CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Cladding Nickel-Based Coating on Shell-Breaking Hammer Head: Microstructure and Properties
Literature Overview
The research on "Plasma Transferred Arc Cladding of Nickel-Based Coatings on Shell-Breaking Hammer Heads: Microstructure and Properties" addresses a significant industrial problem in the foundry and metal casting industry. Shell-breaking hammer heads are subjected to extreme impact and abrasion during the separation of sand molds from castings, leading to rapid wear and frequent replacement. The study investigates the application of Plasma Transferred Arc (PTA) cladding technology to deposit nickel-based wear-resistant coatings, evaluating the resulting microstructure, mechanical properties, and service performance.
PTA Cladding Process Parameters
Plasma transferred arc cladding offers unique advantages for this application due to its precise heat input control, low dilution rates, and ability to deposit high-quality coatings on complex geometries. The study investigates a range of process parameters to optimize coating quality:
| Parameter | Optimized Range | Effect on Coating Quality |
|---|---|---|
| Plasma current | 150–250 A | Controls deposition rate and penetration |
| Arc voltage | 20–28 V | Influences arc stability and powder melting |
| Powder feed rate | 300–500 g/min | Determines coating thickness per pass |
| Travel speed | 200–400 mm/min | Affects dilution and microstructure refinement |
| Powder nozzle distance | 5–10 mm | Influences powder utilization efficiency |
| Shielding gas flow | 15–25 L/min (Ar) | Prevents oxidation of molten pool |
| Interpass temperature | ≤ 200 °C | Controls HAZ thermal cycling |
The nickel-based powder used in the study is a Ni-Cr-Mo-Si-B-C composite powder with a nominal composition providing hardness in the range of 45–55 HRC in the as-deposited condition. The powder particle size distribution (typically 45–150 μm) is critical for achieving complete melting and uniform deposition without excessive spatter or incomplete melting defects.
Microstructural Characterization
The PTA-deposited coating exhibits a distinctive microstructure that differs significantly from both the cast substrate and the powder feedstock. Key microstructural features include:
- Columnar dendritic growth: The rapid solidification rate (10³–10⁴ K/s) promotes columnar grain growth perpendicular to the substrate surface, creating a fine-grained microstructure with grain sizes of 5–15 μm.
- Carbide dispersion: Hard carbide phases (Ni₃B, Ni₃Si, and complex carbides) are uniformly dispersed within the matrix, providing the primary wear resistance mechanism.
- Minimal dilution: The PTA process achieves dilution rates of only 5–15%, preserving the nominal composition of the coating alloy. This is significantly lower than conventional arc welding processes (20–40% dilution).
- Absence of casting defects: Unlike cast overlays, the PTA coating is free from shrinkage porosity, segregation, and hot tearing, resulting in a homogeneous and defect-free microstructure.
The heat-affected zone (HAZ) in the base material is extremely narrow (typically <0.5 mm) due to the concentrated heat input of the plasma arc. This minimal HAZ width is crucial for maintaining the mechanical properties of the underlying hammer head material, which is typically a high-strength alloy steel designed for impact resistance.
Mechanical Properties and Performance
The study presents comprehensive property data for the PTA-cladded hammer head:
| Property | Base Material | PTA Coating | Improvement Factor |
|---|---|---|---|
| Surface hardness | 280 HV | 520 HV | 1.85× |
| Wear resistance | 1.0 (reference) | 3.2–4.5 | 3.2–4.5× |
| Impact toughness | 45 J | 38 J (coating) | 0.84× |
| Bond strength | N/A | 280–350 MPa | N/A |
| Service life | 1× | 4.0–5.5× | 4.0–5.5× |
The wear testing employs both dry sliding (pin-on-disk) and field service evaluation. The laboratory results correlate well with field performance, confirming that the PTA coating extends hammer head service life by 4–5.5 times compared to the uncoated condition. This translates to significant cost savings through reduced replacement frequency and improved production continuity.
Defect Analysis and Quality Control
The study identifies several potential defects in PTA cladding and their countermeasures:
| Defect | Cause | Detection Method | Countermeasure |
|---|---|---|---|
| Porosity | Incomplete powder melting, gas entrapment | RT or UT | Optimize feed rate and arc parameters |
| Cracking | Excessive residual stress, high dilution | MT or PT | Control interpass temperature, reduce heat input |
| Unmelted powder | Insufficient arc energy | Visual, MT | Increase current, reduce travel speed |
| Excessive dilution | High heat input, low travel speed | Spectrographic analysis | Reduce current, increase travel speed |
| Poor bond | Contamination, insufficient penetration | Bond strength test | Clean substrate, optimize parameters |
A critical quality control step is the spectrographic analysis of the coating composition to verify dilution levels. The study recommends that dilution should not exceed 15% for optimal coating performance, requiring careful process parameter selection and monitoring during production.
Engineering Practice Insights
The application of PTA cladding to shell-breaking hammer heads represents a paradigm shift from the traditional approach of periodic replacement to a more sustainable refurbishment strategy. The study demonstrates that even heavily worn hammer heads can be refurbished through grinding and re-cladding, extending their useful life multiple times. This approach aligns with circular economy principles and significantly reduces material waste.
The process also offers advantages in terms of geometry flexibility. Unlike cast overlays or explosive cladding, PTA can be applied to complex hammer head geometries including the working faces, edges, and corners where wear is most severe. The ability to build up worn surfaces and restore original dimensions makes PTA cladding an ideal refurbishment technology for high-value components.
The study concludes that PTA nickel-based cladding provides an excellent solution for extending the service life of shell-breaking hammer heads, offering a favorable combination of wear resistance, impact toughness, and process flexibility. The key to successful implementation lies in careful process parameter optimization, rigorous quality control of the coating microstructure and composition, and proper integration into the maintenance and refurbishment workflow of the foundry operation.