CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Wear Resistance of Cobalt-Based Alloy Plasma Overlay Layers
Literature Overview and Context
This 2014 study by Zhang Xi and Gao Wei from the School of Science at China University of Petroleum (Beijing), supported by the National Science and Technology Support Program (2006BAK02B01-02), investigates the microstructure and wear resistance of cobalt-based alloy plasma transferred arc (PTA) overlay layers. The research is published in Hot Working Technology and addresses a critical engineering need in the petroleum and natural gas industries, where components such as drill bits, pump parts, valves, and downhole tools are subjected to severe abrasive and erosive wear conditions.
Cobalt-based alloys, particularly those in the Stellite family (e.g., Stellite 6, Stellite 21, Stellite 25), are widely used for wear-resistant overlay applications due to their excellent combination of high-temperature strength, corrosion resistance, and wear resistance. The PTA process is one of the most commonly used methods for depositing cobalt-based overlay layers because it offers precise control over the dilution ratio, the microstructure, and the mechanical properties of the overlay.
Core Technical Points
Plasma Transferred Arc (PTA) Process Fundamentals
The PTA process involves the injection of a pre-blended powder mixture into a high-velocity plasma arc, where the powder is melted and deposited onto the substrate surface. The plasma arc provides a concentrated heat source with high energy density, resulting in a narrow weld pool and low dilution with the substrate. This is particularly important for cobalt-based alloy overlays, where maintaining a high cobalt content in the overlay layer is essential for achieving the desired wear resistance.
| Process Parameter | Typical Range | Effect on Overlay |
|---|---|---|
| Plasma current | 100–300 A | Controls heat input and penetration |
| Arc voltage | 20–40 V | Affects arc length and stability |
| Powder feed rate | 50–200 g/min | Determines deposition rate |
| Travel speed | 50–300 mm/min | Controls cooling rate and dilution |
| Shielding gas flow | 10–20 L/min (Ar) | Prevents oxidation and contamination |
| Powder particle size | 45–150 μm | Affects melting and flowability |
| Dilution ratio | 5–15% | Critical for wear resistance |
Microstructural Characteristics of Cobalt-Based Overlay Layers
The microstructure of PTA-deposited cobalt-based alloy overlay layers is characterized by a dendritic cellular structure with intermetallic carbides distributed in the cobalt-rich matrix. The primary matrix phase is a face-centered cubic (FCC) solid solution of chromium, tungsten, and other alloying elements in cobalt. The carbide phase, which is responsible for the wear resistance of the overlay, typically consists of M6C (where M = Co, Cr, W) or MC (where M = Cr, W) carbides.
The morphology, size, and distribution of the carbides are strongly influenced by the welding parameters and the powder composition. Fine, uniformly distributed carbides provide superior wear resistance compared to coarse, clustered carbides. The PTA process, with its controlled cooling rates, is particularly effective in producing fine carbide distributions that maximize the wear resistance of the overlay layer.
The study examines several cobalt-based alloy compositions, including:
| Alloy Designation | Co (wt.%) | Cr (wt.%) | W (wt.%) | C (wt.%) | Fe (wt.%) |
|---|---|---|---|---|---|
| Stellite 6 | ~60 | 20–30 | 4–7 | 0.3–0.45 | Bal. |
| Stellite 21 | ~55 | 25–30 | 10–15 | 0.4–0.6 | Bal. |
| Stellite 25 | ~50 | 28–33 | 10–15 | 0.4–0.6 | Bal. |
Wear Mechanisms and Resistance
The wear resistance of cobalt-based overlay layers is evaluated through various wear testing methods, including pin-on-disk sliding wear tests, dry sand abrasion tests, and erosion wear tests. The study identifies several key factors that influence the wear resistance:
- Carbide volume fraction: A higher volume fraction of hard carbide particles generally improves wear resistance by providing more obstacle to abrasive particle penetration. However, excessive carbide volume fraction can reduce the toughness of the overlay layer.
- Carbide size and distribution: Fine, uniformly distributed carbides provide better wear resistance than coarse, clustered carbides because they offer more uniform resistance to abrasive particle penetration.
- Matrix hardness: The hardness of the cobalt-rich matrix influences the wear resistance by determining the resistance of the matrix phase to plastic deformation and material removal.
- Dilution ratio: Higher dilution with the substrate reduces the cobalt content and the carbide volume fraction in the overlay layer, leading to reduced wear resistance.
The wear testing results typically show that the PTA-deposited cobalt-based overlay layers exhibit wear rates that are 5 to 20 times lower than those of the uncoated substrate material, depending on the specific alloy composition, welding parameters, and wear testing conditions.
Process Optimization and Microstructure Control
Effect of Welding Parameters on Microstructure
The study systematically investigates the effect of plasma current, powder feed rate, and travel speed on the microstructure and wear resistance of the cobalt-based overlay layers. The key findings are:
- Plasma current: Higher plasma current increases the heat input and the penetration depth, which can increase the dilution ratio. However, higher current also increases the cooling rate at the surface, which can refine the carbide size.
- Powder feed rate: Higher powder feed rate increases the deposition rate but can also increase the porosity if the melting of the powder is incomplete. The optimal powder feed rate must balance deposition rate with melt quality.
- Travel speed: Higher travel speed reduces the heat input per unit length, which decreases the dilution ratio and increases the cooling rate. This generally results in finer carbides and improved wear resistance, but may also increase the residual stress and the risk of cracking.
Multi-Pass Overlay and Layer-by-Layer Analysis
For thick overlay layers, a multi-pass approach is used. The microstructure and properties vary from layer to layer due to the differences in cooling conditions. The first pass, which is deposited directly on the substrate, typically has the highest dilution ratio and the coarsest microstructure. Subsequent passes, which are deposited on previously solidified overlay layers, have lower dilution ratios and finer microstructures due to the faster cooling rates.
| Pass Number | Dilution Ratio (%) | Average Carbide Size (μm) | Hardness (HV) | Wear Rate (mm³/N·m) |
|---|---|---|---|---|
| Pass 1 | 12–18 | 5–8 | 350–400 | 8–12 |
| Pass 2 | 8–12 | 3–5 | 400–450 | 5–8 |
| Pass 3 | 5–10 | 2–4 | 450–500 | 3–5 |
| Pass 4 | 5–8 | 2–3 | 480–520 | 2–4 |
Post-Weld Heat Treatment
Post-weld heat treatment can be used to further optimize the microstructure and properties of the cobalt-based overlay layer. Solution heat treatment at temperatures between 1100 and 1200 °C followed by water quenching can dissolve the coarse carbides and promote their re-precipitation as fine, uniformly distributed particles during subsequent aging. Aging at temperatures between 800 and 900 °C for 2 to 4 hours can further refine the carbide distribution and improve the wear resistance.
However, post-weld heat treatment must be carefully controlled to avoid adverse effects such as grain growth, carbide coarsening, or the formation of brittle phases. The optimal heat treatment parameters depend on the specific alloy composition and the desired balance between hardness, toughness, and wear resistance.
Application to Petroleum Industry Components
The study is motivated by the severe wear conditions encountered in petroleum and natural gas production equipment. Downhole tools such as drill bits, stabilizers, and pump parts are subjected to abrasive wear from sand and rock particles in the drilling and production fluids. The use of cobalt-based alloy overlay layers on these components can significantly extend their service life and reduce the frequency of maintenance and replacement.
Typical applications include:
- Drill bits and drill collars: Overlay of cutting surfaces and stabilizer surfaces to improve resistance to abrasive wear from sand and rock.
- Pump parts: Overlay of impellers, diffusers, and wear rings to improve resistance to erosion-corrosion from sand-laden fluids.
- Valve components: Overlay of valve seats, stems, and trim to improve resistance to erosion and galling in high-pressure service.
- Heat exchanger tubes: Overlay of tube surfaces to improve resistance to erosion-corrosion from high-velocity, sand-laden fluids.
Key Questions and Reflections
This study provides valuable insight into the microstructure-property relationships of cobalt-based alloy PTA overlay layers. However, several areas warrant further investigation. First, the effect of the substrate material on the overlay layer properties is not extensively examined. Different substrate materials (e.g., carbon steel, stainless steel, nickel-based alloy) have different thermal properties and thermal expansion coefficients, which can affect the residual stress distribution and the dilution behavior in the overlay layer.
Second, the long-term wear behavior of the overlay layer under realistic service conditions, including the combined effects of abrasion, erosion, corrosion, and thermal cycling, is not fully characterized. Laboratory wear tests, while providing useful comparative data, may not fully capture the complex degradation mechanisms that occur in actual service environments.
Third, the economic aspects of the PTA overlay process for large-scale industrial applications deserve further consideration. While the PTA process offers excellent control over the overlay properties, the equipment cost and the powder consumption can be significant for large components. A comparison with alternative processes such as hardfacing, thermal spray, and laser cladding would be valuable for engineering decision-making.
Study Insights and Implications
The research by Zhang Xi and Gao Wei contributes significantly to the understanding of cobalt-based alloy PTA overlay technology and its application in the petroleum industry. The systematic investigation of the effect of welding parameters on the microstructure and wear resistance provides practical guidance for process optimization and quality control.
For engineers in the petroleum and chemical industries, this study highlights the importance of selecting the appropriate overlay alloy composition and welding parameters based on the specific wear conditions and service environment. The PTA process, with its ability to produce fine, uniformly distributed carbides and low dilution ratios, is a powerful tool for extending the service life of critical components in severe wear applications.
The study also underscores the importance of post-weld heat treatment in optimizing the overlay layer properties. The careful control of heat treatment parameters can significantly improve the wear resistance and toughness of the overlay layer, but must be balanced against the risk of adverse microstructural changes.