Effect of Heat Treatment on Microstructure and Mechanical Properties of Q345B Base Weld Overlay Clad Plate
1. Definition and Fundamental Principles
The study of heat treatment effects on Q345B steel base weld overlay clad plates addresses a critical metallurgical challenge in composite material fabrication. Q345B, governed by GB/T 1591-2018, is a low-alloy high-strength structural steel with a yield strength of ≥345 MPa, widely employed as the structural base in clad plate and clad pipe manufacturing. When a corrosion-resistant or wear-resistant alloy layer is deposited onto Q345B via TIG or MIG weld overlay, the resulting composite interface and base metal Heat-Affected Zone (HAZ) undergo significant microstructural transformations due to the thermal cycles inherent in the welding process.
Post-weld heat treatment (PWHT) is applied to clad plates to achieve several metallurgical objectives: stress relief, grain refinement in the HAZ, tempering of martensitic phases in the overlay weld metal, and improvement of interfacial bonding integrity. The fundamental principle rests on controlled thermal cycling—typically within a range of 550–750°C for Q345B-based systems—whereby residual stresses are reduced through creep relaxation, carbide precipitation is promoted to enhance toughness, and brittle microstructures (such as untempered martensite or bainite) are transformed into more ductile tempered products.
1.1 Metallurgical Mechanisms of Heat Treatment
- Stress Relief: Residual stresses generated during weld overlay (often reaching 200–400 MPa in the HAZ) are reduced through time-temperature-dependent creep mechanisms. At temperatures above 0.4 Tm (absolute melting temperature), dislocation climb and cross-slip enable stress relaxation.
- Grain Growth Control: Excessive grain coarsening in the HAZ during welding can be partially mitigated by controlled reheating, promoting recrystallization and subsequent grain refinement.
- Carbide Precipitation: In the overlay layer (e.g., 309L/310L/6Mo), heat treatment promotes uniform carbide distribution, reducing sensitivity to intergranular corrosion and improving wear resistance.
- Phase Transformation: In the Q345B HAZ, the acicular ferrite and martensite-austenite (M-A) constituent phases formed during rapid cooling are tempered to bainitic ferrite with dispersed carbides, improving ductility and fracture toughness.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s technology framework, the heat treatment study on Q345B base weld overlay clad plates falls under the category of Process Qualification and Metallurgical Optimization. This knowledge base entry represents a systematic learning outcome derived from experimental investigations, directly supporting the company's core business in high-integrity clad plate fabrication for petrochemical, power generation, and nuclear industries.
The business positioning of this technical competency is threefold:
- WPS/PQR Qualification Enhancement: Understanding heat treatment effects enables the company to develop and qualify Welding Procedure Specifications (WPS) that incorporate post-weld heat treatment cycles, ensuring compliance with ASME Section IX, NB/T 20334, and GB/T 19542.
- Product Reliability Assurance: Optimized heat treatment parameters directly correlate to improved service life of clad products, reducing warranty claims and enhancing customer confidence.
- Competitive Differentiation: The ability to deliver clad plates with verified post-heat-treatment mechanical properties and microstructural integrity positions the company favorably in bids requiring NACE MR0175/ISO 15156 compliance or ASME SA-387 Class 2 certification.
3. Technical Purpose and Value
The primary technical purpose of studying heat treatment effects on Q345B base weld overlay clad plates is to establish a scientifically validated relationship between thermal processing parameters and final product performance. This knowledge directly translates into:
3.1 Key Value Deliverables
- Reduced HAZ Hardness: Uncontrolled weld overlay on Q345B can produce HAZ hardness exceeding 300 HV due to martensitic transformation. Proper PWHT reduces this to ≤250 HV, meeting API 579 and ASME B31.3 requirements for carbon/low-alloy steels in pressure piping.
- Improved Impact Toughness: Charpy V-notch impact energy at the HAZ can increase from 15–25 J (as-welded) to 40–60 J (after PWHT), ensuring adequate fracture resistance at service temperatures down to -20°C per GB/T 1591 Q345B classification.
- Enhanced Interfacial Bond Strength: Heat treatment promotes diffusion bonding at the clad-base interface, increasing shear bond strength from approximately 120 MPa to 180–220 MPa, well above the minimum requirements of ASTM A240 and GB/T 13183.
- Reduced Residual Stress: Peak longitudinal residual stresses are reduced from 350–420 MPa to below 100 MPa, minimizing the risk of stress corrosion cracking (SCC) in chloride-containing environments per NACE MR0175.
4. Key Process and Implementation Points
4.1 Typical Heat Treatment Parameters for Q345B Base Clad Plates
| Parameter | Range/Value | Rationale |
|---|---|---|
| Heating Rate | ≤140°C/h (for thickness ≤50 mm); ≤2.5 mm/h inverse rule for thicker sections | Minimize thermal gradients to prevent distortion and interfacial cracking |
| Treatment Temperature | 600–680°C (for Q345B base); 720–760°C (if overlay is 309L/310L) | Above Ac1 for stress relief without full austenitization; below Ac3 to avoid grain coarsening |
| Soak Time | 1 hour per 25 mm of thickness (minimum 2 hours) | Ensure uniform temperature and complete stress relaxation throughout section |
| Cooling Rate | ≤140°C/h from 400°C to ambient; furnace cool below 400°C | Prevent re-introduction of high hardness martensite in the HAZ |
| Maximum Temperature Deviation | ±25°C from setpoint | Ensure uniform microstructural transformation across the clad plate |
| Thermocouple Placement | Base metal, HAZ, overlay surface, and furnace atmosphere (minimum 4 points) | Verify thermal uniformity and document for NDE traceability |
4.2 Microstructural Evolution Before and After Heat Treatment
| Zone | As-Welded Microstructure | Post-Heat-Treatment Microstructure | Hardness Change (HV) |
|---|---|---|---|
| Q345B Base Metal | Ferrite + Pearlite (fine) | Ferrite + Spheroidized Pearlite | 180–200 → 160–180 |
| HAZ (Coarse Grain Zone) | Acicular Ferrite + M-A Islands + Fine Martensite | Tempered Bainite + Dispersed Carbides | 280–320 → 200–240 |
| Weld Overlay Metal (e.g., 309L) | Austenite + Ferrite (dendritic) | Austenite + Spheroidized Ferrite + Carbides | 200–220 → 180–200 |
| Interface Zone | Diffusion layer with mixed phases | Enhanced diffusion bonding, refined interface | 220–260 → 200–230 |
4.3 Implementation Protocol
- Pre-Heat Treatment Inspection: Complete all NDE (MT per ASTM E165, UT per ASTM E269 for clad thickness, RT per ASTM E94 for through-thickness) before heat treatment to establish baseline defects.
- Thermocouple Installation: Embed thermocouples at representative locations including the clad-base interface, HAZ, and overlay surface. Use K-type or N-type thermocouples with calibration certificates traceable to national standards.
- Furnace Atmosphere Control: Maintain a neutral or slightly reducing atmosphere (dew point ≤-40°C or use endothermic gas) to prevent scale formation on the clad surface. For 316L or 304L overlays, avoid oxidizing conditions above 500°C to prevent sensitization.
- Temperature Monitoring: Continuous recording with data logger at intervals ≤1 minute. Deviations exceeding ±25°C require investigation and potential re-treatment.
- Post-Treatment Inspection: Perform hardness survey (ASTM E18), intergranular corrosion testing (ASTM A262 Practice E), and mechanical testing (tensile per ASTM E8, impact per ASTM E23) to verify effectiveness.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Scope of Application | Key Requirement |
|---|---|---|
| GB/T 1591-2018 | Q345B base material specification | Yield strength ≥345 MPa; Charpy impact ≥34 J at 20°C (for B grade) |
| GB/T 13183-2018 | Steel clad plate general specification | Interfacial shear strength ≥120 MPa; No delamination on bend test |
| ASME Section IX, Part QW | Welding procedure and performance qualification | PWHT parameters within qualified range; Post-heat-treatment mechanical tests |
| ASME BPV Code Section VIII Div.1, UG-118 | PWHT for pressure vessels | Temperature, time, and rate requirements based on material group and thickness |
| ASTM A240/A240M | Stainless steel clad overlay specifications | Post-WPHT intergranular corrosion resistance (ASTM A262 Practice E) |
| NB/T 20334-2013 | Nuclear industry weld overlay qualification | Enhanced PWHT documentation; Fracture toughness verification |
| API 579-1/ASME FFS-1 | Fitness-for-service assessment | Post-WPHT residual stress limits for FFS calculations |
| ISO 15156-1 (NACE MR0175) | Materials for H2S environments | Hardness ≤22 HRC for overlay; PWHT required if as-welded exceeds limit |
| GB/T 19542-2008 | Steel and nickel alloy weld overlay procedure qualification | Heat treatment cycle documentation; Post-treatment mechanical verification |
| ASTM E112 | Grain size determination | HAZ grain size ≤ASTM No.5 (≥120 μm) after PWHT |
5.2 Acceptance Criteria Summary
- Hardness: HAZ hardness ≤250 HV10 for Q345B base; overlay hardness per specific alloy specification (typically ≤22 HRC per NACE MR0175 for sour service).
- Impact Toughness: Minimum 47 J at 20°C for base metal (per GB/T 1591 Q345B); HAZ impact ≥30 J at service temperature.
- Interfacial Integrity: No separation on 180° bend test per GB/T 13183; shear bond strength ≥150 MPa.
- Microstructure: No untempered martensite in HAZ; grain size ≤ASTM No.5; carbide morphology acceptable (no continuous grain boundary network).
- Residual Stress: Longitudinal residual stress ≤100 MPa (measured by XRD per ASTM E1426 or hole-drilling per ASTM E837).
6. Common Risks and Controls
6.1 Risk Identification and Mitigation Matrix
| Risk | Cause | Detection Method | Mitigation/Control |
|---|---|---|---|
| Interfacial Cracking | Excessive heating rate; thermal mismatch between clad and base | MT/PT at interface; UT for delamination | Limit heating rate to ≤140°C/h; pre-heat base to 150°C before ramp |
| Overlay Sensitization (400°C–850°C range) | Prolonged exposure at sensitization temperatures for 304/316L overlays | ASTM A262 Practice E intergranular corrosion test | Minimize time in 450–850°C range; use stabilized grades (321/347) or rapid cool through sensitization range |
| Grain Coarsening in HAZ | Excessive PWHT temperature or soak time | ASTM E112 grain size measurement | Limit PWHT temperature to ≤680°C for Q345B; monitor soak time per thickness rule |
| Distortion/Warping | Thermal gradients during heating/cooling; asymmetric thermal expansion | Dimensional inspection (flatness ≤1 mm/m) | Uniform furnace loading; support fixtures; controlled cooling rate |
| Re-introduction of Hard Phase | Rapid air cooling after PWHT; insufficient furnace cool | Hardness survey (ASTM E18) across HAZ | Furnace cool below 400°C; then controlled air cool; verify with post-cool hardness map |
| Oxidation/Scaling of Clad Surface | Oxidizing furnace atmosphere; high temperature exposure | Visual inspection; surface roughness measurement | Neutral/reducing atmosphere; dew point control ≤-40°C; protective coating if necessary |
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Route
In the TIG/MIG weld overlay technology route, heat treatment is an integral part of the manufacturing sequence for Q345B-based clad plates. The multi-pass weld overlay process (typically 2–5 passes depending on clad thickness) introduces significant thermal input, creating a complex HAZ microstructure that requires PWHT optimization. Key applications include:
- Petrochemical Heat Exchanger Tubesheets: Q345B tubesheet with 316L or 6Mo overlay, PWHT at 620°C/2h to achieve ≤22 HRC per NACE MR0175 compliance for sour service.
- Pressure Vessel Linings: Q345B vessel shell with 309L/316L overlay for ASME Section VIII Div.1 construction; PWHT per UG-118 to ensure acceptable HAZ toughness.
- Wear-Resistant Clad Pipes: Q345B pipe with high-chrome overlay; PWHT to temper martensitic overlay while maintaining wear hardness above 40 HRC in the working layer.
The heat treatment study directly supports WPS qualification by establishing the relationship between PWHT parameters and post-treatment mechanical properties, enabling the company to define qualified PWHT ranges in accordance with ASME Section IX QW-407 and GB/T 19542.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (water jet cladding) inherently produces a cold-welded interface without significant thermal input, post-bonding heat treatment may be applied for specific applications:
- Stress Relief of Bonded Assembly: After hydraulic bonding of a 316L strip onto Q345B plate, low-temperature PWHT at 550–600°C relieves residual stresses introduced during the bonding process, improving dimensional stability.
- Subsequent Weld Overlay Compatibility: When hydraulic bonding is used as a pre-cladding step followed by TIG weld overlay (hybrid process), PWHT between steps or after final overlay ensures the combined interface meets mechanical requirements.
- Nuclear-Grade Applications: For NB/T 20334-qualified products, PWHT after hydraulic bonding provides additional assurance of interfacial integrity, as required by nuclear regulatory specifications.
7.3 Explosion Welding Route
In explosion welding of Q345B base plates with stainless steel or nickel alloy clad layers, the high-velocity collision produces a jetted interface with localized plastic deformation. Post-explosion heat treatment serves distinct purposes:
- HAZ Recovery: The explosive bonding process creates a narrow HAZ in the Q345B base with potentially elevated hardness (280–340 HV). PWHT at 620–650°C reduces this to ≤220 HV, restoring base metal toughness.
- Interface Strengthening: Controlled PWHT promotes diffusion at the explosion weld interface, increasing shear bond strength from 140–160 MPa to 200–250 MPa, exceeding ASTM A491 requirements.
- Multi-Layer Composite Fabrication: When explosion welding is combined with subsequent weld overlay (e.g., explosion-welded 304L on Q345B, followed by 316L weld overlay for enhanced corrosion resistance), PWHT after the final overlay ensures uniform properties throughout the composite.
8. Contribution to Qualification Building and Customer Value
8.1 Qualification Building
The systematic study of heat treatment effects on Q345B base weld overlay clad plates directly contributes to the company's qualification portfolio in the following ways:
- WPS Expansion: By establishing qualified PWHT parameter ranges (temperature, time, rate), the company can expand its qualified WPS library to cover thicker sections and additional overlay alloys without requiring full requalification from scratch.
- CNCA/National Certification: Documentation of PWHT effects supports applications for national manufacturing licenses (e.g.,特种设备制造许可证 for pressure vessel components) requiring demonstrated process control.
- Customer-Specific Qualification: Major EPC contractors (e.g., PetroChina, Sinopec, CNPC) require vendor-specific PQRs with PWHT data. The study provides the technical basis for generating these qualification packages efficiently.
- International Market Access: Compliance with ASME, EN 10204, and API certification requirements demands documented PWHT procedures. The study enables the company to meet these international qualification demands.
8.2 Product Delivery Enhancement
- Reduced Rework Rates: Understanding optimal PWHT parameters reduces the incidence of post-treatment failures (cracking, excessive hardness, distortion), decreasing rework by an estimated 30–50%.
- Shorter Production Cycles: Optimized soak times (based on validated thickness rules) reduce furnace occupancy time, improving throughput by 15–25%.
- First-Pass Quality: Predictable post-PWHT properties enable confident first-pass acceptance, reducing the need for supplementary testing and expediting delivery schedules.
8.3 Customer Value Proposition
"The ability to deliver Q345B-based clad plates with fully characterized post-heat-treatment microstructures and mechanical properties provides customers with documented assurance of service integrity. This translates to reduced lifecycle costs, extended equipment availability, and compliance with the most stringent industry codes—directly addressing the customer's primary concerns of safety, reliability, and regulatory compliance."
Customers in the oil and gas, power generation, and nuclear industries increasingly require full traceability of heat treatment parameters and their effects on final product properties. The company's documented expertise in this area positions it as a preferred supplier for critical applications where material performance directly impacts operational safety and economic viability.
9. Conclusion and Forward Recommendations
The study of heat treatment effects on Q345B base weld overlay clad plates represents a foundational technical competency for Cladding Technology Shanxi Co., Ltd. The knowledge derived from this study enables the company to:
- Develop and qualify PWHT procedures that are scientifically validated and code-compliant.
- Deliver clad products with predictable, documented mechanical properties and microstructural integrity.
- Expand into higher-value applications (nuclear, offshore, sour service) that demand rigorous PWHT documentation.
- Reduce manufacturing costs through optimized process parameters and reduced rework.
Recommended next steps include: (1) conducting parametric studies varying PWHT temperature (580°C, 620°C, 660°C, 700°C) with systematic metallographic and mechanical characterization; (2) extending the study to Q345R and Q370R base materials for pressure vessel applications; (3) developing digital twin models of the PWHT process for real-time parameter optimization; and (4) integrating findings into the company's Enterprise Resource Planning (ERP) system for automated WPS selection and heat treatment scheduling.