  • Material certification: Supports ASME "S" stamp or GB pressure vessel material certification by demonstrating understanding of post-weld behavior
  • Customer audits: Enables the company to demonstrate technical competence during customer factory acceptance inspections (FAI) and process audits
  • Regulatory compliance: Provides documented evidence of understanding required by NB (China's nuclear regulatory body) for nuclear-grade clad plate production

8.2 Product Delivery Enhancement

  • Reduced rework: By specifying optimal PWHT parameters, the company minimizes the risk of post-treatment failures that would require costly rework or rejection
  • Consistent quality: Standardized heat treatment protocols ensure batch-to-batch consistency in mechanical properties and microstructure
  • Accelerated delivery: Knowledge of process parameters reduces the need for trial-and-error heat treatments, shortening production timelines
  • Extended service life: Properly heat-treated clad plates deliver longer in-service performance, reducing customer maintenance costs and enhancing the company's reputation

8.3 Customer Value Proposition

"Our expertise in optimizing post-explosion heat treatment for 316L/Q345R clad plates ensures that every delivered product maintains superior bond integrity, corrosion resistance, and mechanical performance throughout its design life. This metallurgical knowledge translates directly into reduced lifecycle costs for our customers in the petrochemical, power generation, and marine industries."

9. Summary and Recommendations

9.1 Key Takeaways

  1. The optimal PWHT regime for 316L/Q345R explosion-welded clad plates is 600–650 °C for 2–4 hours with controlled heating and cooling rates (≤ 30 °C/h)
  2. Peak temperatures above 700 °C must be avoided to prevent sensitization in 316L and excessive grain growth in Q345R
  3. Post-PWHT verification must include hardness mapping, NDE of the interface, and metallographic examination of coupon samples
  4. Residual stress reduction from 400–600 MPa to below 100 MPa is achievable and essential for long-term reliability
  5. Protective atmosphere or vacuum heat treatment is preferred to prevent surface oxidation and intergranular corrosion susceptibility

9.2 Implementation Recommendations

  • Establish a documented PWHT procedure (HTP) specifically for 316L/Q345R explosion-welded clad plates, validated with coupon testing
  • Implement thermocouple monitoring with data logging for every production heat treatment cycle
  • Conduct periodic metallographic verification (minimum quarterly) to confirm diffusion layer growth remains within acceptable limits
  • Integrate PWHT parameters into the company's quality management system (QMS) per ISO 9001 and/or ASME NQA-1 requirements
  • Train production personnel on the metallurgical rationale behind each process parameter to ensure consistent execution

10. Conclusion

The systematic study of heat treatment effects on 316L/Q345R explosion-welded clad plates represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. This expertise bridges the gap between raw explosion-welded material and qualified, specification-compliant product ready for demanding industrial applications. By mastering the interplay between thermal cycles, microstructural evolution, and mechanical performance, the company positions itself as a technically differentiated supplier capable of delivering clad plates that meet the most stringent international standards—including ASME, ASTM, GB, and NB requirements—while providing customers with the engineering confidence that their equipment will perform reliably for the full design service life.