Post-Weld Heat Treatment Effects on Microstructure and Properties of Ultrasonic Excited TIG Welds in Inconel 690
1. Definition and Technical Principles
1.1 Inconel 690 as a Critical Cladding Material
Inconel 690 is a nickel-chromium-iron alloy (UNS N06690) that is the industry-standard cladding material for nuclear power plant steam generator (SG) U-tubes. It offers superior resistance to chloride-induced stress corrosion cracking (SCC), pitting corrosion, and erosion-corrosion in the secondary-side boiler water environment. Its composition typically includes approximately 62% Ni, 29% Cr, 3% Fe, 1.5% Mo, 0.6% Al, and 0.5% Ti. This alloy is the direct successor to Inconel 690GT and is preferred over the legacy Incoloy 800H cladding in modern pressurized water reactor (PWR) designs due to its significantly extended service life (typically 30–40 years versus 10–15 years for Incoloy 800H).
1.2 Ultrasonic Excited TIG Welding Principle
Ultrasonic excited TIG (UETIG) welding represents an advanced variant of conventional gas tungsten arc welding (GTAW) in which ultrasonic vibration is superimposed on the welding process. The ultrasonic energy, typically generated at frequencies of 20–40 kHz, is transmitted through the welding torch or workpiece and introduces several beneficial effects:
- Mechanical stirring of the weld pool: Ultrasonic cavitation and acoustic streaming promote uniform mixing of the molten pool, reducing macrosegregation and columnar dendrite growth.
- Refinement of grain structure: The ultrasonic energy promotes heterogeneous nucleation, resulting in finer and more equiaxed grain structures in both the weld metal and heat-affected zone (HAZ).
- Reduction of solidification defects: Hot cracking susceptibility is diminished due to the suppression of columnar grain growth and the promotion of equiaxed grains.
- Improved wettability: Ultrasonic energy enhances the wettability of the molten metal on the base material, facilitating better fusion and reducing lack-of-fusion defects.
1.3 Post-Weld Heat Treatment (PWHT) in Inconel 690 Weldments
PWHT is a critical post-weld processing step designed to relieve residual stresses, homogenize the microstructure, and optimize the mechanical properties of the welded joint. For Inconel 690, the PWHT typically involves solution treatment at elevated temperatures (commonly in the range of 1010–1150°C) followed by controlled cooling. The purpose is to:
- Dissolve harmful intermetallic phases (such as Laves phase (M23C6) and σ-phase) that precipitate during welding and subsequent cooling.
- Relieve welding-induced residual stresses that can contribute to stress corrosion cracking.
- Restore solid solution strengthening and optimize the balance between strength and ductility.
- Homogenize the microstructure between the weld metal, HAZ, and base metal.
2. Technical Purpose and Value
2.1 Addressing the Weldability Challenges of Inconel 690
Inconel 690 presents significant weldability challenges inherent to nickel-base superalloys:
- High susceptibility to hot cracking: The wide solidification temperature range and the formation of low-melting-point interdendritic films promote solidification cracking.
- Sensitive to intergranular corrosion: Carbide precipitation along grain boundaries during welding and PWHT can lead to sensitization and intergranular attack.
- High residual stresses: The large thermal gradient during welding generates significant residual stresses that can exceed the yield strength of the material.
- Difficult to achieve adequate PWHT: The solution treatment temperature must be high enough to dissolve deleterious phases but not so high as to cause excessive grain growth.
2.2 Research and Development Value
The study summarized in this entry—examining how PWHT parameters interact with the ultrasonic excited TIG welding process to influence microstructure and mechanical performance—provides critical data for:
- WPS/PQR qualification: Establishing the optimal combination of UETIG parameters and PWHT conditions to produce weld joints that meet the acceptance criteria of nuclear-grade specifications.
- Process optimization: Identifying the synergistic effects between ultrasonic excitation and PWHT, enabling the design of more robust welding procedures with wider process windows.
- Quality assurance: Providing metallurgical evidence that the combined UETIG + PWHT approach produces welds with reduced defect susceptibility and improved long-term performance.
3. Key Process and Implementation Points
3.1 Ultrasonic Excited TIG Welding Parameters
| Parameter | Typical Range | Remarks |
|---|---|---|
| Ultrasonic Frequency | 20–40 kHz | Commonly 20 kHz for industrial applications |
| Ultrasonic Power | 0.5–3.0 kW | Higher power increases stirring but may cause spatter |
| Welding Current | 80–180 A (DCEN) | Dependent on plate thickness and joint geometry |
| Travel Speed | 3–8 cm/min | Slower speeds increase heat input and dilution |
| Shielding Gas | 100% Ar or Ar/He mixtures | He addition improves penetration for thicker sections |
| Gas Flow Rate | 12–20 L/min | Ensure complete protection of weld pool and HAZ |
| Filler Wire | Inconel 617 or Inconel 690 | Filler selection affects dilution and final composition |
3.2 Post-Weld Heat Treatment Parameters
| PWHT Parameter | Recommended Value | Purpose |
|---|---|---|
| Heating Rate | ≤ 80°C/h (for thickness > 25 mm) | Minimize thermal distortion and avoid secondary stress |
| Solution Treatment Temperature | 1010–1150°C | Dissolve Laves phase and homogenize microstructure |
| Hold Time | 1–4 hours (depending on section thickness) | Ensure complete phase dissolution throughout cross-section |
| Cooling Rate | Air cool or furnace cool (controlled) | Avoid excessive precipitation during cooling |
| Maximum Furnace Temperature | ≤ 1180°C | Avoid excessive grain coarsening in base metal |
3.3 Microstructural Evolution: UETIG vs. UETIG + PWHT
| Microstructural Feature | UETIG (As-Welded) | UETIG + PWHT | Effect on Properties |
|---|---|---|---|
| Grain Morphology | Fine equiaxed grains (ultrasonic refinement) | Refined equiaxed grains, reduced Laves phase | Improved ductility and fracture toughness |
| Interdendritic Segregation | Reduced by ultrasonic stirring | Further homogenized by solution treatment | Reduced hot cracking and corrosion susceptibility |
| Laves Phase (M23C6) | Present along grain boundaries and dendrites | Dissolved or significantly reduced | Enhanced resistance to intergranular corrosion |
| Residual Stress | High tensile residual stresses (up to 300–400 MPa) | Reduced to near-zero or compressive | Improved SCC resistance and fatigue life |
| Hardness Distribution | Non-uniform; higher in weld and HAZ | More uniform across weld, HAZ, and base metal | Better mechanical property consistency |
3.4 Mechanical Property Comparison
| Mechanical Property | Base Metal (Inconel 690) | UETIG (As-Welded) | UETIG + PWHT | Acceptance Criterion |
|---|---|---|---|---|
| Tensile Strength (MPa) | ≥ 700 | 650–750 | 680–780 | ≥ 0.95 × base metal UTS |
| Yield Strength (MPa) | ≥ 350 | 320–400 | 340–420 | ≥ 0.95 × base metal YS |
| Elongation (%) | ≥ 30 | 25–35 | 28–38 | ≥ 0.90 × base metal elongation |
| Hardness (HV30) | 180–220 | 200–260 | 190–230 | Within 10% of base metal |
4. Applicable Standards and Acceptance Criteria
4.1 Material Standards
- ASTM B564: Standard Specification for Nickel-Chromium-Iron Alloy (UNS N06690) Welding Wire—defines filler metal requirements for Inconel 690 welding.
- ASTM B625: Standard Specification for Nickel-Chromium-Iron Alloy (UNS N06690) Strip and Foil—applicable for thin cladding applications.
- ASTM B138: Standard Specification for Nickel-Chromium-Iron Alloy (UNS N06690) Pipe, Bar, and Shapes—base material specification for SG tubes.
- ASTM B160: Standard Specification for Nickel-Chromium-Iron Alloy (UNS N06690) Sheet, Strip, and Plate.
4.2 Welding Procedure Standards
- ASME Section IX: Qualification of Welding Procedures and Welders—governs WPS/PQR qualification for nuclear and pressure vessel applications.
- ASME Section III, Division 1, Subsection NF: Nuclear Fittings—specific requirements for welding of nickel-base alloy components in nuclear service.
- NB/T 20469: Welding Procedure Qualification for Nuclear Power Plant Components—Chinese nuclear industry standard for WPS qualification.
- GB/T 985: Designation of Welding Positions—applies to all welding procedures.
- GB/T 3375: Terms and Definitions in Welding, Soldering and Brazing.
4.3 Post-Weld Heat Treatment Standards
- ASME Section III, Appendix A: Post-Weld Heat Treatment—prescribes PWHT requirements for nuclear components.
- ASME Section IX, QW-406: Heat Treatment—procedures for PWHT qualification and application.
- NB/T 20335: Post-Weld Heat Treatment for Nuclear Power Plant Components—Chinese nuclear standard for PWHT.
- ASTM A388: Standard Specification for Post-Weld Heat Treatment of Carbon Steel and Low-Alloy Steel Pressure Vessels (analogous principles applied).
4.4 Non-Destructive Examination Standards
- ASME Section V, Article 2: Radiographic Examination—acceptance criteria for weld radiography.
- ASME Section V, Article 4: Ultrasonic Examination—UT acceptance criteria for welds.
- ASME Section V, Article 8: Magnetic Particle Examination—surface defect detection.
- ASME Section V, Article 9: Dye Penetrant Examination—surface defect detection.
- NB/T 47013: Non-Destructive Testing Methods for Pressure Vessels—Chinese nuclear NDT standard series.
- GB/T 3323: Radiographic Testing of Welds.
- GB/T 11345: Ultrasonic Testing of Welds.
4.5 Acceptance Criteria Summary
| Examination Method | Acceptance Level | Standard Reference |
|---|---|---|
| RT (Radiographic) | Level 1 (no linear defects; porosity < 0.5 mm) | ASME Section V, Article 2 / NB/T 47013.2 |
| UT (Ultrasonic) | Level 1 (no indications above acceptance threshold) | ASME Section V, Article 4 / NB/T 47013.3 |
| PT (Penetrant) | No indications of linear defects | ASME Section V, Article 9 / NB/T 47013.5 |
| MT (Magnetic Particle) | No indications of linear defects (if applicable) | ASME Section V, Article 8 / NB/T 47013.4 |
| Mechanical Testing | UTS ≥ 0.95×BM, Elongation ≥ 0.90×BM | ASME Section IX / NB/T 20469 |
| Hardness Testing | Within 10% of base metal hardness | ASME Section IX, QW-452 |
| Macro/Micro Examination | No cracking, no excessive Laves phase, full fusion | ASTM E3 / ASTM E112 |
5. Common Risks and Controls
5.1 Welding Process Risks
| Risk | Cause | Control Measures |
|---|---|---|
| Hot Cracking | Columnar grain growth, interdendritic segregation of low-melting phases | Ultrasonic excitation to promote equiaxed grains; appropriate filler selection (Inconel 617); controlled travel speed; preheating to 150–250°C |
| Lack of Fusion | Inadequate heat input, poor joint fit-up, insufficient gas protection | Optimized current and travel speed; proper joint preparation (V-groove with 60° included angle); adequate shielding gas coverage |
| Porosity | Contamination of base metal or filler, insufficient shielding gas | Thorough cleaning of base metal (acetone wipe); proper gas flow rate (12–20 L/min); dry filler wire |
| Excessive Dilution | High travel speed, large weld pool, inappropriate filler selection | Controlled heat input; appropriate filler wire diameter (1.6–2.4 mm); multi-pass welding strategy |
| Ultrasonic Transducer Damage | Thermal damage to ultrasonic horn or transducer | Proper positioning of ultrasonic horn away from arc; cooling of transducer; regular inspection and replacement |
5.2 Post-Weld Heat Treatment Risks
| Risk | Cause | Control Measures |
|---|---|---|
| Excessive Grain Growth | PWHT temperature too high or hold time too long | Limit PWHT temperature to ≤ 1150°C; monitor furnace temperature with calibrated thermocouples; limit hold time based on section thickness |
| Distortion | Uneven heating/cooling, thermal stresses during PWHT | Controlled heating rate (≤ 80°C/h); use of fixtures and supports; uniform furnace loading |
| Incomplete Phase Dissolution | PWHT temperature too low or hold time insufficient | Ensure furnace temperature uniformity (± 10°C); adequate hold time; verify with post-PWHT metallographic examination |
| Re-sensitization | Slow cooling through sensitization temperature range (500–850°C) | Air cool or controlled furnace cool; avoid slow cooling through sensitization range |
| Oxidation/Scale Formation | Exposure to air at elevated temperatures | Perform PWHT in controlled atmosphere furnace (argon or vacuum) if critical; or apply protective coating |
5.3 Quality Assurance Controls
- WPS/PQR qualification: All UETIG + PWHT procedures must be qualified per ASME Section IX and NB/T 20469 before production welding.
- Welder certification: Welders must be certified for UETIG on Inconel 690 per ASME Section IX, Part QW-300.
- In-process monitoring: Real-time monitoring of welding parameters (current, voltage, travel speed, ultrasonic power); automated data logging.
- Post-weld inspection: 100% RT and UT examination of all production welds; PT/MT for surface defects; mechanical testing on qualification coupons.
- PWHT documentation: Complete temperature-time records with calibrated thermocouples; furnace calibration certificates.
- Traceability: Full material traceability from mill certificates through welding to PWHT and final inspection.
6. Application Scenarios Across Technology Routes
6.1 TIG/MIG Weld Overlay Route
The UETIG + PWHT technology is directly applicable to the TIG/MIG weld overlay route, where Inconel 690 is deposited as a corrosion-resistant cladding layer on carbon steel or low-alloy steel substrates. Key applications include:
- Nuclear steam generator tube sheet cladding: Multi-pass UETIG overlay of Inconel 690 on austenitic stainless steel or low-alloy steel tube sheets, followed by PWHT to optimize microstructure and relieve residual stresses.
- Heat exchanger tube-to-tubesheet welds: UETIG welding of Inconel 690 SG tubes into clad tube sheets with PWHT for stress relief and microstructural homogenization.
- Replacement and repair welding: Field repair of Inconel 690 cladding on steam generator tube sheets using qualified UETIG procedures with appropriate PWHT.
- Transition layer welding: UETIG welding of Inconel 690 onto dissimilar substrates (e.g., 304L stainless steel) with PWHT to manage residual stresses and prevent intergranular corrosion at the interface.
6.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (water-assisted explosive cladding) produces solid-state bonds without melting, the PWHT knowledge gained from UETIG studies is relevant in the following ways:
- Post-bonding heat treatment: Clad plates produced by hydraulic explosive bonding may require PWHT to relieve residual stresses introduced during the explosive bonding process and to optimize the microstructure of the bond interface.
- Interface characterization: The metallurgical understanding of Inconel 690 microstructure evolution under thermal cycles (gained from UETIG + PWHT studies) informs the interpretation of bond interface microstructures in explosively clad products.
- Subsequent welding of clad products: When explosively clad Inconel 690 plates are subsequently welded (e.g., for fabrication into pressure vessels or heat exchangers), the PWHT knowledge ensures that the welding and post-weld treatment do not degrade the explosive bond interface.
- Hydrogen embrittlement mitigation: PWHT can be used to relieve hydrogen that may have been introduced during the explosive bonding process, reducing the risk of hydrogen embrittlement in the Inconel 690 layer.
6.3 Explosion Welding Route
Explosion welding produces Inconel 690 clad products through high-velocity impact bonding. The PWHT expertise from UETIG studies contributes to:
- Post-explosion welding PWHT: Exploively welded clad plates often undergo PWHT to relieve the high residual stresses at the bond interface and in the base metal. The optimal PWHT parameters (temperature, time, cooling rate) are informed by the metallurgical understanding developed through UETIG + PWHT research.
- Welding of explosion-welded clad components: When explosion-welded Inconel 690 clad plates are fabricated into structures (e.g., by welding), the UETIG + PWHT qualification ensures that the final welded joints meet nuclear-grade acceptance criteria.
- Repair welding: Field repair of explosion-welded clad products using UETIG with subsequent PWHT, ensuring that the repair weld and PWHT do not compromise the original explosion bond.
- Microstructural compatibility: Understanding how PWHT affects Inconel 690 microstructure (from UETIG studies) ensures that PWHT of explosion-welded clad products does not cause detrimental phase transformations or grain coarsening at the bond interface.
7. Contribution to Qualification Building, Product Delivery, and Customer Value
7.1 Qualification Building
- WPS/PQR qualification: The study provides the metallurgical basis for qualifying UETIG + PWHT welding procedures for Inconel 690, enabling the company to obtain WPS/PQR certifications required by nuclear regulatory bodies (NRC, CNSA) and utility customers.
- Procedure qualification for nuclear service: Demonstrates the ability to produce welds that meet the stringent acceptance criteria of ASME Section III, NB/T 20469, and utility-specific requirements.
- Welder certification support: Provides the technical foundation for developing and certifying welders qualified in UETIG on Inconel 690, a specialized skill set required for nuclear-grade cladding fabrication.
- Supplier qualification: Enables the company to qualify as a supplier of nuclear-grade Inconel 690 cladding products to utility customers and OEMs.
7.2 Product Delivery
- Improved weld quality: The combination of UETIG and optimized PWHT produces welds with finer microstructures, reduced defect susceptibility, and superior mechanical properties, leading to higher first-pass quality and reduced rework rates.
- Shorter cycle times: Optimized PWHT parameters (informed by UETIG studies) can potentially reduce PWHT time and temperature, accelerating production schedules.
- Reduced NDT rejection rates: Higher-quality welds result in fewer NDT indications, reducing inspection costs and schedule delays.
- Extended service life: Welds produced with UETIG + optimized PWHT exhibit improved resistance to SCC, fatigue, and creep, extending the service life of nuclear components and reducing the frequency of replacement campaigns.
7.3 Customer Value
- Nuclear safety assurance: The metallurgical understanding and process control demonstrated through this study provide confidence to nuclear utility customers that the company can deliver high-integrity cladding products for critical safety-related applications.
- Regulatory compliance: The study supports compliance with nuclear regulatory requirements for welding procedure qualification, welder certification, and quality assurance, facilitating regulatory approval of products.
- Life extension support: For existing nuclear plants undergoing life extension programs, the UETIG + PWHT technology enables the replacement or repair of Inconel 690 cladding with confidence in long-term performance.
- Technical credibility: Demonstrates the company's depth of metallurgical expertise and commitment to continuous improvement, enhancing its reputation in the nuclear cladding market.
- Cost optimization: By optimizing the UETIG + PWHT process, the company can deliver high-quality products at competitive costs, providing value to customers who require both quality and cost-effectiveness.
8. Summary and Recommendations
The study on the effects of post-weld heat treatment on the microstructure and properties of ultrasonic excited TIG welds in Inconel 690 represents a critical knowledge asset for Cladding Technology Shanxi Co., Ltd. The key findings and recommendations are summarized as follows:
Key Finding 1: Ultrasonic excitation during TIG welding significantly refines the grain structure of Inconel 690 welds, promoting equiaxed grains and reducing columnar dendrite growth. This provides a favorable starting microstructure for subsequent PWHT.
Key Finding 2: PWHT at 1010–1150°C effectively dissolves deleterious Laves phase and homogenizes the microstructure, resulting in improved ductility, fracture toughness, and resistance to intergranular corrosion.
Key Finding 3: The combination of UETIG and optimized PWHT produces weld joints with mechanical properties that meet or exceed the acceptance criteria of ASME Section IX and NB/T 20469, demonstrating the viability of this approach for nuclear-grade applications.
Key Finding 4: Residual stresses are significantly reduced by PWHT, improving the long-term performance and SCC resistance of the welded joint.
Recommendations:
- Formalize the UETIG + PWHT procedure: Develop a formal WPS/PQR based on the study findings, qualified per ASME Section IX and NB/T 20469, for use in production welding of Inconel 690 cladding.
- Standardize PWHT parameters: Establish standard PWHT parameters (temperature, time, cooling rate) for different section thicknesses and joint geometries, documented in company procedures.
- Invest in UETIG equipment: Acquire or upgrade ultrasonic excited TIG welding equipment to enable production use of this technology.
- Train and certify welders: Develop a training program for welders to acquire UETIG skills on Inconel 690, with certification per ASME Section IX requirements.
- Conduct further research: Extend the study to include long-term aging tests, SCC tests, and fatigue tests to validate the long-term performance of UETIG + PWHT welds under nuclear service conditions.
- Integrate with other technology routes: Apply the metallurgical knowledge gained from UETIG + PWHT studies to optimize PWHT of hydraulic explosive bonded and explosion-welded Inconel 690 clad products.
- Publish and present: Publish the study findings in peer-reviewed journals and present at industry conferences to enhance the company's technical reputation and attract nuclear utility customers.
This study and its associated technology represent a significant competitive advantage for Cladding Technology Shanxi Co., Ltd. in the nuclear cladding market, where the demand for high-integrity Inconel 690 cladding products is driven by the global nuclear power plant fleet and life extension programs. By leveraging this knowledge to deliver qualified, high-quality, and cost-competitive products, the company can strengthen its position as a leading supplier of nuclear-grade cladding solutions.