Composite CO₂ Arc Weld Overlay and High Chromium-Molybdenum Alloy Thermal Spray Clad Layer for Deep Plough Plow Blade Wear Resistance
1. Definition and Technical Principles
The technology described in this entry represents a hybrid surface engineering approach that combines CO₂ gas-metal arc welding (GMAW) with thermal spray deposition of high chromium-molybdenum (Cr-Mo) alloy powder to create a composite wear-resistant overlay on deep plough plow blades (plowshares). This dual-process methodology leverages the metallurgical bonding strength of CO₂ arc weld overlay as a substrate preparation and transition layer, followed by thermal spray application of a high-hardness Cr-Mo alloy powder to achieve superior abrasive wear resistance.
The fundamental principle relies on two synergistic mechanisms:
- Metallurgical bonding layer: CO₂ arc welding deposits a transition alloy layer that ensures strong metallurgical adhesion between the base steel substrate and the subsequent spray layer. The molten weld pool penetrates the base material, creating a diffusion bond interface that resists spalling under cyclic loading conditions typical of deep ploughing operations.
- Thermal spray hardening layer: High chromium-molybdenum alloy powder (typically containing 15–30 wt% Cr and 0.5–2.0 wt% Mo) is thermally melted and accelerated onto the pre-welded surface. Upon solidification, the Cr-Mo alloy forms hard carbide precipitates (Cr₇C₃, Mo₂C, and mixed Cr-Mo carbides) that provide exceptional resistance to abrasive soil and rock particles encountered during deep tillage.
The composite structure creates a graded hardness profile — from the ductile base steel through the transition weld layer to the ultra-hard spray topcoat — which effectively manages residual stress and prevents catastrophic delamination under impact and abrasion.
2. Category and Business Positioning
This technology falls within the company's TIG/MIG weld overlay technology route, specifically representing an advanced variant that integrates arc welding with thermal spray processes. In the company's product portfolio, it serves the agricultural and earthmoving equipment segment, targeting OEMs and aftermarket suppliers of deep plough blades, ripper shanks, and tillage tools.
Within the broader business framework, this composite overlay technology positions the company as a specialist in multi-process surface engineering solutions, differentiating from single-process providers by offering:
- Extended service life of plow blades in high-abrasion operating environments
- Reduced total cost of ownership through fewer replacement cycles
- Customizable hardness profiles tailored to specific soil conditions and operating loads
3. Technical Purpose and Value
3.1 Engineering Objectives
The primary engineering objective is to achieve a surface hardness of ≥55 HRC in the spray layer while maintaining a transition zone hardness gradient that prevents crack initiation at the weld/spray interface. The composite overlay must withstand:
- Abrasive wear from soil particles (quartz, feldspar) and embedded rock fragments
- Impact loading from ploughing at depths of 500–800 mm
- Cyclic thermal stress from frictional heating during continuous operation
- Corrosive attack from acidic soil environments (pH 4.5–6.0)
3.2 Quantified Value Proposition
| Performance Metric | Uncoated Base Steel | CO₂ Weld Only | Composite Weld + Spray |
|---|---|---|---|
| Surface Hardness (HRC) | 22–28 | 40–48 | 55–62 |
| Abrasive Wear Life (hours) | 80–120 | 250–350 | 500–700 |
| Adhesive Bond Strength (MPa) | — | 200–300 | 150–250 |
| Impact Resistance (J) | 35–45 | 18–25 | 20–28 |
| Corrosion Resistance (h in 5% NaCl) | 48–72 | 120–168 | 200–280 |
4. Key Process and Implementation Points
4.1 Process Sequence
- Base Material Preparation: Deep plough blade (typically Q345B or Q355B low-alloy steel) is machined to remove mill scale, surface contaminants, and previous weld defects. Surface roughness Ra ≤ 12.5 μm is achieved through grinding or shot blasting.
- Pre-Heating: Base material is preheated to 200–250°C to reduce hydrogen-induced cracking susceptibility and minimize thermal gradient during welding.
- CO₂ Arc Weld Overlay (Transition Layer): A 2–4 mm thick transition layer is deposited using a Ni-Cr alloy wire (e.g., matching ER309L composition or a proprietary Ni-Cr-Mo filler) with 100% CO₂ shielding gas.
- Thermal Spray Application (Hardening Layer): High Cr-Mo alloy powder (typically 15–20 wt% Cr, 0.8–1.5 wt% Mo, balance Fe with trace C, Si, Mn) is applied via flame spray or plasma spray to achieve a final overlay thickness of 1.5–3.0 mm.
- Post-Treatment: Controlled cooling or low-temperature tempering (250–300°C, 1–2 hours) to relieve residual stresses without softening the carbide structure.
4.2 CO₂ Arc Welding Parameters
| Parameter | Recommended Range | Rationale |
|---|---|---|
| Shielding Gas | 100% CO₂ | Deep penetration, good wetting on preheated substrate |
| Filler Wire | Ø1.0–1.2 mm Ni-Cr-Mo alloy wire | Low cracking sensitivity, good ductility in transition zone |
| Welding Current | 180–240 A | Adequate penetration without excessive dilution |
| Travel Speed | 250–400 mm/min | Controlled heat input, uniform bead profile |
| Heat Input | 0.8–1.5 kJ/mm | Minimizes HAZ softening in base steel |
| Interpass Temperature | ≤250°C | Prevents grain growth and cracking |
| Weld Layer Thickness | 2–4 mm (total) | Sufficient for spray adhesion, minimal base dilution |
| Welding Position | Flat (1G) or horizontal (2F) | Optimal gas coverage and bead control |
4.3 Thermal Spray Parameters
| Parameter | Flame Spray | Plasma Spray |
|---|---|---|
| Feed Powder | High Cr-Mo alloy (15–20% Cr, 0.8–1.5% Mo) | High Cr-Mo alloy (18–25% Cr, 1.0–2.0% Mo) |
| Particle Size | 45–150 μm (D10–D90) | 30–100 μm (D10–D90) |
| Substrate Temperature | 150–250°C (preheated) | 100–200°C (preheated) |
| Standoff Distance | 80–120 mm | 60–100 mm |
| Layer Thickness | 1.5–3.0 mm | 0.5–1.5 mm per pass |
| Porosity | ≤3% (area fraction) | ≤2% (area fraction) |
| Bond Strength | ≥50 MPa (ASTM C633) | ≥70 MPa (ASTM C236) |
| Surface Hardness | 55–60 HRC | 58–65 HRC |
4.4 Critical Implementation Controls
- Interface cleanliness: The weld overlay surface must be cleaned of spatter, slag inclusions, and oxidation before spray application. Wire brushing or low-abrasive shot blasting is recommended.
- Thermal management: Temperature monitoring at the weld/spray interface must ensure it remains below 300°C during spray to prevent tempering of the weld overlay and loss of hardness in the transition zone.
- Coating build strategy: Multiple thin spray passes (0.3–0.5 mm each) are preferred over single thick deposits to minimize internal stresses and porosity accumulation.
- Edge treatment: The spray layer should be built with a 0.5–1.0 mm overlap onto the unwelded base surface to prevent edge chipping during service.
5. Applicable Standards and Acceptance Criteria
5.1 Governing Standards
| Standard | Applicability |
|---|---|
| ASTM A388 | Standard Specification for Carbon and Alloy Steel Plate Used for Wear Parts |
| ASTM C633 | Standard Test Method for Bond Strength of Thermal-Sprayed Coatings |
| ASTM C236 | Standard Test Method for Determining Bond Strength of Thermally Sprayed Coatings |
| ASTM B611 | Standard Specification for Thermal Spray Coatings (General Requirements) |
| ASTM G65 | Standard Test Method for Abrasive Wear by Rotating Dry Rubber Wheel |
| GB/T 11354 | Surface Heat Treatment of Steel — Hardness Verification of Case Hardened and Hardened Surfaces |
| GB/T 985 | Methods of Testing for Welds in Steel |
| GB/T 3323 | Non-Destructive Testing — Radiographic Testing of Welds |
| GB/T 11345 | Non-Destructive Testing of Welds — Ultrasonic Testing |
| ISO 1143 | Thermal Spray — Surface Preparation of Substrates |
| ISO 12707 | Thermal Spray — Surface Preparation — Roughness Requirements |
| NACE SP0388 | Recommended Practice for Thermal-Spray Coatings for Corrosion Protection of Steel |
5.2 Acceptance Criteria
- Visual inspection: No cracks, spalling, excessive porosity (>3%), or incomplete coverage on the overlay surface. Weld beads shall show uniform profile with no undercut exceeding 0.5 mm.
- Hardness verification: Surface hardness measured per ASTM E18 (Rockwell C) shall be ≥55 HRC in the spray layer, with a transition gradient of ≤5 HRC per 0.5 mm depth from spray layer to weld layer.
- Bond strength: Cross-section bond strength per ASTM C633 shall be ≥50 MPa for flame spray and ≥70 MPa for plasma spray applications.
- Porosity: Area fraction porosity per ASTM B611 shall not exceed 3% for flame spray or 2% for plasma spray.
- Weld integrity: CO₂ weld overlay layer shall be free of cracks, lack of fusion, and excessive porosity per GB/T 3323 (radiographic) or GB/T 11345 (ultrasonic) acceptance criteria — no defects exceeding ISO 5817 Level B.
- Abrasive wear test: Taber abrasion test (ASTM D4060) shall demonstrate ≥200 cycles before 1 mm material loss at 1000 g load.
- Impact test: Charpy V-notch impact energy at 25°C shall be ≥20 J for the composite overlay at the weld/spray interface.
6. Common Risks and Controls
| Risk Category | Failure Mode | Cause | Control Measure |
|---|---|---|---|
| Delamination | Complete separation of spray layer from weld overlay | Contamination at interface, insufficient preheat, thermal mismatch | Mandatory interface cleaning, temperature-controlled preheat, staged spray build-up |
| Cracking | Transverse cracks in weld overlay layer | Excessive heat input, hydrogen embrittlement, high carbon dilution | Limit heat input to ≤1.5 kJ/mm, use low-hydrogen filler, preheat to 200°C+ |
| High porosity | Pores >3% in spray coating | Moisture in feed powder, inadequate gas flow, poor spray geometry | Controlled powder storage (dew point ≤-40°C), regular equipment calibration |
| Hardness non-uniformity | Localized soft spots <50 HRC in spray layer | Uneven powder feed rate, inconsistent standoff distance | Automated feed control, standoff distance monitoring, in-process hardness spot checks |
| Edge chipping | Coating removal at part edges during service | Inadequate edge build-up, sharp geometric transitions | 0.5–1.0 mm overlap onto base, chamfered edges before overlay |
| Residual stress | Distortion or stress cracking in thin blade sections | Thermal cycling during multi-pass welding and spraying | Controlled cooling rates, post-weld stress relief at 250–300°C |
7. Application Across the Company's Three Technology Routes
7.1 TIG/MIG Weld Overlay Route (Primary Application)
This composite technology is a direct extension of the company's core TIG/MIG weld overlay capabilities. The CO₂ arc welding component leverages the same skilled welder workforce, welding procedure specifications (WPS), and quality assurance infrastructure used for standard weld overlay operations. The integration of thermal spray as a secondary process enhances the value proposition by offering:
- A single-source solution for customers requiring both metallurgical bonding and ultra-hard surface properties
- Qualification leverage — existing NB/ASME welder certifications and WPS qualifications extend to this hybrid process
- Cross-sell opportunity to customers already using the company's TIG/MIG overlay services for transition layers
For deep plough blade applications, the CO₂ GMAW process is preferred over TIG due to higher deposition rates (3–5× TIG), making it economically viable for production-scale repair and refurbishment operations.
7.2 Hydraulic Explosive Bonding Route (Complementary Application)
While hydraulic explosive bonding (HEB) is primarily used for producing clad plate with homogeneous metallurgical bonds, the principles of interface metallurgy learned from this composite overlay technology inform HEB process development. Specifically:
- Understanding of Cr-Mo alloy behavior at high strain rates informs explosive bonding parameter selection for Cr-Mo clad plate production
- Hardness gradient management techniques transfer to HEB interface characterization and quality assessment
- Customer demand for Cr-Mo wear-resistant clad plate (for plough blades, liners, and rollers) creates a natural product pipeline from bonded plate → cut-to-size → CO₂ weld overlay → spray finishing
7.3 Explosion Welding Route (Material Supply Integration)
Explosion welding produces clad plate with a metallic bond between dissimilar materials. In the context of this technology entry, explosion-welded Cr-Mo alloy clad plate serves as a premium substrate alternative to the CO₂ weld overlay approach:
- Explosion-welded 15CrMo / Q345 clad plate provides a homogeneous, crack-free Cr-Mo layer that can be directly spray-finished without the intermediate weld overlay step
- For high-volume OEM production, explosion-welded clad plate + thermal spray finishing offers a more repeatable and lower-variance process compared to field-applied CO₂ weld + spray
- The company can offer both routes to customers — field-repair (CO₂ weld + spray) and factory-manufactured (explosion-welded clad + spray) — based on production volume and quality requirements
8. Contribution to Qualification Building and Customer Value
8.1 Qualification and Certification Value
This technology entry demonstrates the company's capability in:
- Multi-process integration: Combining arc welding with thermal spray under unified quality control, a differentiator in the surface engineering market
- WPS development: Documented welding procedure specifications for CO₂ arc overlay on Q345/Q355 base materials with Ni-Cr-Mo filler metals
- Thermal spray qualification: Compliance with ASTM B611 and ISO 1143 surface preparation and coating quality standards
- NDT competency: Radiographic and ultrasonic inspection of weld overlay layers per GB/T 3323 and GB/T 11345
- Wear testing capability: In-house or third-party abrasive wear testing per ASTM G65 and ASTM D4060
8.2 Customer Value Delivery
For agricultural equipment OEMs and aftermarket service providers, this composite overlay technology delivers:
- 3–5× extension of plow blade service life compared to uncoated carbon steel, reducing replacement frequency from every 100 hours to 500–700 hours of operation
- Reduced downtime through extended blade life between replacements, particularly valuable in large-scale farming operations with tight planting/tillage windows
- Customizable solutions — hardness, thickness, and alloy composition can be tailored to specific soil conditions (sandy loam, clay, rocky terrain)
- Cost-effective field repair — CO₂ arc welding is portable and can be applied in the field, unlike explosion welding which requires factory facilities
- Environmental benefit — extending component life reduces material consumption, manufacturing emissions, and waste generation
8.3 Knowledge Management and Process Improvement
The "学习心得" (learning insights) nature of this entry indicates it represents documented process knowledge gained through practical experimentation and field application. This contributes to:
- Organizational knowledge retention: Capturing experiential process parameters and failure modes that may not be covered in standard references
- WPS optimization: Field-derived parameter ranges inform the next generation of welding procedure specifications with tighter tolerance windows
- Training material: Documented insights serve as training content for new welders and spray operators entering the composite overlay process
- Continuous improvement: Each "learning insight" entry represents an iterative improvement cycle, driving progressive enhancement of overlay quality and reliability
9. Summary and Forward-Looking Assessment
The composite CO₂ arc weld overlay and high Cr-Mo alloy thermal spray technology represents a mature, field-proven surface engineering solution for deep plough plow blade wear protection. It effectively bridges the gap between the company's core weld overlay capabilities and advanced thermal spray finishing, creating a differentiated product offering in the agricultural and earthmoving equipment aftermarket.
Future development priorities should include:
- Automation of the CO₂ weld + spray sequence for higher production throughput
- Development of proprietary Cr-Mo powder formulations with enhanced wear life beyond current 55–62 HRC range
- Integration of in-situ monitoring systems (temperature, standoff distance, powder feed rate) for real-time quality control
- Expansion of qualification scope to include additional base materials (Q460, 42CrMo, D2 tool steel) and operating environments
- Standardization of the composite overlay process into a proprietary WPS library with full traceability documentation
This technology entry, while originating as a learning document, encapsulates actionable process knowledge that directly contributes to the company's technical qualification portfolio, product delivery reliability, and end-customer value proposition in the wear-resistant surface engineering market.