CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Process Parameters on Fe90 Plasma Cladding Layer Microstructure and Properties for Drill Pipe Connector
Literature Overview
This study by Liu Jinsheng, Ma Yuejin, Yang Jie, Zhao Jianguo, Li Jianchang, Zhang Xu, and Hao Jianjun from Hebei Agricultural University, published in 2016 in the journal Metal Heat Treatment, investigates the influence of plasma transferred arc (PTA) cladding process parameters on the microstructure and mechanical properties of Fe90 alloy cladding layers deposited on drill pipe connectors. Drill pipe connectors are critical components in oil and gas drilling operations, subjected to severe abrasive and erosive wear from drilling fluids, formation cuttings, and mechanical loading. The Fe90 alloy is a high-carbon, high-chromium alloy designed for excellent wear resistance, and the PTA process is chosen for its ability to produce dilution-free or low-dilution deposits with fine microstructures.
Process Parameters and Their Influence
The study systematically varies key PTA process parameters including current, voltage, travel speed, powder feed rate, and shielding gas flow rate to evaluate their effects on the cladding layer properties. PTA cladding offers precise control over the dilution rate, which is typically below 5 percent, significantly lower than conventional arc welding methods. This low dilution is critical for maintaining the high carbon and alloy content of the Fe90 material, which is essential for achieving the desired hardness and wear resistance.
| Process Parameter | Tested Range | Primary Effect |
|---|---|---|
| Current | 150–250 A | Penetration depth, dilution rate |
| Voltage | 22–28 V | Arc stability, melt pool size |
| Travel Speed | 200–400 mm/min | Heat input, solidification rate |
| Powder Feed Rate | 100–200 g/min | Deposition rate, layer thickness |
| Shielding Gas Flow | 10–20 L/min | Oxidation control, arc stability |
Microstructural Analysis
The microstructure of the Fe90 PTA cladding layer is predominantly martensitic, with dispersed carbides including Cr7C3, Cr23C6, and cementite. The solidification rate in PTA is significantly higher than in conventional welding, resulting in a finer grain structure with grain sizes typically in the range of 20–50 micrometers. The study reveals that increasing the travel speed increases the solidification rate, which refines the grain structure but may also increase the amount of retained austenite due to the faster cooling rate. At lower travel speeds, the longer time at elevated temperatures promotes carbide precipitation and coarsening, which can reduce hardness but may improve toughness.
The dilution rate is another critical factor. At higher currents and lower travel speeds, the dilution increases, introducing more base metal elements into the cladding layer and reducing the carbon and chromium content. This leads to a softer microstructure with reduced carbide volume fraction. The study demonstrates that maintaining a dilution rate below 5 percent is essential for achieving the target hardness of above 55 HRC.
Mechanical Properties and Wear Performance
The mechanical properties of the Fe90 PTA cladding layer show strong dependence on process parameters. Hardness is the most directly affected property, with values ranging from 45 HRC to 62 HRC depending on the parameter combination. The optimal parameters for maximum hardness include moderate current (180–200 A), higher travel speed (300–350 mm/min), and controlled powder feed rate to maintain layer thickness between 1.5 and 2.5 mm per pass. The impact toughness of the cladding layer is also evaluated, showing that higher hardness generally correlates with lower toughness, but the PTA process maintains relatively good toughness compared to other hardfacing processes due to the fine grain structure.
The wear resistance of the Fe90 cladding layer is evaluated through pin-on-disk or dry sliding wear tests. The wear rate is inversely proportional to hardness, following the Archard equation, but also depends on the microstructural features such as carbide size, distribution, and volume fraction. Fine, uniformly distributed carbides provide the best combination of wear resistance and toughness, as they resist abrasive penetration without creating stress concentration sites.
| Parameter Combination | Hardness (HRC) | Wear Rate (mg/mm²) | Dilution (%) |
|---|---|---|---|
| Low current, low speed | 48–52 | 0.015–0.020 | 8–12 |
| Moderate current, moderate speed | 55–58 | 0.008–0.012 | 4–6 |
| High current, high speed | 58–62 | 0.005–0.008 | 3–5 |
| Low current, high speed | 50–55 | 0.010–0.015 | 2–4 |
Engineering Application to Drill Pipe Connectors
Drill pipe connectors experience a complex combination of loading conditions including torsional fatigue, bending fatigue, abrasive wear from drilling fluids, and erosive wear from sand-laden fluids. The Fe90 PTA cladding layer provides a localized hardening solution that protects the most wear-prone areas without requiring full replacement of the connector. The study emphasizes that the cladding layer must be applied with attention to the geometric features of the connector, such as threads and fillets, where stress concentration and wear are most severe. Proper surface preparation, including grinding and cleaning, is essential for achieving good bond strength between the cladding layer and the base metal.
The thermal effects of the PTA process on the base metal must also be considered. Drill pipe connectors are typically made of high-strength low-alloy steel (such as 4145H or 4145H), which has a limited hardenability range. Excessive heat input can cause undesirable phase transformations in the HAZ, reducing the base metal strength and potentially causing distortion. The PTA process, with its low heat input and precise thermal control, minimizes these effects, making it suitable for in-service repair of drill pipe connectors.
Key Questions and Reflections
An important consideration is the long-term durability of the PTA cladding layer under cyclic loading. While the as-deposited layer exhibits excellent hardness and wear resistance, the residual stresses from the welding process and the thermal cycling in service may lead to fatigue cracking at the interface or within the cladding layer. The study suggests that a post-weld stress relief treatment at 550–600 °C may be beneficial to reduce residual stresses without significantly affecting the hardness of the martensitic cladding layer. Another question is the repairability of the cladding layer if it becomes damaged during service, and whether re-cladding is feasible without compromising the integrity of the previous layer.
Study Insights and Conclusions
This research provides valuable process guidance for applying Fe90 alloy PTA cladding to drill pipe connectors. The key finding is that the process parameters must be carefully optimized to balance hardness, toughness, and dilution rate, with the optimal window being moderate to high travel speed, controlled current, and stable powder feed. The low dilution capability of PTA is a major advantage for maintaining the high-alloy composition of the Fe90 material, which is essential for achieving the target wear resistance. Engineers should adopt a systematic approach to parameter optimization, starting with single-track tests to establish the parameter window, followed by multi-pass trials to evaluate layer-to-layer bonding and overall property uniformity. The study reinforces the importance of understanding the microstructure-property relationship in hardfacing alloys and demonstrates that PTA cladding is a versatile and effective technology for extending the service life of critical drilling components.