Impact of Cladding Weld Overlay Method Combinations on Heat-Affected Zone Microstructure of Low-Alloy Steel for Nuclear Power Plant Applications

1. Definition and Fundamental Principles

1.1 Technical Definition

The subject matter concerns a systematic technical study examining how different combinations of surface corrosion-resistant cladding weld overlay methods affect the microstructural evolution within the heat-affected zone (HAZ) of low-alloy steels employed in nuclear power plant (NPP) components. This investigation spans multiple overlay processes—including TIG (Gas Tungsten Arc Welding, GTAW), MIG/MAG (Gas Metal Arc Welding, GMAW), and submerged arc welding (SAW)—applied in sequential or hybrid configurations to deposit corrosion-resistant alloy layers onto low-alloy steel substrates such as SA-508 Cl.3, SA-387 Gr.11/22 Cl.2, P91, and P92 grades.

1.2 Microstructural Mechanisms in the HAZ

When multiple weld overlay passes or process combinations are applied sequentially, the HAZ experiences complex thermal cycling. Each subsequent pass subjects previously formed HAZ microstructures to additional thermal exposure, resulting in:

• Grain growth in the coarse-grained heat-affected zone (CGHAZ) due to cumulative heat input
• Phase transformation sequences: austenitization, recrystallization, and precipitation hardening/reversion
• Carbide dissolution and re-precipitation at grain boundaries, particularly Cr-rich M23C6 and Cr7C3 phases
• Tempering of martensite in high-strength low-alloy (HSLA) substrates
• Sensitization at prior austenite grain boundaries leading to intergranular corrosion susceptibility

1.3 Multi-Pass Thermal Cycling Effect

The critical technical insight from this study is that the HAZ microstructure is not determined solely by the final pass parameters but by the entire thermal history accumulated across all overlay layers. A single-pass TIG overlay produces a narrow, steep thermal gradient HAZ, whereas multi-pass combinations (e.g., TIG打底 + MIG盖面 + TIG精修) create a broader thermal influence zone with heterogeneous microstructural bands. The inter-pass temperature control and process sequencing become decisive factors in HAZ integrity.

2. Category and Business Positioning

2.1 Classification within the Technology Portfolio

This technical capability falls squarely within the company's TIG/MIG weld overlay technology route, specifically addressing the engineering qualification of multi-process overlay sequences for nuclear-grade components. It represents a high-value knowledge asset that bridges metallurgical research with production process optimization.

2.2 Business Positioning

• Qualification Enabler: Provides the metallurgical justification required for WPS (Welding Procedure Specification) qualification of multi-method overlay sequences on nuclear components
• Design Authority Support: Generates data packages that design institutes (e.g., CNNC-1st Institute, SWRI, AECID) require for material selection and cladding specification approval
• Competitive Differentiator: Demonstrates deep metallurgical understanding that distinguishes the company from generic welding contractors in the nuclear supply chain
• Standardization Input: Contributes technical data to industry standards and nuclear regulatory guidance documents

3. Technical Purpose and Value

3.1 Primary Technical Objectives

1. Microstructural Mapping: Establish quantitative relationships between overlay method combinations (process type, sequence, parameters) and resulting HAZ microstructural features (grain size, phase distribution, hardness profile, retained austenite content)
2. Performance Prediction: Correlate HAZ microstructure with mechanical properties (yield strength, impact toughness, creep resistance) and corrosion resistance (intergranular corrosion, stress corrosion cracking susceptibility)
3. Process Optimization: Identify optimal overlay method combinations that minimize adverse HAZ effects while maintaining required cladding thickness and corrosion performance
4. Risk Identification: Pinpoint critical process parameters and sequencing errors that lead to unacceptable HAZ degradation in nuclear service conditions

3.2 Value Chain Contribution

Value Dimension	Specific Contribution
Product Delivery	Enables confident specification of multi-pass overlay sequences for reactor pressure vessel (RPV) internals, steam generator tubes, pressurizer nozzles, and coolant piping without HAZ performance degradation
Qualification Building	Provides metallurgical evidence packages for NQA-1/ISO 27700 quality system audits and regulatory qualification submissions
Customer Value	Reduces design margin requirements by providing verified HAZ performance data; shortens qualification timelines through pre-validated process combinations
Cost Optimization	Identifies process combinations that achieve equivalent performance with lower total heat input, reducing distortion and post-weld heat treatment requirements

4. Key Process and Implementation Points

4.1 Overlay Method Combinations Evaluated

Combination Sequence	Base Layer (TIG)	Filler Layers (MIG)	Capping Layer (TIG)	Total Heat Input	HAZ Width (mm)
TIG only (single method)	0.3-0.5 mm	—	—	1.5-3.0 kJ/mm	2-4
TIG + MIG (2 methods)	0.5-0.8 mm	2.0-4.0 mm	—	5.0-10.0 kJ/mm	5-9
TIG + MIG + TIG (3 methods)	0.5-0.8 mm	2.0-4.0 mm	0.3-0.5 mm	5.5-11.0 kJ/mm	5-10
MIG + TIG + MIG (3 methods)	—	1.5-2.5 mm	0.5-0.8 mm	4.0-8.0 kJ/mm	4-8
TIG + SAW + TIG (3 methods)	0.5-0.8 mm	3.0-6.0 mm	0.3-0.5 mm	8.0-15.0 kJ/mm	8-14

4.2 Critical Process Parameters

4.2.1 Inter-Pass Temperature Control

• Maximum inter-pass temperature: ≤150°C for P91/P92 substrates; ≤200°C for SA-508 Cl.3; ≤100°C for sensitization-sensitive austenitic overlay alloys
• Temperature measurement method: Infrared pyrometer with thermocouple verification at a minimum of 3 points per pass
• Heat input per pass: Strictly controlled to prevent cumulative thermal damage; individual pass heat input should not exceed 3.5 kJ/mm for nuclear-grade applications

4.2.2 Process Sequencing Principles

1. First pass (TIG打底): Establishes metallurgical compatibility between base metal and overlay alloy; minimal heat input prevents excessive HAZ grain growth; ensures full fusion without dilution issues
2. Intermediate passes (MIG/SAW填充): Build volume efficiently; higher deposition rates reduce total number of passes and cumulative thermal cycles; controlled wire feed and travel speed maintain uniform bead geometry
3. Final pass (TIG精修): Achieves surface quality requirements (Ra ≤ 1.6 μm for nuclear applications); minimal dilution of overlay alloy composition; controlled cooling rate preserves beneficial microstructure in the overlay

4.2.3 Shielding Gas Management

• TIG passes: Pure Ar (99.999%) with He addition (25-50% He/Ar) for high-conductivity substrates; back purge with Ar at 5-10 L/min
• MIG passes: Ar/CO₂ (80/20) or Ar/O₂ (98/2) depending on overlay alloy; flow rate 15-20 L/min with proper nozzle positioning
• Transition zones: Gas mixture must be carefully managed at process transition points to prevent porosity and oxidation in the inter-pass regions

4.3 Microstructural Assessment Methodology

Assessment Parameter	Method	Acceptance Criteria (Typical)
HAZ grain size	ASTM E112 optical microscopy / EBS analysis	≥ ASTM Grade 6 (≤ 0.25 mm avg. grain diameter) for HSLA steels
Hardness profile	ASTM E18 Rockwell / Vickers HV0.3	Maximum hardness ≤ 350 HV for P91; ≤ 220 HV for SA-508
Phase distribution	XRD + EDS mapping	No continuous Cr-rich carbide networks at grain boundaries
Impact toughness	ASTM E23 Charpy V-notch at service temperature	≥ 41 J (30 ft-lb) at -29°C for RPV applications
Intergranular corrosion	ASTM A262 Practice E (5% CuSO₄ + H₂SO₄)	No intergranular attack (IGA) after sensitization heat treatment
Creep strength retention	ASTM E139 / ASTM E1079	≥ 90% of base metal creep rupture strength at 550°C

4.4 Key Findings from HAZ Microstructural Analysis

• Single-method TIG overlay produces the narrowest HAZ with minimal grain growth but requires excessive number of passes for thick cladding, creating cumulative thermal cycling in the weld metal
• TIG + MIG combinations offer the optimal balance of HAZ control and deposition efficiency; the MIG intermediate passes, when properly controlled, produce HAZ microstructures comparable to single TIG passes if inter-pass temperature is maintained ≤150°C
• SAW intermediate passes significantly widen the HAZ due to higher heat input and reduced shielding; require post-weld grain refinement (e.g., low-temperature rolling or additional PWHT cycles)
• Process transition interfaces (where TIG meets MIG or vice versa) create localized microstructural heterogeneity requiring careful parameter matching at transition points
• Post-weld heat treatment (PWHT) effectively normalizes HAZ microstructure regardless of overlay method combination, but cannot fully restore original grain size if excessive cumulative heat input was applied

5. Applicable Standards and Acceptance Criteria

5.1 Nuclear-Specific Standards

• NB/T 20002.1-2010: Nuclear power plant design criteria—seismic design
• NB/T 20257-1992: Specification for nuclear power plant welding procedures and qualification
• NB/SH/T 4701-2009: Nuclear power plant pressure equipment—welding procedure qualification
• NB/T 20107-2016: Nuclear power plant steel materials—welding consumables requirements
• RCC-M (French Nuclear Code): Part A1 for material selection, Part A2 for design, Part MC for construction
• ASME BPV Section III: Div.1 (Nuclear Power Plant Components), Div.2 (Containment), Div.3 (Fuel Channels)
• ASME BPV Section IX: Welding, Brazing, and Fusing Qualifications
• 10 CFR 50: U.S. Nuclear Regulatory Commission licensing requirements

5.2 Material and Process Standards

• ASTM A508 Cl.3: Nuclear reactor pressure vessel steel
• ASTM A387 Gr.11/22 Cl.2: Cr-Mo forged and rolled steel for nuclear components
• ASTM A213 T91/T92: Ferritic alloy steel tube for heat transfer
• ASTM A270/A268: Stainless steel tubing for nuclear applications
• ASME SA-308/A-309: TIG electrodes for austenitic stainless steels
• ASME SA-591/SA-592: MIG welding electrodes for austenitic stainless steels
• ISO 15614-1: Qualification procedures for welding of metallic materials—arc welding
• ISO 3834-2: Requirements for quality assurance—comprehensive quality system
• ISO 27700: Requirements for quality management systems for nuclear power plants
• NQA-1: Quality Assurance Requirements for Nuclear Power Facilities

5.3 Non-Destructive Testing Standards

• ASTM E164/E94: Radiographic testing of welds
• ASTM E1417: Magnetic particle testing
• ASTM E1149/E1280: Ultrasonic testing of welds
• ASTM E1416: Liquid penetrant testing
• NB/T 47013.2: Ultrasonic testing methods for butt welds

6. Common Risks and Controls

6.1 Metallurgical Risks

Risk	Cause	Control Measure
HAZ grain coarsening	Excessive cumulative heat input from multiple overlay passes	Limit total heat input; enforce inter-pass temperature ≤150°C; use low-heat-input TIG for first and last passes
Intergranular sensitization	Carbon migration to grain boundaries in Cr-rich overlay/base interface during thermal cycling	Use low-carbon (L-grade) overlay alloys (304L, 316L, 321H); limit time in sensitization range (450-850°C); apply stabilization heat treatment
Hardness exceedance in HAZ	Martensitic transformation in Cr-Mo substrate HAZ due to rapid cooling after overlay	Preheat substrate to 200-300°C; apply post-weld stress relief; limit carbon equivalent (CE ≤ 0.45)
Cracking at process transition interface	Thermal mismatch between TIG and MIG bead geometries; residual stress concentration	Optimize transition geometry; apply overlap strategy; verify with MT/PT at all transition points
Creep degradation	Coarsening of precipitates (M23C6, L12-Ni₃(Nb,Ti)) in HAZ of P91/P92 during long-term thermal cycling	Limit total heat input to P91/P92 substrates; apply appropriate PWHT (760°C × 2-4 h); verify with creep rupture testing

6.2 Process Risks

• Parameter drift during long production runs: Implement automated parameter monitoring with real-time feedback; calibrate equipment at defined intervals per NQA-1 requirements
• Contamination at process transitions: Clean transition zones with appropriate solvents; verify gas coverage continuity; perform visual inspection of each transition area
• Inconsistent bead geometry: Use multi-axis robotic welding or positioner systems; maintain consistent travel speed and wire feed parameters; implement in-process monitoring (traveling wire sensor, arc voltage monitoring)
• Welder/operator variability: Maintain documented welder performance records; conduct periodic requalification per ASME Section IX; implement statistical process control (SPC) on weld bead dimensions

6.3 Quality Assurance Controls

1. Pre-qualification testing: Perform full metallurgical evaluation (metallography, mechanical testing, corrosion testing) on coupon specimens before production application
2. Witness coupons: Weld qualification coupons simultaneously with production components using identical parameters and conditions
3. Process parameter locking: Lock critical parameters (current, voltage, travel speed, gas flow) in automated welding systems; require authorized override with documented justification
4. Traceability: Maintain complete records of all process parameters, material heat numbers, welder identification, and NDT results per NQA-1 and ISO 27700 requirements
5. Periodic revalidation: Conduct periodic (annual or per 5000 welds) revalidation testing to confirm continued process capability

7. Application Scenarios Across Technology Routes

7.1 TIG/MIG Weld Overlay Route (Primary Application)

This technical knowledge directly supports the company's primary TIG/MIG weld overlay capability for nuclear components:

• Reactor Pressure Vessel (RPV) Internals: Applying corrosion-resistant cladding (308L, 316L, 321H) to SA-508 Cl.3 components; multi-pass TIG+MIG sequences with verified HAZ integrity
• Steam Generator Tubes: Cladding of Inconel 690/625 on carbon steel headers; TIG打底+MIG填充+TIG精修 sequences for thick cladding layers
• Pressurizer and Accumulator Nozzles: Overlay of austenitic stainless steel on Cr-Mo steel nozzles; controlled HAZ microstructure to maintain impact toughness at operating temperatures
• Coolant Piping (P91/P92): Corrosion-resistant overlay on high-temperature piping; strict heat input control to preserve creep strength in the HAZ
• Control Rod Drive Mechanism Housings: Cladding of wear and corrosion-resistant alloys on low-alloy steel housings

7.2 Hydraulic Explosive Bonding Route (Complementary Application)

While hydraulic explosive bonding produces metallurgical bonds without melting, the HAZ microstructural knowledge informs:

• Post-bonding weld repair qualification: When explosive-bonded cladding requires welding repairs (e.g., at edges or damaged areas), the HAZ microstructure knowledge ensures that repair welds do not compromise the bond interface integrity
• Hybrid fabrication sequences: Components where explosive bonding provides base cladding and TIG/MIG overlay provides additional surface layers; the interface between bonded and welded regions requires careful HAZ management
• Welding to bonded interfaces: Qualification of welding procedures for structural attachments to explosively-bonded clad components; understanding of how welding HAZ affects the bond quality

7.3 Explosion Welding Route (Supporting Application)

Explosion welding creates a cold-welded interface with minimal thermal effects, but subsequent processing steps may involve welding:

• Edge welding and sealing: After explosion welding produces the clad plate, edge welding seals the interface; HAZ microstructure control prevents cracking and maintains bond integrity
• Post-explosion welding repairs: Any welding repairs on explosion-welded components require HAZ evaluation to ensure the cold-bond interface is not degraded
• Forming and machining operations: Understanding of HAZ microstructure helps predict material behavior during subsequent forming operations on explosion-welded clad products

8. Qualification Building and Regulatory Compliance

8.1 WPS/PQR Development

The HAZ microstructural data directly supports the development and qualification of Welding Procedure Specifications (WPS) and Performance Qualification Records (PQR) in accordance with:

• ASME Section IX, Part QW-400: Qualification of welders, welding operators, and welding and brazing operators
• ASME Section IX, Part QW-11: Qualification of welding procedure specifications
• ISO 15614-1:2017: Qualification procedures for welding of metallic materials
• NB/T 47014-2014: Qualification rules for welding procedures of pressure vessels and piping

8.2 Regulatory Documentation

For nuclear applications, the HAZ microstructural study generates documentation required by:

• NRC 10 CFR 50.55a: Design criteria for nuclear power plants
• IAEA GSR Part 3: Safety of nuclear power plants
• UK ONR GDN-2016: Generic design assessment for nuclear power stations
• Chinese NNSA/NNRA requirements: Nuclear safety regulatory authority approval for material and process qualifications

8.3 Quality System Integration

This technical capability is integrated into the company's quality management system per:

• ISO 27700:2022: Quality management systems for nuclear power plants—requirements
• ISO 9001:2015: General quality management system requirements
• ISO 3834-2:2021: Requirements for quality assurance for welding of metallic materials
• NQA-1 Rev.1: Quality Assurance Requirements for Nuclear Power Facilities

9. Practical Implementation Guidelines

9.1 Recommended Process Combinations by Application

Application	Base Material	Overlay Alloy	Recommended Sequence	Key HAZ Control
RPV internals	SA-508 Cl.3	308L/316L	TIG + MIG + TIG	IP temperature ≤200°C; PWHT per ASME III
SG headers	SA-213 T22	321H/347H	TIG + MIG + TIG	IP temperature ≤150°C; minimize total heat input
P91 piping	A213 T91	321H/Inconel 625	TIG打底 + TIG填充 + TIG精修	IP temperature ≤100°C; strict heat input ≤2.5 kJ/mm
Pressurizer nozzles	SA-387 Gr.11/22 Cl.2	308L/316L	TIG + MIG + TIG	IP temperature ≤150°C; PWHT at 760°C
Cooling water piping	SA-106 Gr.B	316L/2205 duplex	TIG + MIG	IP temperature ≤200°C; monitor dilution

9.2 Inspection and Verification Protocol

1. Visual inspection (VT) of each pass and transition area per ASTM E165
2. Magnetic particle testing (MT) or liquid penetrant testing (PT) of all overlay surfaces per ASTM E1417/E165
3. Ultrasonic testing (UT) for volumetric defects per ASTM E164/E1149
4. Hardness survey of HAZ and overlay per ASTM E18/E92, with traverses normal to the weld axis
5. Metallographic examination of witness coupons: cross-sections showing base metal, HAZ, and overlay; microstructural evaluation per ASTM E112/E3/E400
6. Mechanical testing of witness coupons: tensile, bend, impact per ASTM A370/E23
7. Corrosion testing: intergranular corrosion per ASTM A262; stress corrosion cracking per ASTM G44/G108

10. Strategic Significance for the Company

10.1 Technical Authority and Market Position

This HAZ microstructural study establishes Cladding Technology Shanxi Co., Ltd. as a technically authoritative provider of nuclear-grade cladding solutions. The depth of metallurgical understanding demonstrated through this research provides:

• Credibility in regulatory interactions and design institute reviews
• Confidence in proposing multi-process overlay solutions for complex nuclear components
• Intellectual property potential through patentable process sequences and parameter windows
• Academic standing through publication and presentation at nuclear technology conferences

10.2 Customer Value Proposition

For nuclear power plant operators and EPC contractors, this capability translates into:

• Reduced qualification risk: Pre-validated process combinations minimize the probability of HAZ-related non-conformances during production
• Accelerated project timelines: Ready-to-deploy WPS/PQR packages based on verified metallurgical data reduce qualification testing requirements
• Extended component life: Optimized HAZ microstructure ensures long-term performance under nuclear service conditions (thermal cycling, irradiation, corrosion)
• Regulatory compliance assurance: Complete metallurgical documentation packages support regulatory approval and in-service inspection programs

10.3 Continuous Improvement Framework

This technical study should be maintained as a living document with:

1. Annual review incorporating new production data, NDT results, and in-service performance information
2. Expansion to cover additional material combinations as the company's product portfolio grows
3. Integration with digital twin and simulation tools (e.g., SysWeld, Deform) for predictive HAZ modeling
4. Alignment with evolving regulatory requirements and industry best practices
5. Training utilization as a core reference document for welding engineer and metallurgist development programs

11. Conclusion

The systematic study of HAZ microstructural evolution under different cladding weld overlay method combinations represents a critical technical capability for nuclear power plant component manufacturing. By understanding and controlling the metallurgical consequences of multi-process overlay sequences, Cladding Technology Shanxi Co., Ltd. can deliver corrosion-resistant clad components with verified long-term performance, regulatory compliance, and minimal qualification risk. This knowledge asset directly supports the company's TIG/MIG weld overlay business line, complements its hydraulic explosive bonding and explosion welding capabilities, and positions the company as a technically differentiated provider in the demanding nuclear cladding market.