Plasma Transfer Arc (PTA) Weld Overlay of Nickel-Based and Cobalt-Based Alloy Clad Layers: Microstructure and Wear Resistance
Plasma Transfer Arc (PTA) weld overlay is a high-precision thermal spray-adjacent process that deposits metallurgically bonded cladding layers onto substrate metals using a focused plasma arc as the heat source. When applied to nickel-based and cobalt-based superalloys, PTA produces clad layers with exceptional resistance to high-temperature oxidation, corrosion, and abrasive wear—properties that make these materials indispensable in demanding industrial environments. The study of microstructure evolution and wear resistance in PTA-deposited Ni-based and Co-based alloy layers represents a critical knowledge base for optimizing process parameters, ensuring metallurgical integrity, and delivering qualified products across multiple industrial sectors.
1. Definition and Fundamental Principles
1.1 Process Definition
Plasma Transfer Arc (PTA) cladding is a variant of gas-shielded arc welding in which the arc is constricted by a plasma nozzle, producing a highly concentrated heat source with temperatures reaching 10,000–30,000 K. This intense, focused energy input enables precise control over the molten pool geometry, dilution rates, and solidification behavior of the deposited overlay material. Unlike conventional TIG or MIG weld overlay, PTA achieves lower substrate dilution (typically 5–15% compared to 20–40% in conventional processes), resulting in clad layers that more closely retain the intrinsic composition and properties of the filler alloy.
1.2 Metallurgical Principles of Ni-Based and Co-Based Cladding
Nickel-based alloys (e.g., Hastelloy, Inconel, Stellite-type Ni) and cobalt-based alloys (e.g., Stellite 6, Stellite 21, CoCrW alloys) are deposited via PTA in powder or wire form. The fundamental metallurgical mechanisms governing their performance include:
- Carbide precipitation: In Co-based alloys, hard carbides (Cr₇C₃, Co₃W, WC, Cr₃C) precipitate during solidification, providing primary wear resistance. In Ni-based alloys, Ni₃(Al,Ti) γ' precipitates and Ni₃Nb γ" phases contribute to strength and creep resistance at elevated temperatures.
- Matrix hardening: Solid solution strengthening from alloying elements (Cr, Mo, W, Co) in the Ni or Co matrix enhances yield strength and microhardness.
- Columnar-to-equiaxed grain transition: PTA's high cooling rates promote fine columnar dendritic structures near the fusion boundary, transitioning to finer equiaxed grains in subsequent passes, influencing crack resistance and mechanical properties.
- Intercrystalline precipitation: Secondary phases such as M₆C, M₇C₃, and M₂₃C₆ carbides along grain boundaries can either enhance or degrade properties depending on morphology, distribution, and volume fraction.
1.3 Microstructural Evolution in Multi-Pass PTA Deposits
The layered nature of multi-pass PTA deposits creates a unique microstructural gradient. Each subsequent pass re-melts the top portion of the previous pass, creating a re-solidification zone with modified grain orientation. Key microstructural features include:
- Epitaxial grain growth: Columnar grains from the substrate propagate through successive passes when thermal gradients favor directional solidification.
- Pass boundary characteristics: Interpass boundaries exhibit localized segregation of alloying elements and secondary phase accumulation, which can serve as preferential crack initiation sites if not properly controlled.
- Porosity and inclusions: Gas porosity (Ar, N₂) and oxide inclusions may form at pass boundaries due to incomplete re-melting or inadequate shielding gas coverage.
2. Technical Purpose and Value
2.1 Engineering Value of Ni/Co-Based PTA Cladding
The primary engineering objectives of PTA overlay with Ni-based and Co-based alloys include:
- Wear resistance enhancement: Achieving microhardness values of 400–800 HV in the clad layer, representing 3–5× improvement over typical carbon or low-alloy steel substrates.
- Corrosion resistance: Ni-based alloys (Hastelloy C-276, Inconel 625) provide exceptional resistance to reducing acids, chlorides, and oxidizing environments.
- High-temperature performance: Co-based alloys maintain strength and oxidation resistance up to 1000°C, making them suitable for hot-section components in gas turbines and petrochemical reactors.
- Repair and restoration: Economic restoration of worn or damaged components without complete replacement, extending service life by 3–10× depending on the application.
2.2 Knowledge Base Contribution
The systematic study of microstructure and wear resistance in PTA-deposited Ni/Co alloy layers provides the following organizational value:
- WPS qualification support: Empirical data on dilution rates, microhardness profiles, and microstructural integrity directly support the development and qualification of Welding Procedure Specifications (WPS) in accordance with ASTM A5.1 and ASME Section IX.
- Process optimization: Understanding the relationship between process parameters (current, voltage, travel speed, powder feed rate) and microstructural outcomes enables systematic optimization for specific alloy-substrate combinations.
- Failure analysis capability: Knowledge of microstructural degradation mechanisms (carbide coarsening, grain boundary embrittlement, interpass cracking) enables root cause analysis of field failures.
- Customer technical support: Detailed metallurgical understanding enables credible technical proposals, design reviews, and performance guarantees for end customers.
3. Key Process and Implementation Points
3.1 Critical Process Parameters
| Parameter | Typical Range (Ni-Based) | Typical Range (Co-Based) | Effect on Microstructure/Properties |
|---|---|---|---|
| Plasma Current | 150–350 A | 150–400 A | Higher current increases dilution; excessive current causes substrate penetration and macrosegregation |
| Travel Speed | 100–300 mm/min | 80–250 mm/min | Higher speed reduces heat input, promotes finer grains; too high causes incomplete fusion |
| Powder Feed Rate | 200–600 g/min | 200–700 g/min | Determines bead width/height ratio; affects dilution and porosity |
| Shielding Gas Flow | 15–30 L/min (Ar or Ar-He) | 15–30 L/min (Ar or Ar-He) | Insufficient shielding causes oxide inclusions and porosity |
| Interpass Temperature | ≤ 200°C | ≤ 250°C | Excessive interpass temperature promotes grain coarsening and carbide precipitation |
| Number of Passes | 2–6 | 2–8 | More passes increase total clad thickness; each pass modifies the prior pass microstructure |
| Substrate Dilution | 5–15% | 5–20% | Lower dilution preserves alloy properties; higher dilution may cause cracking in Co-based alloys |
3.2 Powder Selection and Preparation
The powder form of Ni-based and Co-based alloys used in PTA must meet stringent quality requirements:
- Particle morphology: Spherical powders (produced by gas atomization or plasma atomization) are preferred for uniform flow and consistent melting behavior. Irregular or dendritic powders may cause feed inconsistencies and porosity.
- Particle size distribution: Typically 45–150 μm for PTA; bimodal distributions improve packing density and reduce powder bridging in the feed system.
- Chemical composition: Must conform to ASTM B318 (Ni-based) or ASTM B795 (Co-based) specifications. Trace impurities (S, P, O, N) must be controlled below specified limits.
- Dryness and storage: Powders must be stored in desiccated conditions (dew point ≤ -40°C) to prevent oxide formation and moisture absorption, which would lead to hydrogen porosity.
3.3 Substrate Preparation
- Machining: Substrate surface must be machined to provide a flat, clean deposition surface with appropriate geometry for bead overlap. Typical surface roughness: Ra ≤ 6.3 μm.
- Beveling: A V-groove or J-groove preparation (angle 60°–90°, depth 3–10 mm) is typically required to ensure adequate fusion and mechanical bonding at the clad-substrate interface.
- Cleaning: Removal of all contaminants (oil, rust, paint, scale) using mechanical grinding, chemical cleaning, or solvent wiping. Residual contaminants are a primary cause of lack of fusion and porosity.
- Preheating: Substrate preheating to 100–300°C (depending on alloy and section thickness) reduces thermal stresses and minimizes cracking risk, particularly for high-carbon steels and Co-based overlays.
3.4 Solidification and Heat Treatment
Post-deposition heat treatment is critical for optimizing the microstructure and properties of PTA-clad Ni/Co alloy layers:
- Stress relief: Solution treatment at 1050–1150°C followed by controlled cooling relieves residual stresses and homogenizes the microstructure. For Co-based alloys, solution treatment at 1100–1200°C dissolves coarse carbides for subsequent precipitation hardening.
- Precipitation hardening: Ni-based alloys may receive aging treatments (e.g., 720°C for 8h for Inconel 718-type) to precipitate γ' and γ" phases for enhanced strength.
- Carbide re-distribution: In Co-based alloys, appropriate heat treatment re-distributes and refines carbide phases (Cr₇C₃, Co₃W), transforming them from coarse, irregular intergranular networks into fine, uniformly dispersed particles that maximize wear resistance.
4. Wear Resistance Characterization
4.1 Wear Mechanisms in Ni/Co-Based Clad Layers
The wear resistance of PTA-deposited Ni/Co alloy layers is governed by the interplay of multiple mechanisms:
- Adhesive wear: Material transfer between contacting surfaces due to localized cold welding and subsequent shearing. Ni-based alloys with high Cr content exhibit reduced adhesive wear due to passive film formation.
- Abrasive wear: Material removal by hard particles or surfaces. Hard carbide phases in Co-based alloys (HV 1500–2000 for Co₃W, HV 2000+ for WC) provide primary resistance to abrasive wear.
- Oxidative wear: Oxide scale formation and spallation at elevated temperatures. Co-based alloys with Cr and Al form protective oxide scales (Cr₂O₃, Al₂O₃) that reduce oxidative wear rates.
- Microplowing and plowing: Plastic deformation of the softer matrix around hard carbides, leading to material displacement and eventual loss.
4.2 Microhardness Distribution
Microhardness testing (Vickers, 500g load) across the clad layer thickness reveals characteristic profiles:
- Ni-based alloys (e.g., Hastelloy C-276): 250–400 HV, relatively uniform across the clad thickness with slight softening near the fusion boundary due to dilution.
- Co-based alloys (e.g., Stellite 6): 400–600 HV as-deposited; 600–800 HV after proper heat treatment with refined carbide distribution.
- Transition zone: Gradual hardness decrease from clad to substrate over 0.5–2 mm, indicating metallurgical bonding and gradient dilution.
4.3 Tribological Testing Standards
- ASTM G99: Pin-on-disk wear testing for dry sliding wear characterization.
- ASTM G79.1: Abrasive wear testing using standardized abrasive grit.
- ASTM G111: Reciprocating slider wear testing.
- ASTM G98: Sliding wear testing under controlled load and speed conditions.
- ISO 21748: Dry sliding wear testing methods.
5. Applicable Standards and Acceptance Criteria
5.1 Material Specifications
| Standard | Scope | Key Requirements |
|---|---|---|
| ASTM B318 | Nickel-based cast superalloys and related alloys | Chemical composition, melting practice, heat treatment |
| ASTM B795 | Cobalt-base cast superalloys | Composition, mechanical properties, microstructure |
| ASTM B528 | Welding electrodes of nickel and nickel-base alloys | Chemical composition, mechanical properties of deposited metal |
| ASTM B343 | Welding electrodes of cobalt-base alloys | Composition, deposition quality requirements |
| GB/T 20931 | Welding consumables for Ni-base alloys | Chinese national specification for Ni-base welding materials |
| NACE MR0175/ISO 15156 | Materials for H₂S-containing environments | Hardness limits, microstructure requirements for sour service |
5.2 Welding Procedure and Qualification Standards
- ASTM A5.1 / ASME Section IX: Qualification of welding procedures and welders for PTA overlay, including essential variables (current, voltage, travel speed, powder feed rate, electrode/powder type, shielding gas, interpass temperature).
- ASTM A5.2: Welding procedure qualification for overlay welding specifically.
- ASME Section IX, QW-15: Qualification of procedures for welding other than fusion welding (applicable by analogy for PTA process parameter limits).
- ISO 15614-1: Qualification testing of welding procedures for steels and nickel alloys.
- ISO 9606-1: Qualification testing of welders for steels and nickel alloys.
- NB/T 47014: Chinese national standard for qualification of welding procedures for pressure vessels.
- API 1104: Welding of pipelines and related facilities (relevant for pipeline overlay applications).
5.3 Non-Destructive Examination (NDE) Acceptance Criteria
- ASTM E1417: Magnetic particle examination for surface and near-surface defects in ferromagnetic materials. Acceptance: no linear indications exceeding specified length; no clusters of round indications.
- ASTM E164: Dye penetrant examination for surface-breaking defects. Acceptance: no cracks, lack of fusion, or porosity clusters exceeding specified dimensions.
- ASTM E2378 / ASTM E1092: Ultrasonic examination for internal defects. Acceptance: no indications above background level; no lack of fusion or cracks at clad-substrate interface.
- ASTM E127: Eddy current examination for surface and near-surface defects on non-ferromagnetic (Ni/Co-based) clad surfaces.
- ASME Section V: General NDE acceptance criteria for pressure vessel and piping applications.
5.4 Mechanical and Metallurgical Acceptance
- Microhardness: Clad layer hardness must meet minimum specified values (e.g., ≥ 400 HV for Co-based, ≥ 250 HV for Ni-based per customer specification or material standard).
- Dilution rate: Maximum allowable substrate dilution typically 15–20%, verified by optical emission spectroscopy (OES) or scanning electron microscopy (SEM-EDS) line scans.
- Interface integrity: No cracks, lack of fusion, or intermetallic phases at the clad-substrate interface, verified by metallographic examination (ASTM E3).
- Grain size: Conforming to specified grain size requirements per material standard (e.g., ASTM E112 grain size measurement).
6. Common Risks and Controls
6.1 Microstructural Risks
| Risk | Cause | Consequence | Control Measures |
|---|---|---|---|
| Hot cracking (solidification cracking) | High dilution, slow cooling, low-ductility intermetallics at grain boundaries | Cracks in clad layer or at interface | Limit dilution ≤ 15%; use appropriate interpass temperature; select compatible filler; add Ti/B to refine grain |
| Coarse carbide precipitation | Excessive interpass temperature; slow cooling rates | Reduced wear resistance; embrittlement | Control interpass temperature ≤ 200°C; apply post-weld heat treatment to refine carbides |
| Intergranular embrittlement | Segregation of S, P, Sn at grain boundaries; σ-phase formation in Ni-based alloys | Reduced toughness; intergranular fracture | Control impurity levels in filler; avoid time at 600–800°C for Ni-Cr alloys; apply solution heat treatment |
| Columnar grain propagation | High thermal gradient; insufficient grain refinement | Reduced transverse toughness; anisotropic properties | Optimize travel speed and heat input; use grain refiners (Ti, Zr); apply multi-pass strategy with varying parameters |
6.2 Process Risks
| Risk | Cause | Consequence | Control Measures |
|---|---|---|---|
| Porosity | Insufficient shielding; wet powder; high gas porosity from N₂/H₂O absorption | Reduced density; stress concentration; possible leak paths | Ensure adequate shielding gas flow; store powder in desiccated conditions; control powder moisture content ≤ 0.05% |
| Lack of fusion | Excessive travel speed; insufficient current; poor surface preparation | Weak bonding; delamination risk; premature failure | Optimize heat input; verify surface cleanliness; perform dilution testing; conduct interface examination |
| Excessive dilution | High current; slow travel speed; deep substrate penetration | Loss of alloy properties; possible cracking | Use lower current; increase travel speed; use powder with higher melting point; monitor dilution via OES |
| Warping and distortion | High thermal input; asymmetric welding sequence; thick sections | Dimensional inaccuracy; residual stress; possible cracking | Use balanced welding sequence; apply backing bars; control preheat and interpass temperature; consider stress relief |
6.3 Environmental and Operational Risks
- Plasma torch misalignment: Offset between plasma arc center and powder stream causes asymmetric bead profiles and inconsistent dilution. Control: regular torch alignment verification and maintenance.
- Shielding gas contamination: Moisture or oxygen contamination of shielding gas leads to oxide inclusions and porosity. Control: use gas dryers, monitor dew point, replace gas cylinders regularly.
- Powder feed inconsistency: Clogging or bridging in the powder feed system causes bead height variation and composition inconsistency. Control: use vibratory feeders, maintain powder hopper level, inspect feed lines regularly.
- Operator technique variability: Manual PTA requires skilled operators. Control: implement rigorous operator qualification per ISO 9606-1; use automated PTA systems for consistent production.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Integration
While PTA is a distinct process, the microstructural and wear resistance knowledge gained from Ni/Co-based PTA studies directly informs TIG and MIG weld overlay practices:
- Filler selection: Understanding the dilution behavior of Ni/Co alloys in PTA provides benchmarks for predicting dilution in TIG/MIG overlay, enabling selection of appropriate filler compositions to achieve target properties even at higher dilution rates.
- Multi-layer strategy: The layered microstructure knowledge from PTA applies to multi-pass TIG/MIG overlay, guiding decisions on pass sequence, interpass temperature, and heat input to optimize the final clad microstructure.
- Transition layer design: For dissimilar material overlays (e.g., Ni-based alloy on carbon steel), the knowledge of interface microstructure evolution supports the design of intermediate transition layers (e.g., 309L stainless steel) to minimize cracking and ensure metallurgical compatibility.
- Post-weld heat treatment: Heat treatment parameters developed for PTA deposits can be adapted for TIG/MIG overlay deposits, with adjustments for the typically higher dilution and coarser microstructure characteristic of these processes.
7.2 Hydraulic Explosive Bonding Integration
In hydraulic explosive bonding (liquid explosion welding), Ni-based and Co-based alloy sheets are bonded to substrate sheets using a controlled liquid explosive medium. The PTA microstructure and wear resistance knowledge contributes in the following ways:
- Post-bonding overlay: After hydraulic explosive bonding creates a Ni/Co alloy cladded plate, PTA may be applied to repair bonding defects, add thickness to thin clad layers, or apply additional wear-resistant layers on the bond surface. Understanding the PTA microstructure ensures compatibility with the existing bonded interface.
- Material selection: Wear resistance data from PTA-deposited Ni/Co alloys informs the selection of clad materials for hydraulic explosive bonding, ensuring the bonded cladding meets performance requirements.
- Quality verification: Microstructural examination techniques developed for PTA (metallography, hardness profiling, SEM-EDS) are applied to verify the quality of hydraulic explosive bonded interfaces, ensuring metallurgical bonding and absence of defects.
- Composite structure design: For complex components requiring both high-strength bonding and surface wear resistance, hybrid approaches combining hydraulic explosive bonding (for bulk cladding) and PTA (for surface enhancement) can be designed using the knowledge base from both technologies.
7.3 Explosion Welding Integration
In explosion welding, the high-velocity impact creates a distinctive wavy metallurgical bond interface. PTA knowledge of Ni/Co alloy microstructures and wear properties supports explosion welding in the following contexts:
- Post-explosion welding repair: If explosion welding produces localized bonding defects (separation, voids), PTA can be used to repair these areas. The microstructural compatibility between PTA deposits and explosion-welded interfaces must be verified to prevent cracking at the repair boundary.
- Explosion-welded substrate preparation: When PTA overlay is applied to explosion-welded clad plates, the unique microstructure at the explosion welding interface (shear bands, voids, intermetallic phases) must be considered in process parameter selection to ensure adequate fusion and bonding.
- Performance benchmarking: Wear resistance data from PTA-deposited Ni/Co alloys provides comparative benchmarks for evaluating the wear performance of explosion-welded Ni/Co clad plates, supporting material selection decisions.
- Multi-technology qualification: For products combining explosion welding and PTA overlay, comprehensive qualification per ASTM A5.1 and ASME Section IX must cover all process variables, including the interaction between the explosion-welded interface microstructure and the PTA deposited microstructure.
8. Qualification Building and Customer Value
8.1 WPS Qualification Support
The systematic study of Ni/Co-based PTA overlay microstructure and wear resistance directly supports the development of qualified Welding Procedure Specifications:
- Essential variable definition: Empirical data on the sensitivity of microstructure and properties to process parameters (current, voltage, travel speed, feed rate, interpass temperature) enables the definition of qualified essential variables and their permissible ranges per ASTM A5.1.
- Performance testing: Wear resistance data (ASTM G99, ASTM G79.1) provides quantitative performance verification that clad layers meet customer-specified wear life requirements.
- Material compatibility: Microstructural studies of clad-substrate interfaces establish compatibility matrices for various substrate alloys (carbon steel, stainless steel, low-alloy steel, high-alloy steel) and Ni/Co filler alloys.
- Heat treatment qualification: Post-weld heat treatment parameters optimized for specific Ni/Co alloys are documented in the WPS, ensuring consistent property achievement across production batches.
8.2 Product Delivery Enhancement
- Process capability documentation: Detailed microstructural and tribological data demonstrates process capability to customers, supporting competitive bids for high-value overlay projects in oil & gas, power generation, mining, and chemical processing.
- Quality assurance: Standardized microstructural examination and hardness testing protocols ensure consistent product quality and traceability, meeting customer quality management system requirements (ISO 9001, ASME NQA-1).
- Failure analysis and warranty support: Understanding of microstructural degradation mechanisms enables rapid root cause analysis of field failures, supporting warranty claims and continuous improvement.
- Customization capability: The knowledge base enables tailored overlay solutions for specific customer requirements (wear type, environment, temperature, life expectancy), differentiating the company's offerings from competitors.
8.3 Customer Value Proposition
- Extended component life: PTA-clad Ni/Co alloy surfaces deliver 3–10× life extension over unclad substrates in abrasive and corrosive environments, reducing downtime and maintenance costs.
- Cost avoidance: Overlay repair of worn components avoids complete replacement, saving 50–80% of capital expenditure for critical components such as valve bodies, pump casings, and turbine components.
- Performance reliability: Qualified processes and verified microstructures ensure consistent performance, reducing the risk of premature failure and unplanned shutdowns.
- Technical partnership: Deep metallurgical expertise positions the company as a technical partner rather than a simple contractor, enabling collaborative design of overlay solutions for challenging applications.
9. Conclusion
The study of microstructure and wear resistance in PTA-deposited nickel-based and cobalt-based alloy clad layers represents a cornerstone of technical capability for Cladding Technology Shanxi Co., Ltd. This knowledge base enables the company to qualify welding procedures, deliver high-performance overlay products, and provide credible technical support across its three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding. By maintaining rigorous control over process parameters, microstructural integrity, and mechanical performance, the company ensures that every clad component meets the demanding requirements of industrial customers in oil & gas, power generation, mining, chemical processing, and aerospace applications. The systematic approach to microstructural understanding, wear resistance characterization, and quality assurance establishes a foundation for continuous improvement, qualification expansion, and long-term customer value creation.