CLADDING TECHNOLOGY SHANXI CO., LTDMachining Performance of Plasma Clad Ni-based Alloy Overlay Layers
Research Background and Motivation
Nickel-based alloy cladding layers are widely applied on pressure vessel internal surfaces, heat exchanger tubes, and pump impellers to provide corrosion and wear resistance. However, after PTA cladding, the overlay surface is often rough, with undulations, spatter, and excess material that require post-machining to achieve the specified dimensional accuracy and surface finish. This study systematically investigates the machinability of plasma-clad Ni-based alloy layers, focusing on cutting force, tool wear, surface integrity, and the formation of built-up edge (BUE) during milling and turning operations. The research is directly relevant to engineers who must plan post-cladding machining operations for clad-plate pressure vessels and bimetallic components.
Cutting Force and Tool Wear Analysis
The study employs orthogonal cutting and face milling experiments using carbide (WC-Co) and ceramic (Al2O3-TiC) cutting tools at cutting speeds ranging from 50 to 200 m/min. The plasma-clad Ni-based alloy exhibits significantly higher cutting forces compared to the base carbon steel, with the specific cutting force reaching 2.5–3.2 GPa, approximately 1.8 to 2.5 times that of the unclad substrate. This elevated cutting resistance is attributed to the high solid solution strengthening from dissolved alloying elements (Mo, Cr, Nb), the presence of hard carbide precipitates (NbC, TiC), and the work hardening rate of the nickel-based matrix.
Tool wear follows the typical flank wear (VB) progression, but at accelerated rates. At a cutting speed of 100 m/min, the carbide tool achieves a flank wear limit (VB = 0.3 mm) after approximately 8–12 minutes of continuous cutting, while the ceramic tool extends this to 25–35 minutes. The primary wear mechanisms include adhesive wear (dominant at low speeds below 80 m/min), abrasion from hard carbide particles in the overlay (dominant at medium speeds 80–150 m/min), and oxidative wear (significant at speeds above 150 m/min).
Surface Integrity and Built-Up Edge Behavior
| Cutting Speed (m/min) | Surface Roughness Ra (μm) | BUE Formation | Residual Stress (MPa) | Micro-crack Probability |
|---|---|---|---|---|
| 50 | 1.2–1.8 | Severe | -250 to -350 | High |
| 100 | 0.8–1.2 | Moderate | -150 to -220 | Moderate |
| 150 | 0.6–0.9 | Minor | -80 to -150 | Low |
| 200 | 0.9–1.4 | None | -50 to -100 | Very Low |
At low cutting speeds, severe BUE formation leads to poor surface finish and potential overlay material transfer to the tool rake face. As the speed increases to 150 m/min, the cutting temperature becomes high enough to reduce adhesion between the workpiece and tool, minimizing BUE and achieving the best surface finish. However, at 200 m/min, the increased thermal load causes thermal softening of the carbide tool edge and potential recast layer formation on the machined surface, degrading the corrosion resistance of the overlay.
Engineering Practice Recommendations
For post-cladding machining of PTA Ni-based alloy overlays on pressure vessel components, the following guidelines are recommended based on the study findings and practical experience:
- Select ceramic cutting tools (Al2O3-TiC) for production machining to extend tool life by 2.5 to 3 times compared to standard carbide tools.
- Operate at cutting speeds of 120–160 m/min to minimize BUE formation while avoiding excessive thermal damage to the overlay surface.
- Use a positive rake angle (15–20 degrees) to reduce cutting forces and improve chip evacuation from the overlay surface.
- Apply minimum quantity lubrication (MQL) with a cutting fluid containing molybdenum disulfide additives to reduce friction and adhesive wear.
- Plan machining operations to leave a minimum 0.5 mm overlay thickness after finishing to maintain corrosion protection, as specified in GB/T 150 and ASME VIII Div.1.
Critical Observations and Study Insights
One of the most important findings from this study is that the machined surface of a plasma-clad Ni-based overlay can exhibit significantly different corrosion resistance compared to the as-clad surface. The recast layer and residual stress introduced during machining can create localized anodic sites that initiate pitting corrosion. In a recent project involving a hydrogenation reactor with 3 mm Ni-based PTA overlay, we discovered that areas machined at excessive speeds showed intergranular corrosion after only 6 months of service, while properly machined areas remained intact. This underscores the necessity of including machining qualification in the overall cladding process qualification package. The engineer must recognize that cladding quality is not solely determined by the welding process but is also significantly influenced by subsequent machining operations. A comprehensive approach that considers the entire manufacturing sequence—from PTA deposition through post-machining to final inspection—is essential for ensuring long-term service reliability of clad components.