Research on Reducing Residual Stress at Weld Joints Through Internal Surface Weld Overlay of Nozzles
1. Definition and Fundamental Principles
Residual stress at weld joints constitutes one of the most critical integrity concerns in pressure vessel and piping fabrication, particularly at nozzle-to-shell connections where geometric discontinuities, thermal gradients, and constraint factors converge to produce severe stress concentrations. The research addressed in this study focuses on the application of internal surface weld overlay to nozzle internals as a deliberate engineering strategy to mitigate residual stress levels at critical weld joints through controlled thermal cycling, metallurgical redistribution, and stress-relaxation mechanisms.
The fundamental principle underlying this approach rests on several interrelated phenomena:
- Thermal relaxation mechanism: The controlled deposition of weld metal onto the internal nozzle surface introduces localized heating that induces compressive plastic strains in the base metal adjacent to the existing weld joint, effectively counteracting pre-existing tensile residual stresses.
- Metallurgical compatibility redistribution: A properly qualified overlay alloy (typically austenitic or duplex) creates a diffusion-controlled interface that alters the local stress-strain behavior and reduces susceptibility to stress-corrosion cracking (SCC) and fatigue failure.
- Geometric compensation: The overlay layer modifies the effective section modulus at the internal weld toe, redistributing hoop and axial stresses away from the highest-stress concentration zones.
- Thermal cycling effect: Multiple sequential passes of weld overlay create a repeated heating-cooling cycle analogous to a localized post-weld heat treatment (PWHT), promoting stress relaxation through creep and viscoplastic deformation at elevated temperatures.
2. Category and Business Positioning
This research falls squarely within the company's core competency in weld overlay engineering, specifically positioned at the intersection of:
- Weld overlay qualification engineering — demonstrating the ability to develop, document, and validate proprietary welding procedures that address specific metallurgical and mechanical performance challenges;
- Advanced residual stress management — extending the company's value proposition beyond mere corrosion protection to include structural integrity enhancement;
- Process qualification and WPS development — building a portfolio of qualified procedures that differentiate the company in competitive tenders for high-integrity pressure vessel and piping fabrication.
Within the broader business framework of Cladding Technology Shanxi Co., Ltd, this research entry represents a knowledge-management and process-engineering asset. It documents lessons learned, establishes best-practice parameters, and creates a replicable methodology that can be applied across multiple product lines — from refinery reactor nozzles to nuclear-grade piping spools and cryogenic service vessels.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Quantify residual stress reduction: Establish measurable stress reduction percentages (typically 30–60%) achievable through specified overlay parameters on nozzle internal surfaces.
- Define optimal process windows: Identify the range of heat input, interpass temperature, wire feed rate, and travel speed that maximizes stress relief while maintaining overlay integrity.
- Validate metallurgical compatibility: Confirm that the overlay weld metal does not introduce deleterious microstructural changes (e.g., martensitic transformation, carbide precipitation, or hot cracking susceptibility) in the base metal weld joint.
- Develop acceptance criteria: Establish NDT-based and stress-measurement-based acceptance thresholds for the overlay process.
3.2 Value Contribution
- For qualification building: Provides documented evidence of process capability that supports WPS/PQR packages required for ASME Section IX, GB/T 150, and API 510/API 570 compliance.
- For product delivery: Enables the company to offer "integrity-enhanced" nozzle overlay packages that reduce downstream inspection burden and extend service life, directly improving customer TCO (Total Cost of Ownership).
- For customer value: Reduces the probability of fatigue cracking and SCC at nozzle-to-shell joints — historically among the most failure-prone locations in process vessels — thereby enhancing operational safety and reducing unplanned shutdown risk.
4. Key Process and Implementation Points
4.1 Pre-Overlay Preparation
Successful residual stress reduction through internal nozzle overlay requires meticulous surface preparation and pre-existing weld joint characterization:
- Grind or machine the internal nozzle surface to a minimum Ra of 6.3 μm to ensure consistent heat transfer and bead adhesion.
- Perform baseline residual stress measurement using strain gauge rosettes or ultrasonic methods at the weld toe and HAZ regions prior to overlay.
- Verify that the existing weld joint has passed all required NDT (RT per ASME Section V Article 4, UT per Article 23) and that no defects requiring repair exist.
- Confirm base metal chemistry (CE values, Cr, Mo, Ni content) to select appropriate overlay alloy.
4.2 Overlay Welding Parameters (TIG Method — Recommended for Internal Nozzle Surfaces)
| Parameter | Typical Range | Rationale |
|---|---|---|
| Shielding Gas | 99.99% Ar or Ar/He (75/25) | Prevents oxidation; He blend improves heat input for thick sections |
| Heat Input | 0.8–2.5 kJ/mm | Must be sufficient to induce plastic relaxation without excessive HAZ softening |
| Interpass Temperature | 80–150 °C (low); 150–250 °C (stress-relaxation mode) | Higher interpass temps enhance stress relaxation via creep |
| Wire Diameter | 1.6 mm or 2.4 mm (ER309L, ER316L, or ER2209) | Selected per base metal composition and service environment |
| Travel Speed | 30–60 mm/min | Controls dilution and bead geometry; slower speed increases thermal cycling effect |
| Number of Passes | 2–4 layers | Multiple passes create cumulative thermal cycling for progressive stress relief |
| Current (DCEN) | 80–180 A | Dependent on wire diameter and travel speed |
| Voltage | 12–18 V | Controls arc length and bead width |
4.3 Implementation Sequence
- Baseline stress measurement: Record residual stress distribution at the nozzle weld toe using strain gauges (per ASTM E2228) or the center-hole drilling method.
- First overlay pass: Apply a thin root pass with low heat input to establish a metallurgically sound bond without excessive thermal distortion.
- Subsequent passes: Increase heat input progressively for passes 2–4 to maximize thermal cycling effect on the underlying weld joint.
- Post-overlay stress measurement: Repeat residual stress measurement at identical locations and calculate percentage reduction.
- NDT of overlay: Perform dye penetrant inspection (PT per ASTM E709) and, where required, ultrasonic testing (UT per ASTM E1655) of the overlay layer.
- Documentation: Compile all parameters, measurements, and results into a formal PQR/WPS package.
4.4 Alloy Selection Matrix
| Base Metal | Recommended Overlay | Key Consideration |
|---|---|---|
| Carbon steel (SA-106 Gr.B, SA-516 Gr.70) | ER309L (310S equivalent) | High dilution tolerance; Cr/Ni balance prevents cracking |
| Low-alloy steel (SA-387 Gr.11/22) | ER309L or ER310 | Higher Ni content compensates for higher CE of base metal |
| Austenitic stainless (SAF-304, SAF-316) | ER309L or ER347 | Match or slightly exceed base Cr/Ni; avoid 304L to prevent sensitization |
| Stellite / Hardfacing service | ER309L transition + ER2209 or Stellite 6 | Transition layer required; final layer for wear/corrosion resistance |
| Cr-Mo steels (SA-335 P91/P92) | ER309L (thin) + ER316L | Minimize thermal input; avoid exceeding 250°C interpass |
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 150.1–150.4 (Chinese Pressure Vessel Code) — General requirements for vessel fabrication, welding, and inspection.
- GB/T 12466 — Welding procedure qualification for pressure vessels.
- NB/T 47014 — Welding procedure qualification rules for nuclear-grade pressure equipment.
- ASME Section IX — Welding, Brazing, and Fusing Qualifications (Part QW for procedures, Part QW-300 series for TIG).
- ASME Section V, Article 4 — Radiographic examination acceptance.
- ASME Section V, Article 23 — Ultrasonic examination of welds.
- ASTM E2228 — Standard test method for determining residual stress by the incremental hole-drilling strain gauge method.
- ASTM E1655 — Standard practice for ultrasonic examination of welds (phased array).
- ASTM E709 — Standard practice for liquid penetrant examination.
- API 510 — Inspection Code for Pressure Vessels (in-service assessment of residual stress effects).
- NACE MR0175/ISO 15156 — Materials for H2S-containing environments (relevant when overlay alloy selection must account for sulfide stress cracking).
5.2 Acceptance Criteria for the Overlay Process
| Criterion | Acceptance Threshold | Method |
|---|---|---|
| Residual stress reduction | ≥30% reduction in peak tensile stress at weld toe | Strain gauge (ASTM E2228) |
| Overlay penetration | Full fusion to base metal; no lack of fusion | RT (ASME V Art.4) or MT |
| Overlay surface quality | No porosity > 2 mm, no cracks, no undercut | PT (ASTM E709) |
| Overlay thickness uniformity | ±0.5 mm of nominal; no thin spots < 1.5 mm | UT thickness measurement |
| Microstructure (HAZ) | No martensite in 309L weld; no brittle phases at interface | Macro/Micro examination (ASTM E3) |
| Hardness (overlay) | ≤35 HRC (for austenitic); within spec for hardfacing | ASTM E18 (Rockwell C) |
6. Common Risks and Controls
6.1 Risk Identification and Mitigation
| Risk | Mechanism | Mitigation Strategy |
|---|---|---|
| Hot cracking in overlay | Low-melting-point impurities (S, P) segregate at grain boundaries during solidification | Use low-sulfur/low-phosphorus filler (ER309L); control heat input; preheat if CE > 0.45 | Excessive HAZ softening | Overheating during multiple overlay passes reduces base metal strength | Monitor interpass temperature with thermocouples; limit passes to 4 maximum | Weld distortion of nozzle | Asymmetric thermal input causes angular or bowing distortion | Alternate welding direction; use backing plate; apply symmetric heat input | Incomplete stress relief | Insufficient heat input fails to reach relaxation temperature in base metal | Validate heat input calculations; use thermal imaging to confirm base metal temperature | Contamination of overlay | Internal nozzle surfaces may contain mill scale, oil, or moisture | Mandatory cleaning per AWS D1.1; gas-flow verification before and during welding |
| Post-overlay hydrogen cracking | Dissolved hydrogen diffuses to high-stress regions post-weld | Apply post-weld bake (150–200°C for 2h) or use low-hydrogen consumables |
6.2 Quality Assurance Controls
- Implement a documented hold point at baseline stress measurement to ensure traceability.
- Mandate third-party NDT for final overlay acceptance in critical-service applications (nuclear, pressure vessel codes).
- Maintain a process capability study (Cpk ≥ 1.33) on overlay thickness and stress reduction percentage across multiple production lots.
- Conduct periodic operator re-qualification per ASME Section IX QW-400 requirements (every 6 months for TIG).
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
This research directly strengthens the company's TIG/MIG weld overlay capability by providing:
- A validated methodology for combining corrosion protection with structural integrity enhancement in a single overlay operation — eliminating the need for separate PWHT or shot-peening stress-relief operations.
- Qualified WPS packages that can be submitted to clients requiring ASME Section IX or GB/T 12466 compliance for nozzle overlay on pressure vessels, heat exchangers, and reactor internals.
- A documented basis for claiming "integrated stress-relief overlay" as a differentiated service offering in competitive bids for refinery, petrochemical, and power generation projects.
Specific product applications include:
- Internal overlay of catalyst injection nozzles on FCC reactors (stress reduction + erosion/corrosion protection).
- Nozzle repair overlay on hydrogen service vessels where HTHA (High Temperature Hydrogen Attack) mitigation is required.
- Transition overlay on dissimilar-metal nozzle welds (e.g., CS-to-SS) where residual stress from differential thermal expansion is a known failure driver.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding (hydroforming-assisted bonding) is primarily a solid-state joining process, the residual stress research contributes indirectly through:
- Post-bond stress characterization: The measurement methodologies developed (strain gauge rosette placement, ultrasonic stress measurement) are directly transferable to characterizing residual stress fields in hydraulically bonded clad plates and pipe sections, informing subsequent machining allowances and PWHT requirements.
- Interface integrity validation: Understanding of stress redistribution near geometric discontinuities (nozzle penetrations through clad plates) informs the design of transition zones in hydraulically bonded assemblies where overlay may be required at penetrations.
- Qualification data generation: Stress measurement data from overlay research supports the overall residual stress mapping required in qualification programs for hydraulically bonded components per EN 1561 or ASTM E2770.
7.3 Explosion Welding Route
The research findings extend to explosion welding applications in the following ways:
- Stress-field modeling transfer: The thermal-mechanical modeling approach used to predict stress relaxation during overlay welding is conceptually applicable to understanding the residual stress fields generated in explosion-welded clad plates, particularly at edge regions and near nozzle penetrations.
- Post-explosion welding repair qualification: When explosion-welded clad plates require nozzle penetration with weld overlay at the penetration, the qualified overlay procedures from this research ensure that the repair weld does not compromise the explosion-welded interface integrity.
- Combined process qualification: For products requiring explosion-welded clad plates with overlaid nozzle internals (e.g., nuclear-grade heat exchanger tubesheets), this research provides the procedural basis for a combined WPS covering both the explosion welding interface and the subsequent TIG overlay at penetrations.
8. Conclusions and Strategic Recommendations
The research on residual stress reduction through internal nozzle surface weld overlay represents a significant knowledge asset for Cladding Technology Shanxi Co., Ltd. It transforms the company's overlay capability from a purely corrosion-protection service into a comprehensive structural integrity enhancement offering. Key strategic actions recommended include:
- Formalize the research into a qualified WPS/PQR package compliant with ASME Section IX and GB/T 12466, making it available for client submission and regulatory approval.
- Develop a standardized stress-measurement protocol (per ASTM E2228) to be offered as a value-added service alongside overlay work, providing clients with quantifiable proof of stress reduction.
- Extend the methodology to MIG overlay for thicker sections and higher production-rate applications, developing parallel WPS packages for both TIG (precision) and MIG (throughput) methods.
- Integrate residual stress reduction claims into marketing and technical proposals for high-integrity applications (nuclear, offshore, critical refinery units) where fatigue life extension and SCC resistance are primary client concerns.
- Establish a continuous improvement loop by collecting field performance data from deployed overlay work and updating process parameters based on actual service outcomes.
By leveraging this research to build qualified procedures, generate traceable stress-reduction data, and position the company as a provider of integrity-enhanced overlay solutions, Cladding Technology Shanxi Co., Ltd can command premium pricing in high-specification markets while delivering measurable risk reduction to end-users.