CLADDING TECHNOLOGY SHANXI CO., LTDPlasma Transferred Arc Cladding Process of Fe90 Alloy
Literature Overview
The study by Lu Hailong and Kang Jiandong, published in the Journal of Thermal Processing Technology in 2012, investigates the plasma transferred arc (PTA) cladding process for Fe90 alloy, a high-alloy iron-based material widely used in the oil and gas industry for its exceptional resistance to erosion and corrosion in harsh environments such as sour gas service and high-velocity slurry flow. The research is conducted in collaboration with the Liaohe Oilfield Exploration Bureau Machinery General Factory, reflecting a strong industry-academia partnership and a direct focus on practical engineering problems encountered in petroleum equipment maintenance.
Core Technical Content
Fe90 alloy is a high-chromium, high-nickel, high-molybdenum iron-based alloy that typically contains approximately 27–32% Cr, 12–17% Ni, 4–6% Mo, and 0.5–1.0% C. The PTA cladding process is particularly well-suited for this alloy because it offers precise control over dilution, which is critical for maintaining the required alloy composition in the deposited layer. The study examines the influence of key PTA parameters on the dilution rate, microstructure, and mechanical properties of the cladding layer.
| PTA Parameter | Typical Value | Influence on Cladding Quality |
|---|---|---|
| Plasma current | 100–200 A | Higher current increases dilution and deposition rate |
| Powder feed rate | 150–300 g/min | Higher feed rate reduces dilution but may cause incomplete melting |
| Travel speed | 100–300 mm/min | Faster travel reduces dilution but may cause lack of fusion |
| Powder nozzle height | 5–10 mm | Too high causes powder scatter; too low causes arc instability |
| Shielding gas flow | 10–20 L/min (Ar) | Insufficient flow leads to oxidation and porosity |
| Number of passes | 1–3 | Multiple passes reduce dilution in subsequent passes |
The dilution rate in PTA cladding is typically much lower than in conventional arc welding processes, often in the range of 5–15% for the first pass and decreasing in subsequent passes. This low dilution is one of the primary advantages of PTA for high-alloy cladding applications. The study demonstrates that by carefully controlling the plasma current and powder feed rate, it is possible to achieve a dilution rate below 10%, which is essential for maintaining the corrosion resistance and hardness of the Fe90 overlay.
The microstructural analysis reveals that the PTA-cladded Fe90 layer exhibits a fine dendritic structure with a high density of carbide phases, primarily M7C3 and M23C6, which contribute to the wear resistance. The rapid solidification rate in PTA (typically 10–100 °C/s) promotes fine grain formation and suppresses the growth of coarse carbide networks, resulting in a more homogeneous and tougher microstructure compared to conventional arc welding cladding.
Engineering Practice and Process Optimization
In the oil and gas industry, Fe90 cladding is commonly applied to drill collars, downhole tools, valve bodies, and pump components that are subjected to erosion by high-velocity fluid flow containing abrasive particles. The PTA process is preferred over other cladding methods for these applications because it allows for precise control of the overlay composition and minimizes the risk of dilution-related property degradation.
The study's findings on the relationship between plasma current and dilution rate are particularly practical. A general rule of thumb derived from the research is that for every 50 A increase in plasma current, the dilution rate increases by approximately 3–5 percentage points, assuming other parameters remain constant. This relationship allows process engineers to predict and control dilution during procedure development.
Powder feed rate is another critical parameter. If the feed rate is too low relative to the plasma current, the powder will not fully melt, resulting in unmelted particles in the deposit and poor bond strength. If the feed rate is too high, the powder will scatter and the arc will become unstable. The optimal feed rate is typically in the range of 1.5–2.5 g/min per ampere of plasma current, but this ratio should be verified through trial welds for each specific application.
The shielding gas flow rate must be sufficient to protect both the plasma arc and the molten pool from atmospheric contamination. Insufficient shielding leads to oxidation of the high-alloy deposit, which degrades both the corrosion resistance and the mechanical properties. A minimum flow rate of 10 L/min of argon is recommended, with additional flow if the ambient conditions are windy or if the workpiece geometry creates turbulent flow around the weld zone.
A key insight from this research is that the PTA process requires a high degree of operator skill and equipment stability. Unlike conventional welding processes, PTA is sensitive to minor variations in powder feed consistency, nozzle alignment, and travel speed. Any inconsistency in these parameters can lead to non-uniform dilution, porosity, or incomplete fusion. Therefore, automated or semi-automated PTA systems are preferred for production applications, where reproducibility and quality consistency are paramount.
This study provides a solid technical foundation for the PTA cladding of Fe90 alloy in petroleum industry applications. The emphasis on dilution control and microstructure optimization is directly relevant to engineers responsible for welding procedure qualification and process development. The industry-academia collaboration model demonstrated in this research is a valuable approach for addressing practical engineering challenges with rigorous scientific methodology.