Effect of Shielding Gas on Microstructure and Properties of Tungsten Carbide Flux-Cored Wire Weld Overlay Deposits
1. Definition and Fundamental Principles
Tungsten carbide (WC) flux-cored wire weld overlay is a specialized surfacing technique used to deposit hard, wear-resistant layers onto base metal substrates. The process involves a self-shielded or externally shielded flux-cored wire containing tungsten carbide particles as the primary hard phase, melted and deposited onto the workpiece surface through arc heating. The resulting overlay deposit exhibits extreme hardness (typically 1,200–1,800 HV in the as-welded condition, exceeding 1,800 HV after heat treatment) derived from the retained WC particles and the hard carbide phases formed during solidification.
The shielding gas plays a critical role in controlling the thermal profile, solidification behavior, and ultimately the microstructure of the weld overlay deposit. The primary functions of shielding gas in WC flux-cored wire overlay include:
- Atmospheric protection — Preventing oxidation and nitrogen pickup in the molten weld pool, which would degrade the integrity of WC particles and promote brittle phase formation.
- Arc stability and heat input modulation — Different shielding gases alter arc voltage, arc length, and heat distribution, directly affecting dilution rates and cooling rates.
- Weld pool dynamics — Influencing surface tension, flow patterns, and solidification front advancement, which govern WC particle distribution uniformity.
- Gas-metal interaction — Reactive shielding gases (e.g., CO₂) participate in oxidation-reduction reactions that can either refine the microstructure or introduce deleterious phases.
The solidification microstructure of WC weld overlay deposits typically consists of a dendritic iron-carbon matrix with dispersed WC particles, secondary carbides (Fe₃C, M₆C, M₂₃C₆), and in some cases, residual unmelted WC particles. The shielding gas composition directly influences the relative proportion of these phases, the degree of WC particle dissolution, and the morphology of the interdendritic regions.
2. Category and Business Positioning
This technical study falls within the MIG/TIG Weld Overlay Technology route of Cladding Technology Shanxi Co., Ltd.'s three core technology platforms. Specifically, it belongs to the category of hardfacing and wear-resistant overlay surfacing, which addresses the most demanding tribological challenges encountered in mining, power generation, cement, and material handling industries.
The business positioning of WC overlay technology is as follows:
- High-value-add service — WC overlay deposits command premium pricing due to their exceptional wear resistance (10–50× improvement over base steel) and the specialized process knowledge required.
- Qualification-driven market entry — Demonstrating systematic understanding of shielding gas effects enables the company to qualify for critical applications where microstructure control is mandated by customer specifications.
- Technical differentiation — Competitors often apply generic welding parameters; systematic shielding gas optimization provides a measurable performance advantage in deposit hardness uniformity, crack resistance, and bond strength.
- WPS qualification foundation — Shielding gas selection is a critical variable in Welding Procedure Specifications (WPS) for overlay welding, and its optimization directly supports successful procedure qualification.
3. Technical Purpose and Value
3.1 Primary Technical Objectives
The investigation of shielding gas effects on WC flux-cored wire overlay deposits serves several critical technical objectives:
- Maximize deposit hardness — Achieve and maintain hardness levels ≥1,500 HV through optimal gas selection that minimizes WC particle dissolution while preventing excessive carbide coarsening.
- Minimize dilution — Control base metal dilution to ≤30% to preserve the hard phase content and wear resistance of the overlay.
- Reduce crack susceptibility — Select shielding gas compositions that promote favorable solidification morphologies and reduce hydrogen-induced cracking and thermal cracking.
- Ensure deposit uniformity — Achieve consistent microstructure across multi-pass builds through stable arc characteristics and uniform heat input.
- Optimize cost-effectiveness — Balance gas cost, wire consumption, and deposition efficiency to deliver economical solutions without sacrificing performance.
3.2 Value to Product Delivery and Customer Satisfaction
Systematic shielding gas optimization translates directly into improved product reliability, longer service life of overlaid components, and reduced maintenance intervals for end users. This technical knowledge enables the company to provide data-backed WPS recommendations, reduce rework rates, and deliver overlay deposits that consistently meet or exceed customer-specified hardness and wear performance criteria.
4. Key Process and Implementation Points
4.1 Shielding Gas Selection Matrix
| Shielding Gas Composition | Arc Heat Input | Dilution Rate | WC Particle Retention | Deposit Hardness (HV) | Crack Susceptibility | Weld Pool Behavior |
|---|---|---|---|---|---|---|
| 100% Ar | Low–Medium | Low (15–25%) | High | 1,400–1,700 | Low | Stable, narrow bead |
| 100% CO₂ | High | High (30–45%) | Moderate–Low | 1,100–1,400 | Moderate (hot cracking) | Wide, fluid pool |
| Ar + 5% CO₂ | Medium | Moderate (20–30%) | High | 1,500–1,750 | Low | Stable, moderate width |
| Ar + 10% CO₂ | Medium–High | Moderate (25–35%) | Moderate | 1,300–1,600 | Low–Moderate | Slightly wider bead |
| Ar + 20% CO₂ | High | High (30–40%) | Moderate | 1,200–1,500 | Moderate | Fluid, wider bead |
| Ar + 2% O₂ | Medium | Moderate (22–30%) | High | 1,450–1,700 | Low | Stable, good wetting |
4.2 Critical Process Parameters
| Parameter | Typical Range | Influence on Microstructure |
|---|---|---|
| Welding Current (DCRP) | 180–320 A | Higher current increases dilution and WC dissolution; promotes coarser microstructure |
| Voltage | 22–32 V | Affects arc stability and penetration depth; interacts with gas composition |
| Travel Speed | 200–500 mm/min | Higher speed reduces heat input, lowers dilution, preserves WC particles |
| Wire Diameter | 1.2–1.6 mm | Thicker wire increases heat input and dilution; requires higher current |
| Interpass Temperature | ≤150°C (recommended ≤100°C) | Excessive interpass temperature promotes grain coarsening and phase coarsening |
| Gas Flow Rate | 15–25 L/min | Inadequate flow allows contamination; excessive flow causes turbulence and backdrafting |
| Arc Length | 3–5 mm | Affects heat distribution and gas coverage quality |
4.3 Microstructural Evolution Mechanisms
The shielding gas influences the overlay microstructure through several interconnected mechanisms:
- Thermal effect pathway — CO₂-rich shielding gases increase arc heat due to the endothermic dissociation of CO₂ at high temperatures. This elevated heat input increases the temperature gradient at the solidification front, promoting faster WC particle dissolution and higher dilution rates.
- Chemical effect pathway — CO₂ dissociates into CO and atomic oxygen at arc temperatures. The atomic oxygen reacts with carbon in the weld pool, reducing available carbon for carbide precipitation and potentially dissolving WC particles (WC + O → WO₃ + C). Conversely, pure Ar provides no chemical interaction, preserving the original WC particle integrity.
- Fluid dynamics pathway — Gas density differences (CO₂ is 1.5× denser than Ar) affect the protective envelope geometry. CO₂ provides better coverage for vertical and overhead positions but creates turbulence at higher flow rates. Ar's lower density requires higher flow rates but provides more stable coverage on horizontal surfaces.
- Solidification pathway — The cooling rate, influenced by gas-mediated heat transfer, determines dendrite spacing, interdendritic carbide morphology, and the degree of microsegregation of carbon and alloying elements.
4.4 Optimal Shielding Gas Recommendations by Application
| Application Type | Recommended Gas | Rationale |
|---|---|---|
| Slurry pumps and impellers | Ar + 5% CO₂ | Optimal hardness retention with acceptable dilution; good bead profile for curved surfaces |
| Mining bucket teeth and blades | 100% Ar or Ar + 2% O₂ | Maximum WC retention for extreme abrasion; low dilution preserves hard phase content |
| Cement mill liners and rollers | Ar + 5–10% CO₂ | Balanced hardness and toughness; moderate dilution acceptable for thick overlay builds |
| Valve seats and seats (high-pressure) | 100% Ar | Minimum dilution, maximum hardness, best surface finish; critical for sealing applications |
| Heavy-duty structural wear parts | Ar + 10% CO₂ | Good penetration for thick base metals; adequate hardness with improved toughness |
5. Applicable Standards and Acceptance Criteria
5.1 Welding Procedure Standards
- ASME Section IX — Qualification requirements for weld overlay procedures, including essential variables (shielding gas classification, heat input, preheat, interpass temperature).
- AWS D10.9M/D10.9 — Specification for Welding Procedure and Performance Qualification for Weld Overlay.
- GB/T 985 — General technical requirements for welding, applicable to overlay welding procedures in Chinese standard systems.
- GB/T 19866 — Specification for welding procedure qualification for overlay welding.
- ISO 15614 — Qualification testing of welding procedures for metallic materials, Part 9: Additional requirements for welding overlay.
- EN ISO 15614-9 — European qualification standard for overlay welding procedures.
5.2 Material and Performance Standards
- ASTM A538/A538M — Standard specification for weld overlay metals for wear resistance (includes WC-based classifications).
- ASTM A520/A520M — Specification for weld overlay metals for corrosion resistance (relevant when WC overlay is used in combined wear/corrosion environments).
- GB/T 17116 — Classification and technical requirements for welding consumables for hardfacing.
- NACE MR0175/ISO 15156 — Materials for use in H₂S-containing environments (relevant when WC overlay is specified in sour service).
5.3 Acceptance Criteria
| Property | Typical Acceptance Criterion | Test Method |
|---|---|---|
| Hardness (as-welded) | ≥1,200 HV (surface), ≥1,000 HV (subsurface) | ASTM E92 / ASTM E384 (Vickers) |
| Hardness (after HT) | ≥1,500 HV (surface) | ASTM E92 / ASTM E384 |
| Hardness uniformity | ±15% variation across deposit | ASTM E384 (grid pattern) |
| Hardness profile (dilution) | Hardness drop-off ≤50% within 1 mm from surface | ASTM E384 (depth profile) |
| Bond strength | ≥450 MPa (transverse tensile) | ASTM A538 / GB/T 2651 |
| Crack length | No cracks ≥1.5 mm; total crack length ≤5% of weld length | Visual + dye penetrant (ASTM E709) |
| Porosity | Acceptance per AWS D1.1, Level 2 maximum | Visual + radiographic (ASTM E165) |
5.4 Non-Destructive Testing Requirements
- Visual Inspection (VT) — Per ASTM E165; checking bead profile, overlap, surface defects, and gas porosity indicators.
- Dye Penetrant Testing (PT) — Per ASTM E709; detection of surface cracks, particularly hot cracks in WC-rich regions.
- Magnetic Particle Testing (MT) — Per ASTM E709; surface and near-surface crack detection on ferromagnetic substrates.
- Radiographic Testing (RT) — Per ASTM E165/E94; volumetric defect detection (porosity, lack of fusion) for critical applications.
- Ultrasonic Testing (UT) — Per ASTM E2398/E2718; bond line integrity verification and subsurface defect detection.
6. Common Risks and Controls
6.1 Process Risks
| Risk | Cause | Consequence | Control Measure |
|---|---|---|---|
| Excessive WC dissolution | High heat input (CO₂-rich gas, high current) | Reduced hardness, loss of wear resistance | Use Ar-based gas; limit current; increase travel speed |
| Hot cracking (intergranular) | High dilution, high carbon gradient, CO₂-induced oxidation | Overlay failure, loss of wear protection | Minimize dilution; use Ar + 5% CO₂; control interpass temperature |
| Hydrogen-induced cracking | Ambient moisture ingress, inadequate gas shielding | Delayed cracking, bond line separation | Maintain ≥15 L/min gas flow; use gas lens; preheat to 50–100°C |
| Uneven hardness distribution | Inconsistent arc parameters, gas flow variation | Premature localized wear failure | Automate gas flow control; use constant-current power source; train operators |
| Excessive dilution | High current, low travel speed, CO₂-rich gas | Hardness below specification, reduced service life | Optimize gas composition; reduce current; increase travel speed; use multiple thin passes |
| Spatter and porosity | Excessive arc length, gas turbulence, wet base metal | Surface roughness, reduced fatigue life | Maintain 3–5 mm arc length; optimize gas nozzle geometry; clean base metal |
6.2 Quality Control Measures
- Pre-weld verification — Confirm shielding gas composition via gas analyzer; verify flow rate with calibrated flowmeter; inspect gas supply lines for leaks using soap solution or electronic detector.
- In-process monitoring — Track welding parameters (current, voltage, speed) continuously; record gas flow rate; monitor interpass temperature with infrared thermometer.
- Post-weld verification — Perform hardness testing on representative coupons; conduct NDT per WPS; document all results in traceable quality records.
- Gas supply management — Implement first-in-first-out gas cylinder rotation; inspect cylinders for pressure levels; maintain backup supply to prevent mid-build gas interruption.
7. Application Scenarios Across Technology Routes
7.1 TIG/MIG Weld Overlay Route
This technical entry directly supports the MIG weld overlay route, which is the primary delivery mechanism for WC hardfacing applications. The shielding gas knowledge enables:
- WPS development and qualification — Shielding gas selection is an essential variable in ASME Section IX and AWS D10.9 qualification. Systematic understanding allows the company to develop and qualify WPS packages for specific WC overlay applications with confidence.
- Multi-pass overlay optimization — Different gas compositions can be used for different passes (e.g., Ar for the first pass to minimize dilution, Ar + 5% CO₂ for subsequent passes to improve bead profile and wetting).
- Robotized overlay implementation — Shielding gas parameters are critical for automated/robotic overlay systems where consistency is paramount. Gas composition optimization ensures repeatable results across thousands of production cycles.
- Transition layer integration — When WC overlay is applied over a transition layer (e.g., 309L or Ni-based), shielding gas selection must be optimized for both dilution control at the transition/overlay interface and microstructure control within the WC deposit.
7.2 Hydraulic Explosive Bonding Route
While WC overlay is not typically applied via hydraulic explosive bonding (which is primarily used for ductile metal-to-metal cladding such as stainless steel over carbon steel), the shielding gas knowledge contributes indirectly:
- Post-bonding surface preparation — WC overlay may be applied to the clad surface of hydraulically bonded plates to provide additional wear resistance. Shielding gas optimization ensures proper bonding of the overlay to the clad surface without damaging the explosive bond interface.
- Composite construction — In hybrid constructions where hydraulic explosive bonding provides corrosion resistance and WC overlay provides wear resistance, the shielding gas must be selected to prevent undercutting or damage to the underlying clad layer.
- Thermal management — Understanding gas-mediated heat input helps control the thermal budget during overlay application on explosive-bonded substrates, preventing bond line degradation from excessive heat.
7.3 Explosion Welding Route
Similar to hydraulic explosive bonding, explosion welding is primarily used for ductile metal cladding. However, the shielding gas expertise contributes to:
- Post-explosion weld repair and reinforcement — Defect repair and localized reinforcement of explosion-welded joints using WC flux-cored wire requires optimized shielding gas to ensure proper fusion and hardness without compromising the explosion weld bond.
- Overlay on explosion-welded assemblies — When explosion-welded pipe or plate assemblies require additional wear protection, WC overlay with optimized shielding gas provides a reliable surfacing solution.
- Process parameter correlation — Understanding the thermal and metallurgical effects of shielding gas in MIG overlay provides transferable knowledge for evaluating post-weld heat treatment requirements on explosion-welded components.
8. Contribution to Qualification Building and Competitive Advantage
8.1 Qualification and Certification Support
This technical study directly supports the company's qualification building in the following ways:
- WPS Qualification Packages — Shielding gas is classified as an essential variable in welding procedure qualification. Demonstrated understanding of gas effects enables successful qualification of WPS packages under ASME Section IX, AWS D10.9, and GB/T 19866.
- Performance Qualification — Systematic gas optimization data supports performance qualification testing (AWS D10.9) by demonstrating consistent achievement of hardness, dilution, and bond strength requirements.
- Customer Audit Readiness — Documented technical understanding of shielding gas effects demonstrates engineering competence to customers during qualification audits, particularly in regulated industries (oil & gas, power generation, mining).
- Third-Party Certification — Technical documentation supports certification bodies in evaluating the company's capability for specialized overlay welding services.
8.2 Competitive Differentiation
- Data-driven recommendations — Unlike competitors who may rely on generic wire manufacturer data, the company can provide application-specific shielding gas recommendations backed by microstructural analysis.
- Reduced warranty risk — Optimized gas selection minimizes crack susceptibility and hardness non-uniformity, reducing warranty claims and enhancing customer confidence.
- Technical consulting capability — Deep understanding of gas effects enables the company to provide value-added technical consulting to customers on overlay process optimization, positioning the company as a technical partner rather than a mere service provider.
- IP and knowledge protection — Proprietary gas optimization data and process know-how create intellectual property barriers that are difficult for competitors to replicate.
9. Implementation Recommendations
- Establish a shielding gas optimization matrix for each WC flux-cored wire grade in the company's product portfolio, documenting microstructural and mechanical performance data for at least three gas compositions per wire grade.
- Integrate gas composition into WPS documentation as a controlled parameter with specified tolerance ranges (e.g., Ar + 5% ±1% CO₂), ensuring traceability and consistency across production.
- Implement online gas monitoring in production facilities using gas composition analyzers to detect drift in gas supply quality, particularly for mixed gas blends.
- Develop application-specific gas selection guides for key customer segments (mining, power, cement, oil & gas) to streamline quoting and process planning.
- Conduct periodic re-qualification of overlay WPS when gas supply sources change or when wire chemistry modifications are made by consumable manufacturers.
- Train welding operators on the relationship between gas composition, arc behavior, and deposit quality to enable real-time troubleshooting and parameter adjustment.
10. Conclusion
The systematic study of shielding gas effects on WC flux-cored wire weld overlay deposits represents a critical technical competency for Cladding Technology Shanxi Co., Ltd. in the MIG weld overlay technology route. By understanding and controlling the interactions between shielding gas composition, arc behavior, solidification microstructure, and final deposit properties, the company can deliver consistently high-quality wear-resistant overlay solutions that meet the most demanding customer specifications. This technical knowledge directly supports WPS qualification, reduces production risk, enhances customer value, and builds a defensible competitive position in the specialized hardfacing and overlay welding market.
The key takeaway for operational implementation is that shielding gas is not merely a protective medium but a process control variable that must be selected and managed with the same rigor as welding current, voltage, and travel speed. Treating gas composition as an integral part of the welding procedure specification — rather than an afterthought — is the fundamental principle that separates technically competent overlay operations from unreliable, inconsistent production.