Microstructure and Properties of Ni-Based Cladding Layers Deposited by TIG Arc Weld Overlay
1. Definition and Fundamental Principles
Nickel-based weld overlay by TIG (Gas Tungsten Arc) welding refers to the deliberate deposition of nickel alloy consumables onto a substrate—typically carbon steel, low-alloy steel, stainless steel, or cast iron—using a non-consumable tungsten electrode and an inert shielding gas (argon or helium). The resulting cladding layer is designed to impart specific functional properties to the base material surface, including resistance to corrosion, erosion, wear, and high-temperature oxidation, without altering the bulk mechanical integrity of the substrate.
The fundamental metallurgical principle governing Ni-based TIG overlay lies in the controlled solidification of a partially melted interface between the molten weld pool and the solid substrate. As successive passes are deposited, the thermal history creates a columnar-to-equiaxed grain transition that directly influences hardness distribution, crack resistance, and corrosion performance. The dilution ratio—the percentage of base metal incorporated into each deposited layer—remains the single most critical variable controlling the final composition and properties of the cladding.
Ni-based overlay alloys fall into several metallurgical categories:
- Type I (Ni-Cr): High-temperature oxidation and sulfur corrosion resistance (e.g., Alloy 6, Alloy 600, Alloy 625)
- Type II (Ni-Cr-Si): General corrosion resistance in oxidizing acids (e.g., Alloy 800, Hastelloy C)
- Type III (Ni-Fe-Cr): Enhanced mechanical strength with moderate corrosion resistance (e.g., Alloy 718, Alloy 617)
- Hardfacing Ni-Cr-Mo / Ni-Cr-B-Si: Erosion and abrasive wear resistance (e.g., Stellite 6, Stellite 21, Carbide-Stellite)
2. Category and Business Positioning
This research entry belongs to the Weld Overlay Technology category within the company's three principal technology routes: TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. It represents the foundational metallurgical knowledge base that underpins the company's ability to qualify, execute, and certify TIG overlay processes for Ni-based cladding systems.
In terms of business positioning, mastery of Ni-based TIG overlay microstructure and properties enables the company to:
- Develop and qualify Welding Procedure Specifications (WPS) for customer-specific Ni-base overlay requirements
- Provide technically substantiated selection guidance for overlay alloy selection in aggressive service environments
- Support NDE interpretation by understanding expected microstructural features and their relationship to signal indications
- Address customer audit questions regarding dilution control, hardness mapping, and corrosion testing protocols
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The study of Ni-based cladding layer microstructure and properties under TIG arc welding conditions serves several critical engineering purposes:
- Dilution prediction and control: Understanding how heat input, interpass temperature, and preheating affect base metal dilution enables accurate prediction of the final cladding composition and its resultant properties.
- Crack resistance assessment: Identifying the microstructural conditions (e.g., sigma phase formation, Laves phase precipitation, martensite in Ni-Fe alloys) that promote solidification cracking or hot cracking allows proactive process parameter adjustment.
- Property optimization: Correlating solidification rate, cooling rate, and grain morphology with hardness (HV), tensile strength, and corrosion resistance enables systematic optimization of the overlay process.
- Transition zone characterization: Evaluating the metallurgical compatibility at the substrate-overlay interface ensures that the cladding will not delaminate or crack under thermal cycling or mechanical loading.
3.2 Value to the Organization
This knowledge base directly supports the company's value proposition of delivering certified, reliable cladding solutions. When customers require Ni-based overlay on critical components—such as heat exchanger tubes, pump impellers, valve seats, or pressure vessel internals—the company's demonstrated understanding of microstructure-property relationships provides the technical confidence needed for qualification approval and long-term service assurance.
4. Key Process and Implementation Points
4.1 Critical TIG Process Parameters for Ni-Based Overlay
| Parameter | Typical Range (Ni-Cr Alloys) | Typical Range (Ni Hardfacing) | Effect on Microstructure |
|---|---|---|---|
| Current (DCEN) | 80–200 A | 100–250 A | Higher current increases dilution and grain size |
| Travel Speed | 20–60 mm/min | 25–70 mm/min | Slower speed increases heat input and columnar grain length |
| Shielding Gas | 99.99% Ar or 75% Ar/25% He | 99.99% Ar | Helium increases heat input; affects arc stability and penetration |
| Preheat Temperature | 150–300°C (low dilution) | 200–400°C (hardfacing) | Higher preheat reduces cooling rate, promotes equiaxed grains |
| Interpass Temperature | ≤ 150°C | ≤ 100°C | Excessive interpass temp increases dilution and softening |
| Welding Position | PA (flat, 1G) | PA/PB (flat/vertical) | Position affects bead profile and dilution geometry |
| Filler Wire Diameter | 1.6–3.2 mm | 2.4–4.0 mm | Larger wire allows higher deposition rate |
4.2 Microstructural Features and Their Significance
| Microstructural Feature | Formation Conditions | Impact on Properties | Mitigation Strategy |
|---|---|---|---|
| Columnar dendrites | Low cooling rate, high heat input | Reduced transverse toughness; crack propagation path | Reduce heat input; use backing plate; multiple thin passes |
| Sigma phase (σ) | Ni-Cr alloys with Cr > 25%; slow cooling in 500–800°C range | Severe embrittlement; reduced ductility and toughness | Limit Cr content; control interpass temp; post-weld aging treatment |
| Laves phase (Ni₃Mo) | Ni-Mo alloys; slow solidification | Brittle intergranular phase; reduced corrosion resistance | Rapid cooling; reduce Mo content; solution treatment |
| Martensite (α′) | Ni-Fe alloys with high Fe content; rapid cooling | Hard but brittle; potential cracking | Control Fe dilution; tempering post-weld heat treatment |
| Carbide network (Cr₇C₃, Ni₃B) | Hardfacing alloys; eutectic solidification | Enhanced wear resistance but reduced toughness | Acceptable for wear applications; control for corrosion service |
| Equiaxed grains | High cooling rate; grain refiner presence (Ti, Zr, Al) | Improved isotropic properties; better crack resistance | Optimize travel speed; use appropriate filler composition |
4.3 Dilution Control Methodology
Dilution is defined as the mass percentage of base metal incorporated into the deposited weld metal. For Ni-based overlay applications, dilution is typically targeted as follows:
- Corrosion-resistant overlay: Dilution ≤ 10–15% (to maintain Ni-Cr alloying effects)
- Hardfacing overlay: Dilution ≤ 20–30% (limited impact on carbide distribution)
- Transition layer: Dilution 30–50% (intentional metallurgical bridge between substrate and overlay)
Key factors controlling dilution include:
- Weld bead geometry (width-to-depth ratio)
- Number of passes and build-up strategy
- Preheat and interpass temperature management
- Substrate thermal conductivity (steel vs. cast iron vs. stainless)
- Electrode angle and arc force
4.4 Typical Build-Up Strategy
- Surface preparation: Grind substrate to remove oxide, rust, and contaminants; expose clean metal surface within 1 hour of welding.
- Transition layer (if required): Deposit 1–2 passes of a compatible transition alloy (e.g., 309L for carbon steel substrates) to reduce dilution of subsequent Ni-based passes.
- Overlay passes: Deposit 2–5 passes of Ni-based alloy in a stringer bead pattern, maintaining interpass temperature below the specified limit.
- Surface finishing: Grind or machine the overlay surface to specified profile, ensuring minimum remaining overlay thickness is maintained.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard Number | Title / Scope | Relevance to Ni-Based TIG Overlay |
|---|---|---|
| GB/T 19403 | Welding procedure specification for weld overlay | Defines WPS qualification requirements for overlay welding in China |
| GB/T 8110 | Welding consumables – Filler metals for arc welding | Classification and specification of Ni-base filler metals |
| GB/T 10125 | Corrosion tests in artificial atmospheres – Salt spray tests | Corrosion resistance evaluation of Ni-based overlay surfaces |
| ASTM A388 | Standard Specification for Nickel Alloy Welding Electrodes | Filler metal qualification for Ni-base overlay consumables |
| ASTM A511 | Standard Specification for Nickel and Nickel Alloy Castings | Reference composition for Ni-base overlay alloy selection |
| ASME Section IX, Part QW-441 | Qualification of Welding Procedure Specifications for Weld Overlay | WPS qualification procedure for overlay welding |
| ASME B31.3 / B31.1 | Piping Code – Process / Power Piping | Acceptance criteria for overlay on piping components |
| NACE MR0175 / ISO 15156 | Sour Service Materials – H₂S Environment | Hardness limits (≤ 22 HRC) and NACE testing for Ni-base hardfacing |
| ISO 9530 | Welding consumables – Classification of filler metals | International classification of Ni-base filler wires |
| EN ISO 14432 | Welding consumables – Nickel and nickel alloys | European specification for Ni-base overlay consumables |
5.2 Acceptance Criteria for Ni-Based Overlay
- Visual inspection (VT): No cracks, porosity, undercut, or incomplete fusion visible on overlay surface (per ASME BPVC Section V, Article 2)
- Magnetic particle inspection (MT) or penetrant testing (PT): No linear indications exceeding 6 mm (¼ inch) in length (per ASME BPVC Section V, Articles 7 & 6)
- Hardness testing: Overlay surface hardness within specified range (e.g., 25–45 HRC for corrosion overlay; 45–60 HRC for hardfacing; ≤ 22 HRC for NACE sour service)
- Dilution verification: Spectrographic analysis confirming Ni, Cr, Fe content within specified limits at overlay surface and at 50% depth
- Corrosion testing: Salt spray test (ASTM B117) or immersion testing demonstrating acceptable corrosion rate for service environment
- Adhesion/shear testing: Overlay layer must withstand specified shear stress without delamination (typically ≥ 200 MPa for structural overlay)
- Microstructural examination: No excessive brittle phases (sigma, Laves) at substrate-overlay interface; acceptable grain morphology
6. Common Risks and Controls
| Risk | Root Cause | Detection Method | Control / Prevention |
|---|---|---|---|
| Solidification cracking (hot cracking) | High S/P content in substrate; excessive dilution; improper filler selection | MT/PT inspection | Use low-S consumables; limit dilution; pre-heat to reduce cooling rate; select crack-resistant filler |
| Excessive dilution | High heat input; insufficient interpass cooling; wrong bead geometry | Spectrographic analysis; hardness mapping | Reduce current; increase travel speed; use transition layer; enforce interpass temperature limits |
| Sigma phase embrittlement | Slow cooling in 500–800°C range; high Cr content in Ni-Cr alloys | Microstructural examination; hardness increase at interface | Limit interpass temperature; apply PWHT below 500°C; select lower-Cr filler; rapid cooling |
| Porosity (gas inclusion) | Inadequate shielding; contaminated base metal; moisture in consumables | RT/UT; visual inspection | Maintain gas flow rate (15–25 L/min); clean surface thoroughly; dry filler wire; use trailing gas cup |
| Incomplete fusion / lack of bonding | Low current; excessive travel speed; poor joint preparation | MT/PT; shear/adhesion testing | Optimize current-to-speed ratio; ensure proper surface preparation; use backing plate |
| Delamination under thermal cycling | CTE mismatch; residual stress; brittle interface phases | Thermal cycling test; UT thickness measurement | Use transition layer; apply stress-relief PWHT; select compatible alloy system |
| Tungsten inclusion | Electrode contact with molten pool; improper stick-out | Visual inspection; radiography | Maintain proper electrode stick-out (6–12 mm); use correct electrode diameter; avoid arc wandering |
7. Application Scenarios Across the Three Technology Routes
7.1 TIG/MIG Weld Overlay Route
Ni-based TIG weld overlay is the primary application domain for this technology. Key use cases include:
- Corrosion-resistant cladding: Ni-Cr alloys (Alloy 6, Alloy 625) applied to carbon steel heat exchanger tubes, pump casings, and chemical reactor internals exposed to sulfuric acid, hydrochloric acid, or mixed acid environments.
- Erosion-corrosion protection: Ni-Cr-Mo alloys (Hastelloy C-276, Alloy C-22) applied to slurry pump impellers, valve seats, and piping elbows in oil and gas production.
- Wear-resistant hardfacing: Stellite 6, Stellite 21, or Carbide-Stellite applied to crusher jaws, mill liners, and mining equipment components.
- Transition layers: Ni-base alloys used as transition layers between dissimilar materials (e.g., carbon steel to Hastelloy C-276 piping) to reduce residual stress and prevent cracking.
- Repair and reclamation: Restoration of worn or corroded surfaces on existing equipment without full component replacement.
7.2 Hydraulic Explosive Bonding Route
While hydraulic explosive bonding produces mechanically bonded clad plates without melting, the knowledge of Ni-based overlay microstructure informs the selection and qualification of the bonding interface. Specifically:
- Understanding of Ni-base alloy solidification behavior helps predict post-bonding heat treatment requirements for bonded Ni-steel clad plates.
- Knowledge of dilution-related microstructural effects supports the design of hybrid solutions where hydraulic explosive bonding provides the bulk clad plate and TIG overlay provides localized surface reinforcement at high-wear or high-corrosion zones.
- Microstructural characterization techniques (SEM, EBSD, TEM) developed for overlay studies are directly transferable to bonding interface analysis.
7.3 Explosion Welding Route
Explosion welding produces clad plates through high-velocity impact bonding. The Ni-based overlay microstructure research contributes to explosion welding applications in the following ways:
- Post-weld overlay on explosion-welded clad plate: When additional corrosion or wear protection is needed beyond the explosion-welded layer, TIG Ni-based overlay is applied to the clad surface. Understanding microstructure-property relationships ensures compatibility between the explosion-welded interface and the overlay.
- Interface characterization: The same metallurgical evaluation methods used for TIG overlay interfaces (microhardness traverse, SEM fractography, EDS mapping) are applied to characterize explosion welding wave interfaces and assess bonding quality.
- Thermal management: Knowledge of how heat input affects Ni-base microstructure informs post-explosion-welding stress relief and solution treatment procedures for Ni-steel clad plates.
8. Contribution to Qualification Building, Product Delivery, and Customer Value
8.1 Qualification Building
Mastery of Ni-based TIG overlay microstructure and properties is essential for:
- WPS/PQR qualification: ASME Section IX Part QW-441 requires demonstration that the overlay process produces acceptable mechanical and metallurgical properties. Understanding microstructure enables rational selection of process variables within the qualification envelope.
- Welder qualification: Welder performance qualifications for overlay welding require understanding of how operator technique affects dilution and microstructure, enabling effective supervision and training.
- Material qualification: Selection of appropriate Ni-base filler metals requires knowledge of how composition affects solidification behavior and final properties under TIG welding conditions.
- Process capability documentation: Demonstrated knowledge of microstructure-property relationships supports technical submissions for customer audits and regulatory inspections.
8.2 Product Delivery
This technical knowledge directly improves product delivery through:
- First-time-right execution: Understanding how process parameters affect microstructure reduces rework rates and improves schedule adherence.
- NDT interpretation: Knowledge of expected microstructural features enables more accurate interpretation of NDT indications, reducing false calls and unnecessary repairs.
- Non-destructive dilution monitoring: Hardness mapping combined with microstructural knowledge allows real-time dilution assessment without destructive sampling.
- Process optimization: Systematic understanding of microstructure-property relationships enables continuous improvement of deposition rates, dilution control, and property uniformity.
8.3 Customer Value
The technical depth demonstrated through this research capability provides measurable customer value:
- Extended service life: Properly qualified Ni-based overlay can extend component life by 3–10× compared to uncladded substrate, reducing unplanned shutdowns.
- Reduced total cost of ownership: While overlay adds initial cost, the extended service interval and reduced replacement frequency significantly lower lifecycle costs.
- Technical confidence: Customers benefit from the company's demonstrated metallurgical expertise when selecting overlay solutions for novel or critical applications.
- Regulatory compliance: Documented understanding of microstructure-property relationships supports compliance with industry standards (NACE, ASME, API) and regulatory requirements.
- Custom solution development: Ability to tailor overlay composition and process parameters to specific service conditions (temperature, pressure, chemical environment) provides competitive differentiation.
9. Summary and Forward Direction
The study of Ni-based cladding layer microstructure and properties under TIG arc welding conditions represents a cornerstone of the company's technical competence in weld overlay manufacturing. This knowledge base enables the company to qualify processes, deliver reliable products, and provide technically substantiated value to customers across the oil and gas, chemical processing, power generation, and mining industries.
Future development directions include:
- Integration of computational metallurgy (Thermo-Calc, JMatPro) with experimental microstructural data for predictive dilution and property modeling
- Development of real-time process monitoring systems using arc voltage/current signatures and thermal imaging for automated dilution control
- Extension of Ni-based overlay expertise to advanced alloys (single-crystal superalloys, high-entropy alloys) for next-generation aerospace and energy applications
- Integration of overlay microstructural knowledge with explosion welding and hydraulic bonding interface characterization for comprehensive clad product qualification
Key Takeaway: The microstructure of a Ni-based TIG weld overlay is not merely an academic curiosity—it is the direct determinant of the cladding's functional performance in service. Every process parameter (current, speed, preheat, interpass temperature, shielding) ultimately manifests as a microstructural feature that either enhances or degrades the intended properties. Mastery of this relationship is what separates a competent overlay welder from a qualified cladding technology provider.