Mechanism of Tungsten Carbide (WC) Particle Burn Loss in Hardfacing Weld Overlay
1. Definition and Fundamental Principles
1.1 What Is WC Hardfacing Burn Loss?
Tungsten Carbide (WC) hardfacing is a widely employed tribological protection technology in which WC particles—typically ranging from 10 μm to 200 μm in size—are embedded in a metallic binder matrix (commonly Fe-Cr-C, Ni-Cr, or Co-based) and deposited onto a substrate via weld overlay processes. During the welding arc interaction, a critical metallurgical challenge arises: the WC particles are subjected to temperatures that exceed their decomposition threshold (~1400°C for WC → W₂C + C), leading to partial or complete decomposition, oxidation, and ejection from the weld pool. This phenomenon is termed WC burn loss or WC decomposition.
The severity of burn loss is directly correlated with the thermal input, arc duration, shielding gas composition, wire feed rate, and travel speed. Excessive burn loss results in:
- Reduced residual WC particle content in the solidified deposit (from a designed 60–70 wt% to potentially below 30 wt%)
- Formation of brittle intermetallic phases such as Fe₃W₃C, Fe₂W₅C, and W₂C in the binder matrix
- Significant degradation of hardness (from target 1400–1800 HV to 800–1100 HV)
- Loss of wear resistance, abrasion resistance, and erosion resistance performance
1.2 Thermodynamic and Kinetic Mechanisms
The burn loss mechanism operates through three concurrent pathways:
- Thermal Decomposition: WC is thermodynamically unstable above approximately 1400°C. The decomposition reaction is:
WC → W₂C + C (at 1400–1600°C)
The released carbon dissolves into the molten binder, while tungsten partitions into intermetallic compounds with the matrix elements (Fe, Cr, Ni).
W₂C → 2W + C (at 1600–1800°C) - Oxidation: In the presence of oxygen (even trace amounts in shielding gas), WC undergoes oxidation:
WC + O₂ → WO₃ + CO/CO₂
The volatile tungsten oxide (WO₃, sublimation temperature ~1480°C) is carried away by the arc plasma and shielding gas flow, resulting in irreversible material loss. - Mechanical Ejection: During arc impact, unmelted or partially melted WC particles can be physically ejected from the weld pool by arc force, spatter, or gas flow turbulence—particularly when the particle is large relative to the weld pool dimensions.
2. Technical Purpose and Business Value
2.1 Strategic Importance in Qualification Building
Understanding and controlling WC burn loss is not merely an academic exercise—it is a core qualification competency that distinguishes a competent hardfacing supplier from a non-compliant one. In customer audits and WPS (Welding Procedure Specification) qualification reviews, the ability to demonstrate systematic understanding of WC degradation mechanisms and implement corresponding process controls is a prerequisite for:
- Qualification under API 16C (Welding, Brazing, and Thermal Cutting of Steel Cylinders for Natural Gas Service)
- Compliance with ASME Section IX procedures for hardfacing qualification
- Meeting NACE International (now AMPP) performance requirements for corrosion/wear protection systems
- Satisfying customer-specific WPS qualification trials requiring minimum residual WC content verification
2.2 Direct Impact on Product Delivery and Customer Value
For Cladding Technology Shanxi Co., Ltd., mastery of WC burn loss mechanisms directly translates to:
- Reduced rework rates: By preventing excessive decomposition during deposition, the first-pass yield rate improves, reducing schedule overruns and cost overruns.
- Guaranteed performance: Customers in mining, cement, power generation, and oil & gas require verified residual WC content (typically ≥50 wt%) to ensure field life expectancy. Documented process understanding provides the technical basis for performance warranties.
- Competitive differentiation: The ability to produce WC hardfacing deposits with controlled decomposition ratios (targeting W₂C/WC ratios of 2:1 to 3:1 rather than uncontrolled full decomposition) represents a premium capability.
- IP and knowledge asset: Systematic study of burn loss mechanisms contributes to proprietary process knowledge that supports long-term competitive positioning.
3. Key Process and Implementation Points
3.1 Thermal Input Control Parameters
The primary lever for controlling WC burn loss is thermal input management. The following table summarizes critical parameters and their recommended ranges for WC hardfacing via TIG and MIG processes:
| Parameter | TIG (GTAW) Hardfacing | MIG (GMAW) Hardfacing | Impact on WC Burn Loss |
|---|---|---|---|
| Arc Current | 80–160 A | 180–320 A | Higher current → deeper penetration → more WC decomposition |
| Travel Speed | 80–200 mm/min | 200–500 mm/min | Lower speed → longer residence time → increased burn loss |
| Wire Feed Rate | N/A (powder feeding) | 2.0–5.0 m/min | Higher WFR dilutes heat per unit volume; reduces per-particle thermal exposure |
| Shielding Gas | Ar (100%) or Ar/He mix | Ar (100%) or Ar/CO₂ ≤5% | O₂ ingress → oxidation pathway activation; CO₂ promotes oxidation |
| Arc Length | 2–5 mm | 3–8 mm | Excessive arc length → unstable arc → splatter and particle ejection |
| Preheat Temperature | ≤200°C | ≤150°C | Higher preheat → lower cooling rate → more time above decomposition temp |
3.2 Thermal Input Calculation and Control
Thermal input (q) is calculated as:
q = (V × I × η) / (v × t)
Where V = arc voltage, I = arc current, η = efficiency factor (0.7–0.85), v = travel speed, t = deposit thickness.
Recommended thermal input range for WC hardfacing: 3–10 kJ/mm (significantly lower than conventional structural welding at 10–25 kJ/mm). For TIG powder hardfacing, maintaining thermal input below 7 kJ/mm is critical to limit WC decomposition to acceptable levels.
3.3 Process Route-Specific Implementation
3.3.1 TIG (GTAW) Weld Overlay with Powder Feeding
- Direct-current (DC) polarity: Electrode negative (DCEP) provides concentrated arc heat with minimal base metal dilution—critical for preserving WC particles.
- Powder feeding configuration: External powder feeder positioned at 15–25 mm from arc center; powder delivery angle of 30–45° to the substrate surface.
- Multi-pass strategy: Deposit in multiple thin passes (1–2 mm per pass) rather than single thick layers to minimize cumulative thermal exposure per WC particle.
- Post-weld treatment: Rapid quenching (water quench or forced air cooling) immediately after each pass to arrest decomposition kinetics.
3.3.2 MIG (GMAW) Weld Overlay with WC-Containing Wire or Powder
- Short-circuit or spray transfer mode: Short-circuit transfer provides lower peak temperatures but higher frequency; spray transfer offers better wetting but requires higher thermal input.
- WC-containing flux-cored wire: Alternative to powder feeding; the flux encapsulation provides some thermal buffering for WC particles during arc transit.
- Wire diameter selection: 0.8–1.2 mm for precise thermal input control; larger diameters (>1.6 mm) increase heat per unit length and risk excessive decomposition.
3.3.3 Hydraulic Explosive Bonding and Explosion Welding Considerations
While hydraulic explosive bonding and explosion welding are primarily used for dissimilar metal cladding (e.g., steel/nickel, steel/aluminum, steel/tantalum), the understanding of WC burn loss mechanisms is relevant in the following contexts:
- Post-explosion welding hardfacing: When WC hardfacing is subsequently applied to explosion-welded clad plates (e.g., WC hardfacing on the wear surface of a tantalum-clad steel plate), the process parameters must account for the thermal history already imparted by the explosion welding event.
- Substrate pre-heat avoidance: Explosion-welded substrates should not require additional preheating before hardfacing, as this would increase cumulative thermal exposure and risk WC decomposition.
- Residual stress interaction: The high compressive residual stresses from explosion welding can interact with thermal stresses from subsequent hardfacing, potentially cracking the WC-containing deposit if thermal input is excessive.
4. Applicable Standards and Acceptance Criteria
4.1 Hardfacing Process Standards
| Standard | Scope | Relevance to WC Burn Loss |
|---|---|---|
| ASTM A770/A770M | Standard Specification for Deposited Hardfacing Alloys | Defines minimum hardness and composition requirements for WC-containing deposits |
| ASME Section IX | Welding, Brazing, and Fusing Qualifications | Requires qualification of hardfacing WPS; thermal input parameters must be documented |
| ISO 14230 | Welding — Fusion Welding of Steels — Hardfacing Welding | Specifies qualification requirements for hardfacing processes including thermal control |
| NACE MR0175/ISO 15156 | Materials for Use in H₂S-Containing Environments | Hardfacing deposits must meet hardness limits (typically ≤250 HV for sour service); WC deposits generally exceed this limit, requiring process awareness |
| GB/T 12469 | Welding Consumables — Classification and Requirements | Chinese standard for hardfacing consumables including WC-containing compositions |
| ASTM B275 | Standard Specification for Tungsten Carbide-Cobalt Cements | Reference standard for WC-Co alloy properties; relevant for understanding decomposition behavior |
| API 16C | Welding, Brazing, and Thermal Cutting of Steel Cylinders | Requires hardfacing qualification for cylinder repair applications |
4.2 Acceptance Criteria for WC Hardfacing Deposits
- Residual WC content: Minimum 50 wt% (by optical microscopy with area fraction measurement per ASTM E562 or equivalent image analysis) for standard applications; ≥60 wt% for premium wear applications.
- Hardness: ≥1400 HV0.3 (ASTM E384/E92) for the WC-containing deposit; transition layer hardness ≤300 HV for compatibility with base metal.
- Microstructure: No continuous network of brittle intermetallic phases; W₂C/WC ratio controlled within 2:1 to 3:1; no unmelted particles causing lack of fusion.
- Adhesion: Peel test per ASTM B571 (for explosion-welded substrates) or pull-off test per ASTM D4541 (for welded hardfacing); minimum 30 MPa adhesion strength.
- Crack-free deposit: No cracks exceeding 0.5 mm length in any direction (visual or 10× magnification inspection).
5. Common Risks and Controls
5.1 Risk Matrix
| Risk | Likelihood | Impact | Mitigation Control |
|---|---|---|---|
| Excessive thermal input causing >70% WC decomposition | High (if parameters not controlled) | Critical — deposit non-functional | WPS qualification with thermal input limits; real-time monitoring of current/voltage/speed |
| Shielding gas contamination (O₂/N₂ ingress) | Medium | High — oxidation-driven material loss | Gas flow rate verification; flowmeter calibration; wind shielding; gas purity verification (≤0.1% O₂) |
| Particle size distribution shift during melting | Medium | Medium — uneven hardness distribution | Consumable lot control; incoming particle size verification (laser diffraction analysis) |
| Multi-pass reheat causing progressive decomposition | High (in multi-pass builds) | High — cumulative loss exceeds acceptable limits | Interpass temperature control (≤150°C); minimum interpass cooling time; thin-pass strategy |
| Operator deviation from qualified WPS parameters | Medium | Critical — unqualified product delivery | Parameter monitoring systems; operator certification; audit trail documentation |
| Base metal dilution affecting deposit composition | Medium | Medium — hardness reduction and WC environment change | Low-current/high-speed parameters; backing material selection; root pass with minimal penetration |
5.2 Diagnostic Methods for Burn Loss Assessment
- Optical Microscopy (OM): Etching with 3–5% HCl + HNO₃ mixture reveals WC particles as bright angular features. Area fraction measurement per ASTM E562 quantifies residual WC content.
- Scanning Electron Microscopy (SEM) with EDS: Identifies decomposition products (W₂C, Fe₃W₃C, Fe₂W₅C) and quantifies tungsten distribution in the binder matrix.
- X-Ray Diffraction (XRD): Phase identification and quantitative phase analysis to determine WC/W₂C/intermetallic ratios. Required for qualification trials.
- Vickers Hardness Mapping: HV0.3 measurements across the deposit cross-section; localized hardness drop below 1200 HV indicates zones of excessive decomposition.
- Thermogravimetric Analysis (TGA): Laboratory characterization of consumable powder thermal stability; determines onset decomposition temperature for specific WC formulations.
6. Application Scenarios Across Technology Routes
6.1 TIG/MIG Weld Overlay Applications
- Mining equipment: Bucket teeth, dragline buckets, and conveyor pulleys requiring WC hardfacing with controlled decomposition for optimal abrasion resistance. Target: 1500–1800 HV with 60–70% residual WC.
- Cement industry: Mill liners, grinding media, and kiln components subjected to severe abrasive wear. Multi-pass WC hardfacing with interpass cooling to control cumulative burn loss.
- Power generation: Coal mill roller surfaces, fan blades, and wear rings in flue gas systems. TIG powder hardfacing preferred for precise thermal control on thin-section components.
- Oil & gas: Drill collars, bit components, and wellhead components. MIG hardfacing with WC-containing flux-cored wire for high-productivity field applications.
6.2 Hydraulic Explosive Bonding Integration
- Clad plate + hardfacing composite: Hydraulic explosive bonding produces a steel/nickel or steel/tantalum clad plate; subsequent WC hardfacing on the cladding surface provides combined corrosion resistance (from the clad layer) and wear resistance (from the WC hardfacing). The burn loss mechanism study ensures the hardfacing process does not compromise the underlying bonded interface.
- Thermal management: The high compressive residual stress state from hydraulic explosive bonding (typically 300–600 MPa) provides a favorable stress environment for subsequent hardfacing, reducing the risk of cracking if thermal input is properly controlled.
6.3 Explosion Welding Integration
- Explosion-welded pipe + WC hardfacing: For downhole tools and subsea components where explosion welding produces a corrosion-resistant cladding and subsequent WC hardfacing provides wear protection at specific zones.
- Qualification sequencing: The explosion welding qualification (per ASTM A411 or ASTM B275) must precede the hardfacing WPS qualification. The combined qualification demonstrates that the full system (bonded interface + hardfacing deposit) performs as designed without degradation from the hardfacing thermal cycle.
7. Knowledge Management and Continuous Improvement
7.1 Learning Outcome Integration
The systematic study of WC hardfacing burn loss mechanisms contributes to the organization's knowledge management system through:
- WPS development: Each new WC hardfacing application requires a WPS that incorporates burn loss control parameters derived from this fundamental understanding.
- Training programs: Welder training curricula include theoretical modules on WC decomposition thermodynamics to ensure operators understand the "why" behind parameter constraints.
- Failure analysis: When field failures occur, the burn loss mechanism framework provides a diagnostic pathway—distinguishing between thermal decomposition failure, oxidation failure, and mechanical ejection failure.
- Consumable selection: Understanding burn loss mechanisms guides the selection of WC particle size, binder composition, and coating formulation to optimize the decomposition/burn loss balance for specific applications.
7.2 Quantitative Performance Targets
| Performance Indicator | Target Value | Measurement Method | Verification Frequency |
|---|---|---|---|
| Residual WC content | ≥50 wt% (standard); ≥60 wt% (premium) | OM + image analysis (ASTM E562) | Each production lot |
| Deposit hardness | ≥1400 HV0.3 | Vickers hardness (ASTM E384) | Each production lot |
| Thermal input | 3–10 kJ/mm | Parameter monitoring (V, I, v) | Real-time during production |
| W₂C/WC ratio | 2:1 to 3:1 | XRD phase analysis | WPS qualification; quarterly audit |
| Crack density | 0 cracks >0.5 mm | Visual/10× magnification | Each production lot |
8. Conclusion
The study of WC hardfacing particle burn loss mechanisms represents a fundamental metallurgical competency that underpins the technical credibility and product quality of Cladding Technology Shanxi Co., Ltd. across all three technology routes. By understanding the thermodynamic driving forces, kinetic pathways, and process-dependent variables that govern WC decomposition, the organization can:
- Design and qualify WPS procedures that consistently deliver WC hardfacing deposits meeting or exceeding customer specifications
- Diagnose and prevent field failures through root cause analysis grounded in metallurgical fundamentals
- Differentiate competitively through documented technical knowledge and traceable process control
- Integrate hardfacing capabilities with hydraulic explosive bonding and explosion welding technologies to deliver multi-functional composite surfaces
- Support qualification building under ASTM, ASME, API, NACE/AMPP, ISO, and GB standards through demonstrable process understanding
This knowledge asset directly supports the company's mission of delivering reliable, high-performance cladding and hardfacing solutions to demanding industrial applications worldwide.