CLADDING TECHNOLOGY SHANXI CO., LTDCO2 Welding and Spray High-Chromium Molybdenum Alloy Powder Composite Cladding Layer Wear Resistance for Deep Plough Shovels A Study Note
Introduction and Technical Background
Deep plough shovels operate under extreme abrasive and adhesive wear conditions in mining, quarrying, and heavy earthmoving applications. The shovel teeth and cutting edges are subjected to continuous impact, abrasion by hard rock and soil, and adhesive wear from soil materials. The topic reviewed in this study examines a composite cladding approach that combines CO₂ gas metal arc welding (GMAW-C) with spray application of high-chromium molybdenum alloy powder to create a wear-resistant composite overlay layer on deep plough shovel components.
This hybrid approach leverages the high deposition rate and cost-effectiveness of CO₂ welding for the base overlay layer, combined with the superior wear resistance properties of high-chromium molybdenum alloy powder applied by thermal spray processes. The resulting composite structure is designed to provide a gradient of hardness and toughness from the base metal through the overlay, optimizing the balance between wear resistance and impact toughness.
Material System and Metallurgical Design
Base Overlay Layer (CO₂ Welding)
The base overlay layer is deposited using CO₂ gas metal arc welding with a flux-cored wire or solid wire containing medium-carbon manganese steel composition. This layer serves two purposes: providing a transition from the base metal to the wear-resistant overlay, and providing a thick, tough base that can absorb impact loads without cracking.
| Parameter | Specification | Rationale |
|---|---|---|
| Wire composition | Mn-Si steel, 0.5–0.8% C, 1.0–1.5% Mn | Balanced toughness and weldability |
| Shielding gas | 100% CO₂ | Cost-effective, good penetration |
| Deposition rate | 4–6 kg/h | High productivity for thick overlay |
| Overlay thickness | 8–15 mm | Adequate for impact absorption |
| Hardness (as-welded) | 200–280 HB | Sufficient toughness for impact loading |
Wear-Resistant Top Layer (Thermal Spray)
The top wear-resistant layer is applied by thermal spray using high-chromium molybdenum alloy powder. The powder composition is typically based on a high-chromium martensitic steel containing 12–18% Cr, 0.3–0.8% Mo, and 0.5–1.0% C, which forms a hard carbide matrix in the as-sprayed or heat-treated condition.
| Parameter | Specification | Rationale |
|---|---|---|
| Powder composition | 12–18% Cr, 0.3–0.8% Mo, 0.5–1.0% C | Hard carbide formation |
| Powder size | 15–45 μm | Optimal melting and bonding |
| Spray process | Flame spray or plasma spray | Adequate bond strength |
| Top layer thickness | 2–4 mm | Sufficient for wear life |
| Hardness (as-sprayed) | 55–65 HRC | High wear resistance |
| Bond strength | > 70 MPa | Adequate for impact loading |
Composite Structure Design
The composite structure creates a functionally graded material with the following properties from base to surface:
- Base metal — Structural steel with high toughness (200–300 HB).
- CO₂ weld overlay — Medium-hardness transition layer (200–280 HB) providing toughness.
- Thermal spray top layer — High-hardness wear-resistant layer (55–65 HRC) providing abrasion resistance.
This gradient design addresses the fundamental challenge of wear-resistant overlay design: achieving high surface hardness without sacrificing the bulk toughness required to resist impact and cracking.
Process Integration and Interface Considerations
Interface Bonding Between Weld Overlay and Spray Layer
The interface between the CO₂ weld overlay and the thermal spray top layer is a critical region for the long-term performance of the composite cladding. The bond strength at this interface must be sufficient to resist the shear and peeling stresses generated during service.
The factors influencing interface bond strength include:
- Surface roughness of the weld overlay — A roughened surface provides mechanical interlocking with the spray layer.
- Surface cleanliness — Oxide, slag, and other contaminants must be removed before spraying.
- Spray parameters — Particle velocity, temperature, and stand-off distance affect the kinetic energy of the particles and the quality of the bond.
- Interpass temperature — Excessive temperature can soften the weld overlay surface and reduce bond strength.
The recommended surface preparation for the spray application includes:
- Grinding the weld overlay surface to a Ra of 25–50 μm.
- Shot blasting to remove residual slag and surface oxides.
- Cleaning with a solvent wipe to remove any remaining contaminants.
- Applying the spray layer within a time window of less than 4 hours after surface preparation.
Thermal Compatibility and Residual Stress
The thermal expansion mismatch between the CO₂ weld overlay and the thermal spray top layer can generate residual stresses at the interface during cooling. These stresses can be tensile or compressive depending on the relative thermal expansion coefficients of the two materials.
| Material | Thermal Expansion Coefficient (×10⁻⁶/°C) | Residual Stress Contribution |
|---|---|---|
| Base steel | 11–13 | Reference |
| CO₂ weld overlay | 12–14 | Slight tensile stress at interface |
| High-Cr-Mo spray layer | 10–12 | Slight compressive stress at interface |
The net residual stress at the interface is typically a combination of thermal stresses from the welding process and the spray process. In most cases, the compressive stress from the spray layer partially offsets the tensile stress from the weld overlay, resulting in a net compressive or near-neutral stress state at the interface. This is favorable for the long-term durability of the composite cladding.
Wear Performance and Defect Analysis
Wear Mechanisms and Performance
The wear performance of the composite cladding is governed by the following mechanisms:
- Abrasive wear — Dominant in the early stages of wear, controlled by the hardness of the top layer.
- Adhesive wear — Becomes significant as the top layer is worn through, controlled by the friction coefficient and material compatibility.
- Fatigue wear — Occurs at the interface between the top layer and the weld overlay due to cyclic loading.
- Delamination — The ultimate failure mode, occurring when the interface bond is exceeded.
The expected service life of the composite cladding is typically 3–5 times that of a single-material overlay, based on field trials in deep plough shovel applications. The primary failure mode is gradual wear-through of the top layer followed by wear of the weld overlay base layer, rather than catastrophic delamination.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Spray layer delamination | Poor surface preparation, low particle velocity | Improve surface roughness, optimize spray parameters |
| Weld overlay cracking | Excessive heat input, high restraint | Reduce heat input, use skip welding sequence |
| Interface porosity | Surface contamination, poor spray parameters | Clean surface, increase particle velocity |
| Excessive spray layer spatter | Incorrect powder feed rate, wrong gas flow | Optimize powder feed and gas flow parameters |
| Weld overlay undercut | Excessive travel speed, improper torch angle | Adjust travel speed and torch angle |
Engineering Practice and Field Results
Field trials of the composite cladding on deep plough shovels in a copper mining operation demonstrated significant improvements in service life compared to conventional single-material overlays. The composite cladding achieved an average service life of 1,200 operating hours before replacement was required, compared to 300–400 hours for a conventional high-chromium overlay. The primary wear pattern was gradual erosion of the spray top layer followed by wear of the CO₂ weld base layer, with no instances of catastrophic delamination.
The economic analysis showed that the composite cladding approach reduced the cost per operating hour by approximately 60% compared to conventional overlays, primarily due to the extended service life and reduced downtime for repairs. The initial cost of the composite cladding was approximately 30% higher than a conventional overlay, but this was more than offset by the extended service life.
Study Insights and Conclusions
The study of CO₂ welding combined with high-chromium molybdenum alloy powder spray for deep plough shovel cladding demonstrates the power of functionally graded material design in addressing complex wear problems. The composite approach successfully combines the high deposition rate and toughness of CO₂ welding with the superior wear resistance of high-chromium molybdenum alloy, creating a solution that is both technically effective and economically viable.
The most important insight from this study is that the interface between the two layers is the critical design parameter. The long-term performance of the composite cladding depends not on the individual properties of the weld overlay or the spray layer, but on the quality of the interface between them. This insight has direct implications for process development, quality assurance, and failure analysis in composite cladding applications.