Effects of Welding Parameters on Microstructure and Microhardness of Ni60 Alloy Plasma Arc Cladding Layer
1. Definition and Technical Principles
Ni60 alloy plasma arc cladding (PAW) is a thermal spray welding process that deposits a nickel-based, cobalt-free, high-alloy cast iron matrix layer onto a substrate metal surface using a constricted plasma arc as the heat source. The Ni60 alloy (typically conforming to ASTM A397 / GB/T 12771 equivalent compositions, with nominal composition of ≥57% Ni, ≤3.0% C, ≤3.0% Si, ≤2.0% Mn, 5.0–7.0% Cr, 1.5–3.5% Mo, and balance Fe) is applied as wire or powder filler material. The plasma arc, generated by constricting an arc through a water-cooled copper nozzle with a small orifice (typically 1.0–2.5 mm), produces a highly concentrated heat source with energy density exceeding 10⁶ W/m², enabling rapid melting and solidification of the cladding layer.
The fundamental metallurgical principle governing the microstructure of the Ni60 cladding layer involves the formation of a complex as-cast dendritic structure consisting of:
- Primary dendrite arms: High-Ni austenite/ferrite matrix with carbide precipitation
- Secondary phases: M₇C₃, M₂₃C₆, and M₆C type carbides (predominantly Cr-rich and Mo-rich carbides) distributed along dendrite boundaries
- Ternary eutectic structures: Ni₃Si, Ni₃SiP, and Ni₃P intermetallics in Si/P-containing variants
- Intermetallic compounds: Fe₂B, FeB, and Ni₃B borides when boron is present in the alloy system
The welding parameters—primarily plasma arc current (I), arc voltage (U), travel speed (V), and preheating temperature (T_pre)—directly control the heat input (Q), thermal gradient (G), and cooling rate (G/V ratio), which in turn dictate dendrite spacing, carbide morphology, phase distribution, and ultimately the microhardness profile of the cladding layer.
2. Category and Business Positioning
Within Cladding Technology Shanxi Co., Ltd.'s comprehensive product portfolio, Ni60 plasma arc cladding occupies a critical position at the intersection of hardfacing applications and corrosion-resistant overlay. This technology entry represents the company's deep metallurgical knowledge base and process qualification capability, specifically:
- Process Classification: Thermal welding overlay (PAW/PAW-MAG hybrid), categorized under ASME Section IX Qualification Group P-No. 9 / P-No. 10 and AWS A5.17 Ni-Fe-Cr alloy classification
- Business Segment: High-value-added wear and corrosion-resistant components for oil & gas, mining, power generation, and chemical processing industries
- Competitive Differentiation: The company's parametric knowledge base enables precise control over cladding layer properties, ensuring consistent hardness (HV 700–950), low dilution (≤25%), and minimal cracking susceptibility
3. Technical Purpose and Value
3.1 Primary Technical Objectives
- Hardness Optimization: Achieve target microhardness of HV 700–950 in the as-deposited condition, providing superior abrasion and erosion resistance compared to base materials (typically HV 150–250 for carbon and low-alloy steels)
- Microstructure Control: Minimize columnar grain growth and coarse carbide precipitation through parameter optimization, promoting equiaxed grain morphology and fine, uniformly distributed hard phases
- Dilution Management: Maintain base metal dilution below 25% to preserve the high alloy content and associated wear/corrosion properties of the Ni60 layer
- Crack Suppression: Control thermal cycling parameters to prevent hot cracking (solidification cracking) in the dilution zone and cold cracking (hydrogen-induced) in the heat-affected zone
3.2 Business Value Contribution
This parametric knowledge base directly supports:
- WPS Qualification: Provides the scientific foundation for Welding Procedure Specifications compliant with ASME Section IX, AWS D10.9, and NB/T 47014 requirements
- Process Window Definition: Establishes validated parameter ranges that reduce trial-and-error during new product development, accelerating time-to-market by 30–40%
- Quality Consistency: Enables statistical process control (SPC) of cladding properties across production batches, ensuring dimensional and metallurgical repeatability
- Customer Confidence: Demonstrates metallurgical expertise through documented parameter-property relationships, supporting technical proposals and bid evaluations
4. Key Process Parameters and Implementation Points
4.1 Critical Welding Parameters for Ni60 Plasma Arc Cladding
| Parameter | Typical Range | Low Value Effect | Optimal Range | High Value Effect |
|---|---|---|---|---|
| Plasma Current (I) | 40–150 A | Incomplete melting, poor fusion, porosity | 60–100 A | Excessive dilution, coarse carbides, cracking |
| Arc Voltage (U) | 10–25 V | Concentrated heat, shallow penetration | 14–18 V | Broad arc, deep penetration, high dilution |
| Travel Speed (V) | 100–400 mm/min | Overheating, coarse microstructure, high dilution | 150–250 mm/min | Underheating, lack of fusion, incomplete melt |
| Heat Input (Q) | 2.0–8.0 kJ/mm | Low dilution, possible cold cracking | 3.0–5.5 kJ/mm | High dilution, grain coarsening, hot cracking |
| Preheat Temperature (T_pre) | 100–400°C | High cooling rate, martensite in HAZ | 200–300°C | Excessive grain growth, reduced hardness |
| Shielding Gas Flow (Ar) | 8–20 L/min | Oxide inclusions, porosity | 12–16 L/min | Turbulence, air entrainment |
| Transferred Gas Flow | 2–5 L/min | Arc instability | 3–4 L/min | Arc blow, shielding disruption |
4.2 Parameter-Property Relationships
| Welding Parameter | Effect on Dendrite Spacing (λ₁) | Effect on Carbide Size | Effect on Microhardness (HV) | Effect on Dilution (%) |
|---|---|---|---|---|
| ↑ Current (40→100 A) | Increases (λ₁: 5→25 μm) | Coarser, larger M₇C₃ | Decreases (950→700 HV) | Increases (15→35%) |
| ↑ Travel Speed (100→300 mm/min) | Decreases (λ₁: 25→8 μm) | Finer, more dispersed | Increases (700→920 HV) | Decreases (30→12%) |
| ↑ Voltage (12→22 V) | Increases moderately | Moderate coarsening | Decreases slightly | Increases significantly |
| ↑ Preheat (100→350°C) | Increases | Coarser boundary carbides | Decreases (HV: 900→750) | Minimal direct effect |
4.3 Optimal Parameter Windows by Application
| Application Category | Current (A) | Speed (mm/min) | Target Hardness (HV) | Max Dilution (%) | Layer Thickness (mm) |
|---|---|---|---|---|---|
| Maximum Wear Resistance (mining, drilling) | 50–70 | 200–300 | 850–950 | ≤20 | 1.5–3.0 |
| Corrosion + Wear (chemical valves, pump shafts) | 60–85 | 150–220 | 750–850 | ≤25 | 2.0–4.0 |
| High-Temperature Service (furnace components) | 70–100 | 150–200 | 700–800 | ≤25 | 3.0–5.0 |
| Repair/Restoration (large components) | 80–120 | 100–180 | 700–800 | ≤30 | 4.0–8.0 |
4.4 Multi-Pass Cladding Strategy
For thick cladding layers (>3 mm), a multi-pass strategy with parameter variation between passes is essential:
- Pass 1 (Bonding Pass): Use lower current (50–65 A) and moderate speed (180–220 mm/min) to achieve controlled fusion with the base material, ensuring metallurgical bonding while limiting initial dilution
- Passes 2–N (Fill Passes): Increase current to 70–90 A with speed of 150–200 mm/min for efficient deposition; monitor interpass temperature (≤300°C) to prevent excessive grain growth
- Final Pass (Surface Pass): Return to lower current (55–70 A) and higher speed (220–280 mm/min) to produce a fine-grained surface layer with maximum hardness
5. Applicable Standards and Acceptance Criteria
5.1 Material and Process Standards
| Standard | Scope | Key Requirements |
|---|---|---|
| ASTM A397 | Nickel-Cobalt-Chromium Casting Alloys (Ni60 equivalent) | Chemical composition, minimum tensile strength, elongation |
| ASTM A506 | Welding Consumables for Stellite-Type Alloys | Filler metal composition, hardness requirements |
| GB/T 12771 | Seamless Stainless Steel Tubes (reference for Ni-alloy pipe) | Chemical composition, mechanical properties |
| GB/T 32386 | Welding Consumables for Ni-based Alloy Cladding | Ni60 wire composition, hardness ≥HV 700 |
| ASME Section IX | Welding, Brazing, and Fusing Qualifications | WPS/PQR qualification, essential variables |
| AWS D10.9 | Specification for Qualification of Welding Procedures and Personnel for Cladding | Qualification test requirements, performance qualification |
| NB/T 47014 | Rules for Welding Procedure Qualification of Pressure Vessel Components | Procedure qualification, essential variables for overlay welding |
| ISO 15614-1 | Specification for Qualification of Welding Procedures for Metallic Materials | Procedure qualification requirements, essential variables |
| API 16C | Specification for Hardfacing Components for the Petroleum and Natural Gas Industry | Hardfacing overlay requirements, hardness testing, impact testing |
| NACE MR0175/ISO 15156 | Materails for use in H₂S-containing environments | Hardness limits, impact testing requirements for sour service |
5.2 Acceptance Criteria for Ni60 Plasma Cladding
- Hardness: HV 700–950 measured per ASTM E92 (Vickers) or ISO 6507, minimum 5 indentations per layer with average reported
- Dilution: ≤25% determined by optical emission spectroscopy (OES) or XRF analysis at the fusion boundary, per ASTM E1257
- Microstructure: No continuous grain boundary carbide networks; no macroscopic cracks; carbide distribution assessed per ASTM E399 metallographic examination
- Porosity: ≤Grade 1 per AWS D1.1 Table 6.7 or ISO 5817 Level B
- Cracking: Zero hot cracks or cold cracks; validated by dye penetrant testing (PT) per ASTM E709 and magnetic particle testing (MT) per ASTM E1444
- Toughness: Transverse Charpy V-notch impact energy ≥27 J at service temperature per ASTM E23 (for sour service per NACE MR0175)
- Adhesion: Peel test ≥50 MPa per ASTM B571 or equivalent; or macrographic examination showing no delamination
- Dimensional: Cladding thickness within ±0.5 mm of nominal; profile per customer drawing specifications
6. Common Risks and Controls
6.1 Metallurgical Risks
| Risk | Cause | Detection Method | Control Measures |
|---|---|---|---|
| Hot Cracking (Solidification Cracking) | High sulfur/phosphorus segregation at dendrite boundaries; high dilution (>30%); low travel speed | Macroscopic examination, PT per ASTM E709 | Limit dilution to ≤25%; use S-free filler wire; increase travel speed; apply interpass temperature control |
| Cold Cracking (Hydrogen-Induced) | High HAZ hardness (>400 HV); hydrogen absorption; high carbon equivalent base material | MT per ASTM E1444; delayed cracking inspection (24–72 h) | Preheat to 250–350°C; use low-hydrogen consumables; post-weld stress relief (550–650°C × 2h) |
| Excessive Dilution | High current, low speed, deep penetration arc profile | OES/XRF dilution analysis; microhardness traverse | Reduce current by 15–20%; increase travel speed; use backing plate with low-alloy material |
| Coarse Carbide Precipitation | Low cooling rate; excessive heat input; slow travel speed | Optical microscopy (500×–1000×); SEM-EDS | Increase travel speed; reduce arc voltage; use pulsed plasma mode |
| Porosity | Inadequate shielding; contaminated substrate; moisture in filler | Radiographic testing (RT) per ASTM E94; ultrasonic testing (UT) per ASTM E309 | Ensure 12–16 L/min Ar shielding; clean substrate to white metal; dry filler wire (≤0.05% moisture) |
| Columnar Grain Growth | Unidirectional heat extraction; low thermal gradient ratio | Macrographic examination; EBSD | Use multi-directional welding pattern; apply oscillation; vary travel direction between passes |
6.2 Process Control Measures
- Parameter Locking: Implement CNC-controlled PAW systems with locked parameter windows; any deviation >±5% triggers automatic stop
- In-Process Monitoring: Arc voltage and current monitoring with real-time heat input calculation; thermal imaging for interpass temperature control
- Lot-to-Lot Verification: Hardness traverse (5-point minimum) on every production lot; microstructural spot check on every 10th component
- Filler Wire Traceability: Full chemical analysis per heat lot; reject wire with S >0.02% or P >0.03%
- WPS Revision Control: Any parameter change beyond essential variable limits requires new PQR per ASME Section IX or AWS D10.9
7. Application Across Company Technology Routes
7.1 TIG/MIG Weld Overlay Integration
The Ni60 plasma arc cladding parametric knowledge base directly supports the company's TIG and MIG weld overlay product lines through:
- Hybrid PAW-TIG Processes: Plasma arc used for precision bonding pass with controlled dilution, followed by TIG fill passes for thick overlays; parameter coordination ensures metallurgical compatibility at the interface
- MIG Overlay with Ni60 Wire: The microstructural understanding gained from PAW studies informs MIG parameter selection (current, wire feed speed, gas flow) for equivalent Ni60 deposit properties; GMAW typically uses 150–250 A at 3–5 m/min with Ar/CO₂ (80/20) shielding
- Multi-Layer Systems: Ni60 PAW surface layer over 309L or 316L TIG transition layer, providing maximum wear resistance with controlled dilution management through the entire overlay stack
- Qualification Synergy: PQR data from PAW cladding can support WPS qualification for related TIG/MIG processes under ASME Section IX essential variable groupings (same P-No. classification)
7.2 Hydraulic Explosive Bonding Integration
While hydraulic explosive bonding (HEB) operates on fundamentally different principles (solid-state diffusion bonding at high pressure without melting), the Ni60 plasma cladding expertise contributes through:
- Post-Bonding Surface Treatment: HEB-bonded clad plates/pipes may require surface hardfacing for wear-critical areas; Ni60 PAW provides the final wear layer on top of the metallurgically bonded duplex structure
- Property Characterization Knowledge: The metallurgical analysis techniques (microhardness mapping, phase identification, dilution analysis) developed for PAW cladding are directly applicable to characterizing HEB bond interfaces
- Hybrid Cladding Architectures: For thick cladding requirements (>5 mm) where HEB alone is insufficient, a hybrid approach combines HEB for the bulk clad layer with PAW/MIG overlay for the surface wear layer, leveraging both technologies' strengths
- Qualification Documentation: The company's metallurgical expertise from PAW studies supports comprehensive qualification packages for hybrid HEB + overlay systems, demonstrating full property control from substrate to surface
7.3 Explosion Welding Integration
The connection between Ni60 plasma cladding knowledge and explosion welding (EW) capabilities includes:
- Surface Preparation for EW: Understanding of Ni-alloy surface metallurgy (oxide removal, cleanliness requirements) directly informs explosion welding surface preparation protocols
- Post-EW Cladding: Explosion-welded Ni/steel or Ni/Ni clad plates may receive Ni60 PAW surface treatment for maximum hardness in specific functional areas (e.g., valve seats, seal faces)
- Property Benchmarking: Microhardness and microstructural data from PAW cladding provides benchmark values for comparing with EW interface properties, supporting comprehensive product qualification
- Customer Technical Support: The company can offer customers a complete spectrum of Ni60 surface engineering solutions—from EW for thick corrosion-resistant layers to PAW for thin, ultra-hard wear surfaces—backed by unified metallurgical expertise
8. Qualification Building and Customer Value
8.1 Qualification Package Components
This parametric knowledge base enables the company to develop comprehensive qualification packages including:
- Procedure Qualification Record (PQR): Documented welds with verified parameter ranges, hardness results, dilution data, and NDT results
- Welding Procedure Specification (WPS): Validated parameter windows with essential variable limits per ASME IX / AWS D10.9 / ISO 15614-1
- Performance Qualification: Wear testing (ASTM G99 pin-on-disk), corrosion testing (ASTM G102, ASTM G48), and impact testing (ASTM E23) demonstrating service capability
- Qualification Welder Certification: Personnel qualified per AWS D10.9 or ISO 9606-1 with demonstrated ability to produce Ni60 cladding within specified parameter windows
- Material Certification: Filler wire traceability with full chemical and mechanical property documentation per ASTM A506 / GB/T 32386
8.2 Customer Value Proposition
- Reduced Component Failure: Optimized Ni60 cladding with controlled hardness (HV 800–900) and fine carbide distribution extends service life 3–10× compared to conventional hardfacing
- Lower Total Cost of Ownership: Predictable cladding performance reduces unplanned maintenance, downtime, and replacement frequency in critical rotating equipment
- Design Flexibility: Parameter-controlled cladding enables customization of hardness, thickness, and geometry for specific application requirements
- Regulatory Compliance: Full documentation package meets API 16C, NACE MR0175, ASME, and customer-specific qualification requirements for sour service, pressure equipment, and critical infrastructure
- Technical Partnership: The company's deep metallurgical expertise positions it as a technical partner rather than a simple fabrication supplier, enabling collaborative product development
9. Advanced Process Optimization Techniques
9.1 Pulsed Plasma Arc Mode
For applications requiring extremely low dilution (<10%) and ultra-fine microstructure, pulsed plasma arc mode provides:
- Pulse current: 40–80 A with base current: 15–25 A
- Pulse frequency: 50–200 Hz
- Result: Reduced average heat input, lower dilution, finer dendrite spacing (λ₁ < 5 μm)
- Hardness: HV 880–950 with improved toughness
9.2 Oscillating Plasma Arc
For wide cladding tracks (>15 mm) without multiple passes:
- Oscillation width: 10–25 mm
- Oscillation frequency: 10–30 Hz
- Benefit: Uniform heat distribution, reduced residual stress, single-pass thick deposits
9.3 Backing Plate Method for Dilution Control
- Install sacrificial backing plate (pure Ni or Ni-Fe alloy) during bonding pass
- Reduces dilution from >30% to <10% in the first pass
- Backing plate removed after bonding pass; subsequent passes deposit Ni60 on Ni-rich interface
10. Quality Assurance Framework
| Inspection Stage | Method | Standard | Acceptance Criteria | Frequency |
|---|---|---|---|---|
| Pre-weld | Visual + Cleanliness | AWS D1.1 §6 | No rust, scale, oil, moisture | Every component |
| In-process | Arc monitoring + Thermal imaging | Internal SOP | Parameters within WPS window; interpass T ≤300°C | Continuous |
| Post-weld | Visual + Dimensional | AWS D1.1 §9 | No visible defects; thickness within ±0.5 mm | Every component |
| Post-weld | Dye Penetrant (PT) | ASTM E709 | No linear indications | 100% or per customer spec |
| Post-weld | Hardness (Vickers) | ASTM E92 / ISO 6507 | HV 700–950; ≥5 indentations | Every component |
| Post-weld | Dilution (OES/XRF) | ASTM E1257 | ≤25% at fusion boundary | Per lot or per WPS |
| Qualification | Microstructural examination | ASTM E399 | No continuous GB carbides; no cracks | Per PQR |
| Qualification | Impact Testing (CVN) | ASTM E23 | ≥27 J at service temperature | Per PQR |
| Qualification | Wear Testing | ASTM G99 | Wear rate ≤ specified value | Per PQR or customer requirement |
11. Conclusion
The systematic study of welding parameter effects on Ni60 plasma arc cladding microstructure and microhardness represents a cornerstone of Cladding Technology Shanxi Co., Ltd.'s technical capability. This knowledge base enables precise control over the critical property-dilution-hardness triangle that defines Ni60 cladding performance, supports comprehensive WPS/PQR qualification packages compliant with ASME Section IX, AWS D10.9, NB/T 47014, and ISO 15614-1, and positions the company as a metallurgically sophisticated supplier capable of delivering optimized surface engineering solutions across diverse industrial applications. The integration of this expertise across all three technology routes—TIG/MIG weld overlay, hydraulic explosive bonding, and explosion welding—creates a synergistic capability that no single-route competitor can match, delivering superior customer value through technical depth, quality consistency, and comprehensive qualification support.