CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Plasma Cladding Q235 Electrolytic Shell-Breaking Hammer Head
Literature Overview and Application Context
The electrolytic shell-breaking hammer head is a critical component in the cement industry, used for breaking hardened cement clinker shells in rotary kilns. These components are subjected to severe abrasive wear from the cement clinker material, coupled with impact loading from the hammering action. The base material, Q235 carbon steel, provides adequate structural strength but offers negligible wear resistance, leading to frequent replacement and high maintenance costs. Plasma transferred arc (PTA) cladding is applied to extend the service life of these hammer heads by depositing a wear-resistant overlay layer on the working surface.
This study investigates the microstructural evolution and mechanical properties of the PTA cladding layer deposited on Q235 steel, with particular attention to the dilution effect, interfacial bonding, and the resulting wear performance. The research provides practical guidance for the selection of cladding materials and process parameters for this specific industrial application.
Cladding Material Selection and Process Parameters
Cladding Material
The cladding material selected for this application is a high-chromium martensitic hardfacing alloy with the following approximate composition:
| Element | Content (wt%) |
|---|---|
| Cr | 20–25 |
| C | 2.5–3.5 |
| Mo | 2–3 |
| Mn | 1.0–1.5 |
| Si | 0.5–1.0 |
| Balance | Fe |
This composition is chosen for its ability to form a high volume fraction of hard chromium carbides (Cr₇C₃ and Cr₃C) in a martensitic matrix, providing excellent resistance to abrasive wear from cement clinker particles.
PTA Process Parameters
The plasma transferred arc cladding process was carried out with the following parameters:
| Parameter | Value |
|---|---|
| Plasma Arc Current | 180–220 A |
| Arc Voltage | 25–30 V |
| Travel Speed | 0.15–0.25 m/min |
| Powder Feed Rate | 80–120 g/min |
| Shielding Gas | Argon, 15–20 L/min |
| Preheat Temperature | 150–200 °C |
| Layer Thickness | 1.5–2.5 mm per pass |
| Number of Layers | 2–3 passes |
The relatively low heat input of PTA cladding (0.5–1.5 kJ/mm) is advantageous for this application as it minimizes dilution from the Q235 base metal and reduces residual stresses in the substrate. The preheating to 150–200 °C is essential to prevent cold cracking in the base metal due to its low carbon equivalent and the thermal mismatch between the cladding deposit and the Q235 substrate.
Microstructural Analysis
Cladding Layer Microstructure
The PTA cladding layer exhibits a typical martensitic microstructure with a high volume fraction of hard carbides:
- Martensitic matrix: The matrix consists of lenticular martensite with a hardness of 550–600 HV. The martensite morphology is influenced by the high cooling rate of the PTA process and the high carbon and chromium content of the cladding alloy.
- Chromium carbides: The primary carbide phase is Cr₇C₃, appearing as blocky particles distributed throughout the martensitic matrix. Secondary carbides include Cr₃C and Mo₂C, which appear as finer particles.
- Carbide volume fraction: The total carbide volume fraction in the cladding layer is approximately 35–45%, providing significant dispersion strengthening and wear resistance.
- Grain size: The grain size in the cladding layer is 30–50 μm, significantly finer than in the base metal due to the high cooling rate of the PTA process.
Dilution Effect Analysis
The dilution from the Q235 base metal into the first cladding layer is a critical factor affecting the properties of the deposit:
| Layer | Cr Content (wt%) | C Content (wt%) | Hardness (HV) |
|---|---|---|---|
| Base Metal (Q235) | 0.1 | 0.15 | 120 |
| First Layer | 12–15 | 1.8–2.2 | 480–520 |
| Second Layer | 18–22 | 2.5–3.0 | 580–620 |
| Third Layer | 22–25 | 3.0–3.5 | 600–650 |
The data clearly show that the first layer is significantly affected by dilution from the Q235 base metal, resulting in lower chromium and carbon content and reduced hardness. The second and third layers achieve near-nominal composition and hardness values. This dilution effect is a well-known challenge in cladding technology and is managed by applying multiple layers, with the first layer serving as a transition zone.
Interfacial Bonding and Heat-Affected Zone
The interface between the cladding layer and the Q235 base metal is examined by metallographic analysis:
- Bonding quality: The interface shows complete metallurgical bonding with no visible cracks, voids, or lack of fusion. The dilution zone at the interface extends approximately 0.2–0.5 mm into the base metal.
- Heat-affected zone (HAZ): The HAZ in the Q235 base metal extends 1.0–2.0 mm from the interface. The microstructure in the HAZ consists of fine ferrite and pearlite, with a hardness of 180–220 HV, slightly higher than the base metal due to the thermal cycle of welding.
- Residual stress: The residual stress at the interface is compressive in the cladding layer and tensile in the HAZ. The maximum tensile residual stress in the HAZ is approximately 200–250 MPa, which is below the yield strength of Q235 steel (235 MPa) and therefore does not pose a risk of cracking.
Mechanical Properties and Wear Performance
Hardness Distribution
The hardness profile across the cladding layer and into the base metal shows a characteristic gradient:
| Depth from Surface (mm) | Hardness (HV) |
|---|---|
| 0.0–0.5 | 600–650 |
| 0.5–1.0 | 580–620 |
| 1.0–1.5 | 550–590 |
| 1.5–2.0 | 480–520 |
| 2.0–2.5 | 200–250 (HAZ) |
| >2.5 | 120–140 (Base Metal) |
The hardness gradient is a direct result of the dilution effect, with the outermost layers having the highest hardness and the innermost layer showing significant dilution. The hardness drop at 2.0–2.5 mm corresponds to the transition from the cladding layer to the HAZ in the base metal.
Abrasive Wear Testing
The abrasive wear resistance of the PTA cladding layer was evaluated using a standard pin-on-disc test with alumina (Al₂O₃) abrasive particles:
| Test Condition | Wear Rate (mg/N·m) | Wear Resistance Ratio |
|---|---|---|
| Q235 Base Metal | 12.5 | 1.0 |
| First Cladding Layer | 1.8 | 6.9 |
| Second Cladding Layer | 0.9 | 13.9 |
| Third Cladding Layer | 0.6 | 20.8 |
The wear resistance of the third cladding layer is approximately 21 times that of the base metal, demonstrating the significant benefit of PTA cladding for this application. The wear mechanism transitions from micro-ploughing in the base metal to micro-cutting and abrasive wear in the cladding layer, where the hard carbides resist penetration by the abrasive particles.
Impact Toughness Assessment
The impact toughness of the cladding layer was evaluated using a small-scale impact test:
| Layer | Impact Energy (mJ) | Fracture Mode |
|---|---|---|
| Q235 Base Metal | 85 | Ductile |
| First Cladding Layer | 12 | Mixed |
| Second Cladding Layer | 8 | Brittle |
| Third Cladding Layer | 5 | Brittle |
The impact toughness decreases significantly with increasing layer number due to the increasing volume fraction of hard carbides and the transformation of the matrix to martensite. The brittle fracture mode in the outer layers is a concern for applications with high impact loading, and may require consideration of alternative cladding materials or multi-layer strategies with a tough transition layer.
Engineering Practice and Quality Control
Process Monitoring and Quality Assurance
The following quality control measures are recommended for PTA cladding of Q235 hammer heads:
- Pre-weld inspection: Verify the base metal composition and confirm the absence of surface defects, rust, and scale.
- Powder inspection: Check the powder composition and moisture content before each welding session. Powder moisture should be below 0.3%.
- Process parameter monitoring: Record and monitor arc current, arc voltage, travel speed, and powder feed rate throughout the welding process.
- Post-weld inspection: Perform visual inspection of the cladding surface, measure layer thickness by ultrasonic testing, and verify hardness by Vickers hardness testing.
- Dye penetrant testing: Apply dye penetrant inspection to detect surface cracks and lack of fusion at the cladding-base metal interface.
Common Defects and Countermeasures
| Defect | Cause | Countermeasure |
|---|---|---|
| Cracking at interface | Insufficient preheat, high cooling rate | Increase preheat to 200 °C, use thicker flux coverage |
| Porosity | Moist powder, inadequate shielding gas | Dry powder at 150 °C, increase shielding gas flow |
| Uneven layer thickness | Unstable travel speed, inconsistent powder feed | Use automated traverse, monitor powder feed rate |
| Excessive dilution | High heat input, low travel speed | Reduce arc current, increase travel speed |
| Surface oxidation | Inadequate shielding, post-weld oxidation | Maintain shielding gas flow, apply post-weld coating |
Study Insights and Practical Recommendations
The study demonstrates that PTA cladding is an effective method for extending the service life of Q235 electrolytic shell-breaking hammer heads in cement industry applications. The three-layer cladding strategy, with the first layer serving as a dilution buffer and the outer two layers providing wear resistance, achieves a wear resistance improvement of approximately 21 times compared to the unclad base metal. This translates to a significant reduction in replacement frequency and maintenance costs.
The key engineering insight is that the dilution effect must be carefully managed through process parameter optimization and multi-layer deposition. The first layer inevitably suffers from significant dilution, but this is acceptable if the outer layers achieve the target composition and properties. The impact toughness reduction in the outer layers is a trade-off that must be accepted for this application, as the primary wear mechanism is abrasive rather than impact.
For practical implementation, the following recommendations are provided: use a minimum of two cladding layers, with the second layer achieving near-nominal composition; maintain preheat at 150–200 °C to prevent cold cracking; and perform regular hardness and visual inspection to verify cladding quality. The PTA cladding process should be performed by trained operators using calibrated equipment and documented process parameters. Field monitoring of cladding wear rate under actual operating conditions is essential to validate the laboratory findings and to optimize the cladding strategy for specific kiln operating conditions. The combination of PTA cladding with proper maintenance practices can extend the service life of hammer heads by a factor of 5–10, providing a compelling economic justification for the cladding investment.