CO₂ Shielded Arc Welding and Thermal Spraying of High Carbon Chromium-Iron Alloy Powder for Wear-Resistant Composite Overlay Layers
1. Definition and Fundamental Principles
CO₂ shielded metal arc welding (SMAW/FCAW) and thermal spraying of high carbon chromium-iron (Fe-Cr-C) alloy powder constitute a dual-process approach to producing wear-resistant composite weld overlay layers. The underlying metallurgical principle relies on the formation of hard carbide phases—predominantly M₇C₃, M₆C, and M₂₃C₆ chromium carbides—within a martensitic or austenitic matrix. The high carbon content (typically 2.0–4.0 wt%) combined with chromium (18–30 wt%) creates a metastable microstructure that, upon controlled cooling or post-weld heat treatment, transforms into a hard, wear-resistant composite layer.
In CO₂ welding, the active shielding gas (carbon dioxide) interacts with the arc plasma to produce a CO/O₂ environment that promotes controlled decarburization at the fusion boundary while allowing carburization of the overlay material. The resulting composite layer exhibits a gradient microstructure: a transition zone with moderate hardness at the base metal interface, transitioning to a high-hardness carbide-rich zone at the surface. Thermal spraying (flame spray, plasma spray, or HVOF) of the same alloy powder produces a splat-based microstructure with retained carbide particles dispersed in a metallic binder matrix, offering an alternative route to wear resistance without excessive dilution of the base substrate.
The composite nature of these overlay layers—combining a ductile metallic matrix with hard ceramic-like carbide reinforcements—provides superior abrasion and erosion resistance compared to homogeneous weld deposits, particularly under conditions involving sliding wear, impact abrasion, and high-temperature oxidation.
2. Category and Business Positioning
This technology falls under the company's MIG/CO₂ weld overlay capability portfolio and thermal spray surface engineering services. It represents a cost-effective, high-productivity approach to surface hardening that complements the company's premium TIG weld overlay services and solid-state bonding technologies (hydraulic explosive bonding and explosion welding). The positioning is as follows:
- Cost-Performance Segment: CO₂ welding offers significantly lower consumable and labor costs compared to TIG welding, making it suitable for large-area, high-thickness overlay applications where extreme metallurgical purity is not required.
- Thermal Spray Complement: Spraying of high carbon chromium-iron powder provides a non-heat-affected-zone (HAZ) alternative for substrates sensitive to thermal input, such as thin-walled components or pre-stressed structures.
- Qualification Building: This technology supports the development of welding procedure specifications (WPS) and qualification records that demonstrate the company's breadth of surface engineering capability, particularly for the mining, cement, and power generation industries.
- Customer Value: Enables rapid restoration of worn components with extended service life (typically 2–5× the life of unhardened steel), reducing unplanned downtime and maintenance costs.
3. Technical Purpose and Value
The primary technical purpose is to achieve surface hardness in the range of 55–70 HRC (weld overlay) or 45–60 HRC (sprayed coating) with sufficient adhesion strength and mechanical integrity to withstand severe abrasive and erosive service conditions. Key performance targets include:
- Hardness: Surface hardness ≥ 60 HRC for weld overlay; ≥ 50 HRC for thermal spray coatings
- Adhesion Strength: Peel test ≥ 25 MPa (sprayed coatings); full fusion with no delamination (weld overlay)
- Crack Resistance: No transverse or longitudinal cracks in visual and magnetic particle inspection
- Wear Life Extension: Minimum 2× improvement over baseline carbon steel substrate in standardized wear testing
The value proposition extends beyond raw performance metrics to include process flexibility (applicable to both ferrous and certain non-ferrous substrates), scalability (suitable for field repair and shop fabrication), and compatibility with existing manufacturing infrastructure.
4. Key Process Parameters and Implementation Points
4.1 CO₂ Shielded Arc Weld Overlay Parameters
| Parameter | Recommended Range | Notes |
|---|---|---|
| Shielding Gas | 100% CO₂ or 80% Ar / 20% CO₂ | Pure CO₂ provides deeper penetration and higher dilution; mixed gas reduces spatter |
| Electrode/Wire Diameter | 1.2 mm – 2.0 mm | Larger diameter for thicker deposits; smaller for detailed work |
| Welding Current | 180 A – 320 A | Dependent on wire diameter and deposition rate requirements |
| Voltage | 20 V – 28 V | Lower voltage for narrower bead; higher for wider, flatter profile |
| Travel Speed | 200 mm/min – 400 mm/min | Higher speed reduces heat input and dilution |
| Heat Input | 0.8 kJ/mm – 2.5 kJ/mm | Critical for controlling dilution and microstructure transformation |
| Interpass Temperature | ≤ 250°C (typically 150°C – 200°C) | Prevents excessive grain growth and tempering of martensitic structure |
| Deposition Layers | 2 – 5 passes | First pass for transition; subsequent passes for full alloy chemistry |
| Preheat Temperature | 100°C – 250°C (substrate-dependent) | Reduces cracking risk; higher for high-carbon or low-alloy steels |
4.2 Thermal Spraying Parameters (Flame/Plasma/HVOF)
| Parameter | Flame Spray | Plasma Spray | HVOF |
|---|---|---|---|
| Particle Size | 30 – 150 μm | 15 – 63 μm | 15 – 45 μm |
| Standoff Distance | 100 mm – 150 mm | 100 mm – 200 mm | 50 mm – 100 mm |
| Coating Thickness per Pass | 0.1 mm – 0.3 mm | 0.05 mm – 0.2 mm | 0.05 mm – 0.15 mm |
| Final Coating Thickness | 0.5 mm – 3.0 mm | 0.3 mm – 2.0 mm | 0.2 mm – 1.5 mm |
| Substrate Temperature | ≤ 250°C | ≤ 200°C | ≤ 150°C |
| Achieved Hardness | 45 – 55 HRC | 50 – 60 HRC | 55 – 65 HRC |
| Adhesion Strength | ≥ 20 MPa | ≥ 30 MPa | ≥ 40 MPa |
4.3 Critical Implementation Points
- Substrate Preparation: Surface must be grit-blasted to Sa 2.5 (ISO 8501-1) or equivalent. For weld overlay, a V-groove or U-groove preparation with 60° included angle is recommended to ensure full fusion and minimize dilution. Surface contaminants (oil, rust, mill scale) must be completely removed.
- Dilution Control: Dilution of the overlay alloy by base metal is the single most critical factor affecting final hardness. For CO₂ welding, dilution typically ranges from 30% to 50% for the first pass, decreasing to 10%–20% for subsequent passes. Multi-pass deposition is essential to achieve nominal alloy chemistry in the final layers.
- Weld Geometry: Stringer beads (narrow, high reinforcement) should be used for the transition layer to minimize dilution. Wider, flatter beads are appropriate for subsequent overlay passes. Bead overlap should be 25%–30% to ensure complete coverage and uniform thickness.
- Post-Weld Heat Treatment (PWHT): For weld overlay deposits, a tempering treatment at 500°C–600°C for 1–2 hours is recommended to reduce residual stress and improve toughness while maintaining acceptable hardness. Rapid quenching (furnace cooling followed by water quench) can be used to maximize martensite formation, but increases cracking risk.
- Spray Coating Build-Up: For thermal spray applications, coatings should be built up in multiple passes with interpass cleaning (light grinding or grit blasting between passes) to ensure inter-layer adhesion. Total thickness should be built to specification with ±0.1 mm tolerance.
- Direction of Travel: For large areas, a systematic travel pattern (herringbone, zigzag, or circumferential) should be used to ensure uniform heat input distribution and avoid thermal distortion of the substrate.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
- GB/T 11345 — Ultrasonic testing of welds (for subsurface defect detection in overlay layers)
- GB/T 2651 — Magnetic particle testing (for surface and near-surface crack detection)
- GB/T 1388 — Visual inspection of welds (surface quality, undercut, porosity)
- GB/T 6394 — Metallographic examination procedures (microstructure verification)
- GB/T 230 — Rockwell hardness testing (HRC measurement for overlay verification)
- GB/T 18244 — Thermal spray coating terminology and specifications
- GB/T 31433 — Thermal spray coating adhesion testing
- ASTM B733 — Standard Specification for Thermal Spray Coatings of Iron, Nickel, and Cobalt
- ASTM B614 — Standard Specification for Thermal Spray Coatings of Iron, Nickel, and Cobalt (general requirements)
- ASTM A743 — Castings, iron cast, for pressure-containing parts (reference for Cr-Fe-C chemistry)
- ASTM E10 / E18 — Rockwell hardness testing methods
- ASTM E94 — Magnetic particle examination
- ASME Section IX — Qualification of welding procedures (WPS/PQR qualification)
- ISO 14286 — Thermal spray — Metal and metal ceramic coatings — Surface preparation of substrates
- ISO 18589 — Thermal spray — Metal and metal ceramic coatings — Surface preparation
- ISO 2360 — Thermal spray — Adhesion testing
- ISO 6506 — Vickers hardness testing
- NACE SP0388 — Control of internal corrosion in carbon steel tanks by protective coatings
5.2 Acceptance Criteria
| Inspection Item | Acceptance Criterion | Standard Reference |
|---|---|---|
| Visual Inspection (VT) | No cracks, undercut ≤ 0.5 mm, no excessive reinforcement or concavity | GB/T 1388, ISO 5817 (Level B) |
| Magnetic Particle Inspection (MT) | No indications of linear discontinuities (cracks); round indications (porosity) ≤ 2 mm | GB/T 2651, ASTM E94 |
| Hardness (Surface) | ≥ 60 HRC for weld overlay; ≥ 50 HRC for sprayed coatings | GB/T 230, ASTM E18 |
| Hardness Gradient | Gradual transition from substrate hardness to overlay hardness; no abrupt drop | GB/T 6394 (metallographic traverse) |
| Adhesion (Sprayed Coating) | Peel strength ≥ 25 MPa; no coating delamination at interface | GB/T 31433, ISO 2360 |
| Thickness | As specified (typically 2–6 mm weld overlay; 0.5–3 mm spray); tolerance ±0.5 mm | Customer specification |
| Chemical Composition | C: 2.0–4.0%; Cr: 18–30%; Fe: balance; dilution verified by spectrographic analysis | ASTM E415, GB/T 223 series |
| Microstructure | Martensitic matrix with dispersed M₇C₃/M₆C carbides; no untempered martensite or retained austenite > 15% | GB/T 6394 |
6. Common Risks and Controls
| Risk | Cause | Control Measure |
|---|---|---|
| Cracking in overlay layer | Excessive carbon equivalent; rapid cooling; high restraint; hydrogen embrittlement | Preheat to 150–250°C; limit heat input; use low-hydrogen consumables; post-weld stress relief at 500–600°C; control interpass temperature ≤ 250°C |
| Excessive dilution (low hardness) | Too high heat input; single-pass deposition; poor weld geometry | Use multi-pass technique; reduce current/voltage; use stringer beads for transition; verify dilution by spectrographic analysis after first pass |
| Poor adhesion (sprayed coating) | Inadequate surface preparation; contaminated substrate; excessive standoff distance | Grit blast to Sa 2.5; degrease before spraying; maintain standoff distance per WPS; perform adhesion testing on coupon before production run |
| Porosity in weld deposit | Moisture in consumables; inadequate gas shielding; surface contamination | Dry electrodes/wire per manufacturer specifications; use gas lens and proper gas flow rate (15–20 L/min); clean substrate thoroughly |
| Thermal distortion of substrate | Excessive heat input; large area coverage without compensation | Use alternating travel pattern; back-gas cooling; limit continuous welding length; use fixture or backing plate for rigid support |
| Delamination at weld interface | Incomplete fusion; surface oxide contamination; incompatible base metal chemistry | Ensure proper groove preparation; grind to bright metal before welding; verify base metal composition; use first-pass dilution check |
| Hardness loss after PWHT | Over-tempering; excessive PWHT temperature or duration | Control PWHT temperature to 500–550°C for maximum 2 hours; verify hardness after PWHT; adjust PWHT parameters based on post-treatment hardness results |
7. Application Scenarios Across Technology Routes
7.1 CO₂ Weld Overlay Applications
CO₂ shielded arc welding of high carbon chromium-iron alloy powder is particularly suited to the following industrial applications:
- Mining Equipment: Wear plates on excavator buckets, conveyor chute linings, crusher liners, and haul truck chassis components. The high deposition rate of CO₂ welding makes it ideal for large-area restoration of worn mining equipment.
- Cement Industry: Rotary kiln wear plates, preheater tower internals, and fan impellers exposed to abrasive cement particulates. Multi-pass CO₂ overlay provides economical protection for large, low-value components.
- Power Generation: Boiler tube surfaces exposed to fly ash erosion, turbine blade tips, and ash hopper linings. The wear resistance of the Cr-C-Fe overlay extends component life in severe erosive environments.
- Construction Equipment: Bucket teeth, blade edges, and track links on heavy earthmoving equipment. Field-repairable with portable CO₂ welding equipment.
7.2 Thermal Spray Applications
Thermal spraying of high carbon chromium-iron alloy powder complements weld overlay in the following scenarios:
- Thin-Walled Components: Where heat input from welding would cause distortion or weakening of the substrate, thermal spray provides a near-net-shape coating without significant thermal effects.
- Complex Geometry Restoration: Components with intricate shapes (impellers, valve bodies, mold cavities) where weld overlay is impractical but spray coating can achieve uniform coverage.
- Pre-Stressed Components: Springs, pre-loaded shafts, or other components where welding-induced residual stress would compromise structural integrity.
- Multi-Layer Coating Systems: As a wear-resistant top layer over a diffusion-bonded or TIG-welded transition layer, creating a graded composite structure with optimal adhesion and surface performance.
7.3 Integration with Hydraulic Explosive Bonding and Explosion Welding
While CO₂ weld overlay and thermal spraying are primarily surface engineering technologies, they integrate with the company's solid-state bonding capabilities in the following ways:
- Post-Bonding Surface Protection: After hydraulic explosive bonding or explosion welding of dissimilar metal clad plates (e.g., carbon steel/SS316 or steel/nickel alloys), CO₂ weld overlay or thermal spray of high carbon Cr-Fe alloy can be applied to the outer surface to provide additional wear resistance for the bonded assembly.
- Repair of Explosion-Welded Components: When explosion-welded or hydraulic-explosion-bonded clad components experience localized wear or damage, CO₂ weld overlay provides a practical repair method that maintains the integrity of the underlying bonded interface.
- Hybrid Clad Systems: For applications requiring both corrosion resistance (provided by the bonded clad layer) and wear resistance (provided by the overlay/spray layer), a hybrid approach combining explosion welding for the base clad and CO₂ overlay or thermal spray for the wear surface delivers comprehensive protection.
- Qualification Synergy: Demonstrating proficiency in both solid-state bonding and weld overlay technologies positions the company as a comprehensive surface engineering provider capable of delivering multi-functional clad and coated components for complex industrial requirements.
8. Qualification Building and Customer Value
8.1 WPS/PQR Qualification Strategy
Establishing qualified welding procedure specifications (WPS) and performance qualification records (PQR) for CO₂ weld overlay and thermal spray of high carbon chromium-iron alloy powder is essential for regulatory compliance and customer confidence. The qualification program should include:
- Procedure Development: Establish baseline WPS parameters covering the full range of anticipated applications (substrate types, thicknesses, geometries, and service conditions).
- Coupon Testing: Fabricate qualification coupons per ASME Section IX or applicable customer specifications, including hardness traverse testing, chemical analysis for dilution verification, and mechanical testing (adhesion, peel, or tensile).
- Non-Destructive Testing: Perform MT and UT on qualification welds to establish acceptance criteria and verify defect-free execution.
- Wear Testing: Conduct standardized wear testing (ASTM G99 pin-on-disk, ASTM G65 taber abrasion, or customer-specific field trials) to quantify wear life improvement.
- Documentation: Compile complete qualification records including WPS, PQR, test reports, NDT reports, and metallographic examinations for customer submission and regulatory filing.
8.2 Customer Value Proposition
This technology delivers measurable value to customers through:
- Extended Asset Life: 2–5× improvement in wear life over uncoated carbon steel, reducing replacement frequency and maintenance intervals.
- Reduced Downtime: Field-repairable overlay technology minimizes component removal and return-to-shop requirements, enabling in-situ restoration during scheduled maintenance windows.
- Cost Efficiency: CO₂ welding consumables cost 40–60% less than TIG welding equivalents, and thermal spray provides near-net-shape coatings without machining allowances, reducing material and labor costs.
- Technical Versatility: Applicability across a wide range of substrates (carbon steel, low-alloy steel, cast iron, and certain stainless steels) and component geometries makes this technology suitable for diverse industrial applications.
- Comprehensive Solution: Integration with the company's full technology portfolio (TIG overlay, thermal spray, hydraulic explosive bonding, explosion welding) enables delivery of complete, multi-functional surface protection solutions tailored to specific service conditions.
9. Summary and Recommendations
The CO₂ shielded arc welding and thermal spraying of high carbon chromium-iron alloy powder represents a mature, well-understood technology for producing wear-resistant composite overlay layers. Its value lies in the balance of performance, cost, and process flexibility that makes it suitable for high-volume industrial applications where TIG welding would be economically impractical and solid-state bonding would be technically unnecessary.
To maximize the commercial and technical value of this capability, the company should:
- Establish a comprehensive WPS/PQR qualification matrix covering the full range of anticipated substrate types, geometries, and service conditions.
- Develop standardized process documentation and operator training programs to ensure consistent quality across production runs.
- Invest in metrology and NDT capabilities (hardness testing, spectrographic analysis, MT/UT equipment) to support qualification and production quality assurance.
- Conduct targeted wear testing and field trials with key industry customers to generate performance data and case studies that support marketing and sales efforts.
- Explore hybrid approaches combining this technology with the company's solid-state bonding capabilities to develop differentiated, multi-functional clad and coated products for premium market segments.