CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Properties of Plasma-Cladded Nickel-Based Coatings on Shell-Breaking Hammer Heads
Literature Overview
This 2014 publication in the Journal of Mechanical Engineering, authored by Zhang Guodong, Li Li, Liu Nian, Cao Hongmei, and Mao Yan from Wuhan University, Shanghai Aircraft Manufacturing Co., Ltd., and Shanghai Jiao Tong University, investigates the microstructural characteristics and mechanical properties of nickel-based coatings applied to shell-breaking hammer heads via plasma transferred arc (PTA) cladding. The work was supported by the Hubei Provincial Natural Science Foundation (2009CDB300), the Central University Basic Scientific Research Business Fee Special Fund (2012208020201), and the Ministry of Education Doctoral Academic Newcomer Award (5052012208001). Shell-breaking hammer heads are subjected to extreme impact, abrasion, and corrosion conditions in mining and quarrying operations, making them ideal candidates for surface protection through PTA cladding of nickel-based alloys.
Core Technical Points
Nickel-Based Alloy Systems for Impact-Abrasive Service
Nickel-based overlay alloys offer a unique combination of properties that make them suitable for shell-breaking hammer applications:
| Alloy System | Hardness (HV) | Key Advantage | Typical Application |
|---|---|---|---|
| Ni-Cr-Mo-C | 400–700 | Impact resistance with wear protection | Hammer faces, chisels |
| Ni-Si-Cr | 500–900 | High hardness, moderate impact resistance | Abrasive wear zones |
| Ni-Fe-Cr | 350–600 | Good weldability, moderate hardness | General surface protection |
| Ni-Co-Cr | 400–800 | High-temperature strength, oxidation resistance | Hot section components |
| Ni-based amorphous | 800–1200 | Very high hardness, excellent erosion resistance | Severe erosion zones |
The selection of a specific nickel-based alloy system depends on the dominant wear mechanism. Shell-breaking hammer heads experience a combination of impact loading and abrasive wear, which requires a coating that provides both high hardness for wear resistance and adequate toughness to withstand impact without cracking.
Microstructural Analysis of PTA-Cladded Nickel-Based Coatings
PTA cladding of nickel-based alloys produces microstructures that are significantly different from those obtained by conventional arc welding overlay. The rapid solidification rates in PTA (typically 5 to 50 K/s) promote:
- Fine dendritic structures: The primary solidification phase is typically austenite (gamma phase) or martensite, depending on the alloy composition.
- Refined carbide distribution: Hard carbides (Cr7C3, Cr23C6, Mo2C, WC) are finely dispersed in the matrix, providing effective wear resistance without excessive embrittlement.
- Reduced grain size: The rapid cooling suppresses grain growth, resulting in a fine-grained microstructure with higher strength and hardness.
- Limited retained austenite: The rapid solidification may trap austenite that would otherwise transform during slower cooling, contributing to toughness.
Mechanical Properties and Performance
The mechanical properties of PTA-cladded nickel-based coatings on shell-breaking hammer heads were evaluated through:
| Property | Typical Value | Significance |
|---|---|---|
| Surface hardness (HV30) | 400–900 | Directly correlates with wear resistance |
| Impact toughness (J/cm²) | 30–80 | Determines resistance to impact cracking |
| Bond strength (MPa) | 250–450 | Ensures adhesion to substrate |
| Fatigue life (cycles) | 10⁵–10⁷ | Critical for cyclic impact loading |
| Wear rate (mg/N·m) | 0.1–1.0 | Quantifies abrasive wear resistance |
The study demonstrates that PTA-cladded nickel-based coatings provide a significant improvement in service life for shell-breaking hammer heads compared to uncoated or conventionally arc-welded alternatives. The combination of high hardness and adequate toughness achieved through PTA cladding is superior to that of conventional overlay processes, which often sacrifice toughness for hardness.
Process Parameters and Their Influence
The PTA cladding process parameters must be carefully optimized to achieve the desired balance of hardness, toughness, and bond strength:
| Parameter | Typical Range | Influence on Properties |
|---|---|---|
| Arc current (A) | 150–350 | Higher current increases dilution and reduces hardness |
| Travel speed (mm/min) | 200–600 | Higher speed reduces heat input and improves microstructure |
| Powder feed rate (g/min) | 50–150 | Controls layer thickness and composition |
| Powder particle size (μm) | 75–150 | Affects arc stability and deposit uniformity |
| Shielding gas (Ar) (L/min) | 8–12 | Prevents oxidation and contamination |
| Interpass temperature (°C) | <200 | Controls residual stress and microstructure |
A critical finding from this work is that the dilution rate must be carefully controlled to maintain the intended composition of the nickel-based overlay. Excessive dilution with the carbon steel substrate reduces the nickel content in the cladding, leading to lower hardness and reduced corrosion resistance. The optimal dilution rate for nickel-based PTA cladding is typically 5 to 10 percent.
Engineering Practice Integration
Application Context
Shell-breaking hammer heads are used in mining and quarrying operations to break rock and ore into smaller pieces for processing. The operating conditions are extremely demanding:
- Impact loading: Repeated impact with rock at high velocities (typically 10 to 20 m/s).
- Abrasive wear: Contact with hard, abrasive rock surfaces.
- Corrosive environment: Exposure to water, chemicals, and rock dust.
- Thermal cycling: Temperature variations from ambient to several hundred degrees Celsius.
PTA cladding of nickel-based alloys addresses all of these challenges simultaneously, providing a comprehensive surface protection solution.
Quality Assurance and Inspection
The quality of PTA cladding on shell-breaking hammer heads must be verified through comprehensive inspection:
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual inspection | Surface defects, porosity | No visible defects |
| Magnetic particle testing (MT) | Surface and near-surface cracks | No linear indications |
| Ultrasonic testing (UT) | Internal defects, bond quality | No indications exceeding 25% DAC |
| Hardness testing | Verify cladding hardness | Within specified range (±10%) |
| Bond strength testing | Verify adhesion | Minimum 250 MPa |
| Impact testing | Verify toughness | Meets specified impact energy |
| Dimensional inspection | Verify geometry | Within tolerance per drawing |
Common Defects and Countermeasures
| Defect Type | Root Cause | Countermeasure |
|---|---|---|
| Cracking in cladding | High carbon content, excessive cooling rate | Preheat substrate, control interpass temperature |
| Porosity | Inadequate shielding, contaminated powder | Increase gas flow, use dry powder |
| Excessive dilution | Low travel speed, high current | Optimize current-speed ratio |
| Poor bond strength | Inadequate fusion, substrate contamination | Clean substrate, ensure proper fusion |
| Delamination | Residual stress, poor design | Control interpass temperature, apply PWHT |
Study Insights and Reflections
This literature makes a significant contribution to the understanding of PTA cladding technology for mining equipment protection, a sector where the economic impact of premature component failure is substantial. The collaboration between Wuhan University, Shanghai Aircraft Manufacturing, and Shanghai Jiao Tong University demonstrates the multidisciplinary approach required for effective technology development, combining academic research with industrial expertise.
From a materials science perspective, the study highlights the importance of understanding the microstructure-property relationships in nickel-based overlay alloys. The rapid solidification inherent to PTA cladding produces microstructures that are fundamentally different from those obtained by conventional welding processes, and this difference must be understood to fully exploit the potential of the technology. Engineers working with nickel-based cladding must be aware of the sensitivity of the microstructure to process parameters and must implement rigorous process control to ensure consistent performance.
The work also raises important considerations regarding the qualification and certification of PTA cladding processes for mining equipment. While the mining industry has less stringent standards requirements than the pressure vessel industry, the consequences of cladding failure can be severe, including equipment downtime, safety incidents, and significant economic losses. Engineers should develop process qualification procedures that are at least as rigorous as those required by NB/T 47014 for pressure equipment, even when the application is not directly regulated.
In conclusion, this literature provides a comprehensive technical foundation for the application of PTA cladding of nickel-based alloys on shell-breaking hammer heads. The findings demonstrate that PTA cladding is a highly effective technology for protecting mining equipment from impact, abrasive wear, and corrosion, provided that proper process parameters, quality control measures, and inspection protocols are implemented. Engineers working in the mining and quarrying sectors should consider PTA cladding as a preferred technology for extending the service life of impact-loaded components, and should invest in the training and qualification of personnel to ensure reliable and repeatable results.