Flame Spraying and Plasma Surfacing of Ni60 and Ni60-WC Coatings: Microstructure and Performance
1. Definition and Fundamental Principles
Flame spraying and plasma surfacing (plasma arc weld overlay) are two distinct yet complementary thermal spray and weld overlay technologies used to deposit nickel-based hardfacing coatings—specifically Stellite-type Ni60 (UNS N06600/N06602) and Ni60-WC (tungsten carbide-reinforced Ni60)—onto base substrates. These processes are employed to impart exceptional wear resistance, corrosion resistance, and thermal stability to critical components in mining, oil and gas, power generation, and chemical processing industries.
1.1 Flame Spraying (Flame Thermal Spraying)
Flame spraying utilizes the combustion of acetylene-oxygen or propane-oxygen mixtures to generate a high-temperature flame (approximately 3,100–3,400 °C) that melts or partially melts metallic wire or powder feedstock. The molten or semi-molten particles are propelled by compressed gas onto the prepared substrate, forming a thermally bonded coating. The resulting microstructure typically features lamellar morphologies with oxide inclusions, inter-splat boundaries, and residual porosity.
1.2 Plasma Surfacing (Plasma Arc Weld Overlay)
Plasma surfacing employs a high-velocity, high-temperature plasma arc (up to 10,000–20,000 °C at the arc core) generated within a constricted plasma torch. Powder or wire feedstock is injected into the plasma stream and melted to full liquid state before being deposited onto the substrate. This process produces a fully dense, metallurgically bonded coating with a columnar dendritic microstructure and minimal porosity. The dilution ratio between the deposited coating and base metal can be precisely controlled through arc current, travel speed, and powder feed rate optimization.
1.3 Ni60 and Ni60-WC Coating Systems
Ni60 is a classic cast iron-strengthened nickel-based alloy containing approximately 55–65% Ni, 28–35% Cr, and 1.5–3% C. Upon solidification, it forms a matrix of austenite with extensive precipitation of hard carbides (Cr₇C₃, Cr₃C, Ni₃C, and Fe₇C₃) within dendritic cells. Ni60-WC coatings incorporate 20–30% tungsten carbide (WC) particles into the Ni60 matrix, providing synergistic reinforcement through composite hardening mechanisms.
2. Category and Business Positioning
Within the broader cladding and surface engineering technology portfolio of Cladding Technology Shanxi Co., Ltd., flame spraying and plasma surfacing of Ni60/Ni60-WC coatings occupy a strategic position in the weld overlay and thermal spray route, complementing the company's hydraulic explosive bonding and explosion welding capabilities. This technology entry represents:
- Research and Development Capability: Demonstrates deep metallurgical understanding of coating microstructure-property relationships, essential for WPS (Welding Procedure Specification) development and qualification.
- Technical Differentiation: Positions the company as a specialist in surface engineering solutions for extreme wear and corrosion environments where conventional cladding methods are insufficient.
- Qualification Foundation: Provides the scientific basis for customer-specific coating design, process optimization, and performance guarantee documentation.
3. Technical Purpose and Value
3.1 Engineering Performance Targets
The primary engineering objectives of Ni60 and Ni60-WC coatings prepared by flame spraying and plasma surfacing include:
- Hardness of 35–50 HRC (Ni60) and 60–75 HV₀.₃ (Ni60-WC) after appropriate heat treatment
- Abrasion resistance 5–20 times that of base carbon or low-alloy steels
- Corrosion resistance in sulfuric acid, hydrochloric acid, and hot alkaline solutions
- Thermal stability up to 650 °C (Ni60) with minimal hardness degradation
- Adhesion strength exceeding 30 MPa (peel/shear tests)
3.2 Value Proposition to Customers
- Extended Service Life: Coated components in mining crushers, cement mill liners, and pump impellers achieve 3–10× life extension over uncoated equivalents.
- Reduced Downtime: Predictable coating performance enables planned maintenance scheduling rather than emergency replacement.
- Cost Optimization: Coating thickness of 1–5 mm on a high-strength base is significantly more economical than fabricating the entire component from Ni-based superalloy.
- Customization: Ability to tailor coating composition, thickness, and dilution ratio to specific service conditions.
4. Key Process and Implementation Points
4.1 Substrate Preparation Requirements
Proper surface preparation is the single most critical factor determining coating adhesion and integrity:
- Mechanical Grinding: Remove all scale, rust, oil, and contaminants using wire brushing or grinding to bare metal
- Shot Peening / Abrasive Blasting: Achieve surface roughness Ra 6–12.5 μm for flame spraying; Ra 3–6 μm for plasma surfacing
- Surface Temperature: Maintain substrate at 150–350 °C (flame heating or induction preheating) to prevent quench cracking in high-carbon and high-alloy steels
- Preheating for Thick Sections: For base materials thicker than 25 mm, preheat to 250–400 °C to reduce residual stress and prevent hydrogen-induced cracking
4.2 Flame Spraying Process Parameters
| Parameter | Typical Range (Ni60 Wire) | Typical Range (Ni60-WC Powder) |
|---|---|---|
| Flame Temperature | 3,100–3,400 °C (O₂-C₂H₂) | 3,100–3,400 °C (O₂-C₂H₂) |
| Wire/Nozzle Distance | 20–40 mm | N/A (powder feed) |
| Stand-off Distance | 50–150 mm | 80–150 mm |
| Travel Speed | 100–300 mm/min | 100–250 mm/min |
| Wire Feed Rate | 0.5–1.5 m/min | N/A |
| Compressed Air Pressure | 0.3–0.6 MPa | 0.3–0.6 MPa |
| Coating Thickness per Pass | 0.1–0.5 mm | 0.05–0.3 mm |
| Inter-pass Temperature | < 350 °C | < 300 °C |
4.3 Plasma Surfacing Process Parameters
| Parameter | Ni60 Powder | Ni60-WC Powder |
|---|---|---|
| Plasma Arc Current | 200–400 A | 250–450 A |
| Plasma Gas (Ar) | 4–8 L/min | 5–10 L/min |
| Shielding Gas (Ar/He mix) | 15–25 L/min | 20–30 L/min |
| Travel Speed | 300–800 mm/min | 250–600 mm/min |
| Powder Feed Rate | 80–150 g/min | 60–120 g/min |
| Torch Nozzle Distance | 8–15 mm | 10–18 mm |
| Coating Thickness per Pass | 0.2–0.8 mm | 0.15–0.6 mm |
| Inter-pass Temperature | < 300 °C | < 250 °C |
4.4 Post-Deposition Heat Treatment
Heat treatment is essential to optimize the microstructure and properties of both flame-sprayed and plasma-surfaced Ni60 coatings:
- Stress Relief: 550–650 °C for 1–2 hours to relieve residual stresses and prevent delayed cracking
- Carbide Precipitation Treatment: 900–950 °C for 2–4 hours (solution treatment) followed by 600–700 °C for 4–8 hours (aging) to promote fine, uniformly distributed Cr₇C₃ and Cr₃C carbides
- WC Coating Considerations: Limit peak temperature to below 900 °C to prevent WC decomposition into W₂C and free carbon, which degrades hardness and corrosion resistance
4.5 Microstructural Analysis and Characterization
The study of microstructure-property relationships in Ni60 and Ni60-WC coatings involves:
- Optical Microscopy (OM): Examination of columnar dendritic structures, dilution zone morphology, and interfacial bonding characteristics
- Scanning Electron Microscopy (SEM) with EDS: Mapping of Cr₇C₃, Cr₃C, Ni₃C, Fe₇C₃ carbide distribution and composition; analysis of WC particle integrity and interfacial bonding in Ni60-WC coatings
- X-ray Diffraction (XRD): Phase identification of austenite (γ), martensite (α'), carbide phases, and detection of untransformed or oxidized phases
- Vickers Hardness Profiling: Cross-sectional hardness measurements (HV₀.₃) from substrate through dilution zone to coating surface
- Adhesion Testing: Peel test (per ASTM C1024) or shear test to quantify coating-substrate bond strength
- Wear Testing: Pin-on-disk (ASTM G99), dry sand-rubber (ASTM G65), or sliding wear tests against alumina or steel counterfaces
5. Applicable Standards and Acceptance Criteria
5.1 Material Standards
| Standard | Description | Relevance |
|---|---|---|
| ASTM B108 | Standard Specification for Nickel-Chromium-Iron-Molybdenum Alloy (UNS N06602) Casting | Ni60 alloy composition and casting requirements |
| ASTM B246 | Standard Specification for Nickel-Chromium-Iron Casting Alloys for Investment Casting | Stellite-type alloy specifications |
| GB/T 16496 | Cast Iron Strengthened Nickel-Based Hardfacing Alloys | Chinese national standard for Ni60-type alloys |
| ASME SFA-5.7 | Welding Consumable Specifications for Stellite Surfacing | Welding consumable specifications for Ni-based surfacing |
| GB/T 12469 | Electrodes for Nickel and Nickel Alloy Welding | Chinese standard for Ni-based welding consumables |
5.2 Process and Performance Standards
| Standard | Description | Relevance |
|---|---|---|
| ASTM C1024 | Standard Test Method for Pull-Off Adhesion Strength of Thermal Sprayed Coatings | Adhesion qualification testing |
| ASTM G65 | Standard Practice for Abrasive Wear Testing with Dry Sand/Rubber | Wear performance verification |
| ASTM G99 | Standard Test Method for Wear Testing with a Pin-on-Disk Apparat | Sliding wear evaluation |
| NF EN ISO 14432 | Thermal Spray — Surface Preparation of Substrates | Substrate preparation requirements |
| NF EN ISO 14555 | Thermal Spray — General Recommendations | General thermal spray process guidance |
| GB/T 11366 | Thermal Spray Coatings — General Specifications | Chinese national standard for thermal spray coatings |
| NACE MR0175/ISO 15156 | Materials for Use in H₂S Environments in Oil and Gas Production | Corrosion resistance qualification for oil/gas applications |
| API 5L / API 6D | Pipeline Steel / Pipeline Valve Specifications | Base material compatibility for pipeline coating applications |
5.3 Acceptance Criteria Summary
- Adhesion Strength: ≥ 30 MPa (peel test, ASTM C1024) for flame-sprayed coatings; ≥ 40 MPa for plasma-surfaced coatings
- Hardness: ≥ 35 HRC (Ni60) or ≥ 600 HV₀.₃ (Ni60-WC) after heat treatment
- Porosity: ≤ 5% for flame-sprayed coatings; ≤ 1% for plasma-surfaced coatings
- Coating Thickness: Uniformity within ±10% of specified thickness; minimum 1.5 mm for severe wear applications
- Surface Defects: No cracks, spalling, or excessive spatter; surface roughness Ra ≤ 12.5 μm (as-sprayed) or Ra ≤ 3.2 μm (after machining)
- Dilution Zone: Maximum 15–20% base metal dilution for optimal Ni60 properties; verified by EDS line scan analysis
6. Common Risks and Controls
6.1 Coating Adhesion Failure
Risk: Insufficient substrate preparation, contamination, or excessive inter-pass temperature leads to delamination during service.
- Control: Mandatory surface preparation per NF EN ISO 14432; inter-pass temperature monitoring with infrared pyrometer; adhesion testing on witness coupons for each production batch
6.2 Cracking in Coating or Dilution Zone
Risk: High residual stresses combined with brittle carbide phases cause cracking, particularly in thick coatings or on high-carbon/high-alloy substrates.
- Control: Control inter-pass temperature below 300–350 °C; apply stress relief heat treatment (550–650 °C); use multiple thin passes rather than single thick deposits; select appropriate substrate preheat temperature
6.3 Excessive Dilution
Risk: High dilution (>25%) reduces coating hardness and corrosion resistance by incorporating base metal elements that form softer phases.
- Control: Optimize arc current, travel speed, and powder feed rate; apply transition layer (e.g., 309L or Ni-base) on dissimilar substrates; verify dilution by EDS line scan analysis
6.4 WC Decomposition in Ni60-WC Coatings
Risk: Excessive heat input or post-deposition heat treatment temperatures above 900 °C cause WC to decompose into W₂C and free carbon, resulting in hardness loss and reduced corrosion resistance.
- Control: Limit plasma arc current and powder feed rate to control heat input; avoid post-deposition heat treatment above 900 °C; use rapid quenching techniques; verify WC integrity by SEM/EDS analysis
6.5 Oxide Inclusion Formation (Flame Spraying)
Risk: Flame spraying inherently produces oxide inclusions (particularly Cr₂O₃ and NiO) that act as stress concentrators and reduce adhesion.
- Control: Optimize oxygen-fuel gas ratio for neutral flame; use inert gas shielding around the spray gun; post-machining to remove surface oxide-rich layer; consider plasma surfacing for oxide-sensitive applications
6.6 Hydrogen-Induced Cracking in Substrate
Risk: High carbon equivalent substrates (CE > 0.6) are susceptible to hydrogen-induced cracking from thermal cycling during multi-pass coating deposition.
- Control: Preheat substrate to 250–400 °C; control inter-pass temperature; apply post-weld heat treatment (PWHT) per applicable code requirements; select low-hydrogen process parameters
7. Application Scenarios Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
Ni60 and Ni60-WC coatings can be applied via TIG or MIG arc surfacing as an alternative or complement to flame spraying and plasma surfacing. The microstructural knowledge gained from flame and plasma studies directly informs TIG/MIG process parameter optimization:
- Transition Layer Strategy: Ni60 coatings are commonly applied over a 309L or Ni-base transition layer on carbon steel substrates to prevent cracking and reduce dilution. The microstructural study provides the basis for selecting optimal transition layer thickness and composition.
- Thick Coating Applications: For coating thicknesses exceeding 5 mm (e.g., pump impellers, valve seats), TIG/MIG surfacing is preferred over thermal spray due to better mechanical properties and lower porosity. Flame/plasma studies provide metallurgical understanding for process development.
- Field Repair Applications: Portable plasma surfacing equipment enables on-site repair of worn components without removal, leveraging process knowledge from laboratory-scale studies.
7.2 Hydraulic Explosive Bonding Integration
While hydraulic explosive bonding (HMB) produces solid-state bonds between dissimilar metals without melting, Ni60/Ni60-WC coatings serve complementary roles:
- Post-Bonding Surface Treatment: HMB-bonded clad plates may require surface coating with Ni60 for enhanced wear resistance in specific service zones (e.g., bolted joints, edge areas where the HMB bond is vulnerable to mechanical damage).
- Transition Zone Coating: In HMB clad structures where the base metal is carbon steel and the cladding is stainless steel, Ni60 coatings can be applied at geometric transitions or stress concentration zones to provide local wear protection.
- Process Development Synergy: Understanding of Ni60 microstructure and bonding mechanisms from thermal spray studies informs the design of HMB process parameters for Ni-based alloy bonding.
7.3 Explosion Welding Integration
Explosion welding (EW) produces high-integrity clad plates and pipes through explosive-driven collision. Ni60/Ni60-WC coatings complement EW in the following ways:
- Surface Enhancement of EW Clad Plates: EW-produced clad plates (e.g., SS316/Carbon Steel) can be further surface-treated with Ni60 coatings in high-wear zones to achieve multi-functional performance (corrosion resistance from EW cladding + wear resistance from Ni60 coating).
- Ni60 Base Material for EW: The metallurgical understanding of Ni60 solidification behavior and carbide formation from thermal spray studies informs EW parameter selection when Ni60 is used as the flyer plate material.
- Combined Cladding Strategies: For complex components requiring both thick corrosion-resistant cladding and thin wear-resistant surface protection, the company can combine EW (thick cladding) with plasma surfacing (thin wear coating) in a single fabrication workflow.
8. Contribution to Qualification Building and Customer Value
8.1 WPS/PQR Development Foundation
The microstructure-property study of Ni60 and Ni60-WC coatings provides the scientific foundation for developing and qualifying Welding Procedure Specifications (WPS) and Performance Qualification Records (PQR) for:
- Flame spray coating procedures per NF EN ISO 14555
- Plasma surfacing procedures per ASME SFA-5.7 and applicable company standards
- TIG/MIG weld overlay procedures incorporating Ni60 consumables
8.2 Customer-Specific Coating Design
Deep understanding of coating microstructure enables the company to:
- Design custom coating systems (multi-layer, gradient, composite) tailored to specific wear and corrosion conditions
- Predict coating performance in service based on microstructural analysis rather than empirical trial-and-error
- Provide technical documentation and performance guarantees supported by metallurgical evidence
- Recommend optimal coating thickness, heat treatment, and post-processing for each application
8.3 Quality Assurance and Traceability
Microstructural characterization capabilities enable:
- In-process quality monitoring through hardness profiling and dilution zone analysis
- Root cause analysis of coating failures in the field
- Documentation of coating properties for customer acceptance and warranty purposes
- Process improvement through systematic study of parameter-microstructure-property relationships
9. Conclusion
The study of flame spraying and plasma surfacing preparation of Ni60 and Ni60-WC coatings represents a critical technical competency for Cladding Technology Shanxi Co., Ltd. It provides the metallurgical foundation for process development, WPS qualification, quality assurance, and customer-specific coating design. The knowledge gained directly supports the company's broader technology portfolio across TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding routes, enabling integrated surface engineering solutions that deliver superior performance, extended service life, and reduced total cost of ownership for customers in demanding industrial applications.