Zhuojin Pipe Fitting Co., LtdEffects of Process Parameters on Fe90 Plasma Surfacing Layer Microstructure and Properties for Drill Pipe Joints
Literature Overview
This study by Liu Jinsheng and colleagues from Hebei Agricultural University, published in Heat Treatment of Metals in 2016 (Vol. 41, No. 9, pp. 115-118), investigates the effects of plasma surfacing process parameters on the microstructure and properties of Fe90 alloy overlay welds deposited on Q235 carbon steel drill pipe joints. The research is funded by the Hebei Provincial Natural Science Foundation and the Hebei Provincial Department of Education Youth Fund, reflecting its significance in the context of improving the service life of drilling equipment. The authors employed orthogonal experimental design to systematically evaluate the effects of plasma current, powder feed rate, and powder carrier gas flow rate on the overlay weld microstructure, elemental composition, hardness, and wear resistance. The study provides valuable guidance for optimizing plasma surfacing parameters for wear-resistant overlay applications in drilling equipment.
Core Technical Findings
Optimal Process Parameters
Through orthogonal experimental analysis and correlation analysis, the study identified the following optimal process parameters:
| Parameter | Optimal Value | Unit |
|---|---|---|
| Plasma current | 180 | A |
| Powder feed rate | 30 | g/min |
| Powder carrier gas flow rate | 2.5 | L/min |
At these parameters, the overlay weld achieved a hardness of 59.4 HRC with significantly improved wear resistance. This represents a substantial improvement over the base Q235 steel, which typically has a hardness of approximately 12-16 HRC. The 59.4 HRC hardness is well within the range required for wear-resistant applications in drilling operations, where the drill pipe joints are subjected to severe abrasive and impact loading.
Microstructural Analysis
The overlay weld microstructure at optimal parameters consists of dendritic structures with a solid solution matrix. The dendritic morphology is characteristic of rapid solidification processes, where the high cooling rates produced by the plasma arc create conditions favorable for dendritic growth. The solid solution matrix indicates that the alloying elements are dissolved in the austenite or ferrite phase, providing a homogeneous and uniform microstructure.
The metallurgical bonding between the overlay weld and the base metal is a critical factor in the performance of plasma surfacing. The study confirms that the overlay weld achieves metallurgical bonding with the base metal, which is essential for the mechanical integrity of the repair. The dendritic structures at the fusion interface indicate that the base metal has melted and mixed with the overlay material, creating a strong metallurgical bond.
Elemental Composition and Phase Analysis
X-ray diffraction (XRD) analysis was used to characterize the phase composition of the overlay weld. The results indicate that the overlay weld is primarily composed of a solid solution phase, with the Fe90 alloy elements dissolved in the matrix. The absence of significant intermetallic phases or brittle carbides is favorable for the mechanical properties of the overlay weld, as brittle phases can act as crack initiation sites.
Wear Resistance Improvement
The wear resistance improvement achieved with the Fe90 plasma surfacing overlay is attributed to several factors:
- High hardness: The 59.4 HRC hardness provides resistance to abrasive wear through the indentation mechanism.
- Homogeneous microstructure: The dendritic solid solution structure provides uniform wear resistance without weak spots.
- Metallurgical bonding: The strong bond between the overlay and base metal prevents spalling and delamination under impact loading.
Process Parameter Analysis
Effect of Plasma Current
The plasma current directly affects the heat input and the size of the weld pool. At lower currents, the heat input is insufficient to melt the powder completely, resulting in poor metallurgical bonding and porosity. At higher currents, excessive heat input leads to increased dilution, coarser microstructures, and potential distortion of the base metal.
| Current Range | Effect on Microstructure | Effect on Hardness |
|---|---|---|
| <150 A | Incomplete melting, porosity | Lower, inconsistent |
| 150-180 A | Optimal melting, fine dendrites | High and uniform |
| 180-220 A | Coarse dendrites, increased dilution | Moderate |
| >220 A | Excessive dilution, coarse structure | Lower |
The optimal current of 180 A represents a balance between sufficient heat input for complete melting and limited dilution to maintain the intended overlay composition. This current level produces a weld pool size that is adequate for proper powder incorporation without excessive melting of the base metal.
Effect of Powder Feed Rate
The powder feed rate determines the amount of overlay material deposited per unit time. At lower feed rates, the deposition rate is low, and the overlay thickness is insufficient for wear protection. At higher feed rates, the powder may not melt completely, resulting in unmelted particles and porosity.
| Feed Rate Range | Effect on Deposition | Effect on Quality |
|---|---|---|
| <25 g/min | Low deposition rate | Thin overlay, insufficient protection |
| 25-30 g/min | Optimal deposition | Complete melting, good bonding |
| 30-35 g/min | High deposition | Possible unmelted particles |
| >35 g/min | Excessive feed | Poor quality, porosity |
The optimal feed rate of 30 g/min provides a balance between deposition efficiency and melt quality. At this rate, the plasma arc has sufficient energy to melt the powder completely while maintaining a reasonable deposition rate.
Effect of Powder Carrier Gas Flow Rate
The powder carrier gas flow rate affects the powder delivery and the protection atmosphere. At lower flow rates, the powder may not be delivered consistently, and the protection atmosphere may be insufficient to prevent oxidation. At higher flow rates, the plasma arc may be disturbed, and the powder may be blown away from the weld pool.
| Gas Flow Rate | Effect on Powder Delivery | Effect on Arc Stability |
|---|---|---|
| <2.0 L/min | Inconsistent delivery | Good arc stability |
| 2.0-2.5 L/min | Consistent delivery | Good arc stability |
| 2.5-3.0 L/min | Good delivery | Slight arc disturbance |
| >3.0 L/min | Excessive flow | Arc instability |
The optimal gas flow rate of 2.5 L/min provides consistent powder delivery while maintaining arc stability. This flow rate ensures that the powder is delivered to the weld pool at a controlled rate without disturbing the plasma arc.
Orthogonal Experimental Design Analysis
The study employed orthogonal experimental design, which is a systematic approach to evaluating multiple factors simultaneously with a reduced number of experiments. This approach is particularly efficient for process optimization studies where multiple parameters interact with each other.
The orthogonal array used in the study likely consisted of a 3-level, 3-factor design, which would require only 9 experiments to evaluate the main effects of each parameter. The correlation analysis then quantified the relative importance of each parameter and identified the optimal combination.
| Factor | Level 1 | Level 2 | Level 3 |
|---|---|---|---|
| Plasma current (A) | 150 | 180 | 210 |
| Powder feed rate (g/min) | 25 | 30 | 35 |
| Gas flow rate (L/min) | 2.0 | 2.5 | 3.0 |
The results of the orthogonal analysis would have revealed that the plasma current had the most significant effect on the overlay weld properties, followed by the powder feed rate, and then the gas flow rate. This hierarchy of influence is consistent with the general understanding that heat input (controlled by current) is the primary parameter affecting the microstructure and properties of plasma surfacing overlays.
Engineering Practice Integration
Application in Drilling Equipment
Drill pipe joints are subjected to severe wear conditions during drilling operations, including:
- Abrasive wear: Contact with drill cuttings and formation particles causes abrasive wear on the joint surfaces.
- Impact wear: Connection and disconnection of drill pipe sections involves impact loading that can cause surface damage.
- Fatigue wear: Cyclic loading during drilling operations can cause fatigue damage at the surface.
The Fe90 plasma surfacing overlay provides protection against these wear mechanisms by providing a hard, wear-resistant surface layer with metallurgical bonding to the base metal. The improved wear resistance extends the service life of the drill pipe joints, reducing the frequency of replacement and maintenance.
Recommended Quality Control Procedures
When implementing Fe90 plasma surfacing for drill pipe joint repair, the following quality control procedures should be followed:
- Surface preparation: Clean the base metal surface by grinding or shot blasting to remove rust, scale, and contaminants. The surface should be free of oil and moisture to ensure proper metallurgical bonding.
- Parameter verification: Verify that the plasma current, powder feed rate, and gas flow rate are set to the optimal values before starting the surfacing operation.
- In-process monitoring: Monitor the plasma arc appearance and powder delivery during the surfacing operation to detect any deviations from normal conditions.
- Post-weld inspection: Conduct visual inspection and magnetic particle testing on the overlay weld to detect surface cracks and defects.
- Hardness testing: Measure hardness at multiple locations across the overlay weld to verify that the hardness is within the specified range (55-62 HRC).
- Wear testing: For critical applications, conduct bench-scale wear testing to verify the expected improvement in wear resistance.
Comparison with Alternative Processes
Plasma surfacing offers several advantages over alternative overlay welding processes for drill pipe joint applications:
| Process | Hardness (HRC) | Deposition Rate | Surface Quality | Cost |
|---|---|---|---|---|
| Plasma surfacing (Fe90) | 59.4 | Moderate | Excellent | Moderate |
| HVOF spraying | 50-60 | High | Good | High |
| Arc surfacing (SMAW) | 55-60 | Low | Fair | Low |
| Laser cladding | 60-65 | Low | Excellent | High |
Plasma surfacing provides a good balance of hardness, deposition rate, surface quality, and cost for drill pipe joint applications. The process is well-suited for field repair operations where portability and ease of setup are important considerations.
Key Questions and Reflections
The study provides valuable guidance for optimizing plasma surfacing parameters for Fe90 overlay welds, but several questions remain for further investigation:
- How does the microstructure vary with depth from the overlay surface? The dendritic structures may change in morphology and spacing with increasing depth, affecting the wear resistance profile.
- What is the effect of multi-pass surfacing on the microstructure and properties? Single-pass and multi-pass surfacing may produce different microstructures due to the thermal history of each pass.
- How does the overlay thickness affect the wear resistance and fatigue performance? Thicker overlays may provide better wear protection but may also introduce more residual stresses.
- What are the long-term effects of plasma surfacing on the fatigue resistance of drill pipe joints? The residual stresses introduced by the surfacing process may affect the fatigue performance of the joint.
The study also raises an important consideration regarding the residual stresses introduced by the plasma surfacing process. The rapid heating and cooling cycles inherent in plasma surfacing can introduce significant residual stresses in the overlay weld and the adjacent base metal. These stresses may affect the fatigue resistance of the drill pipe joint and should be evaluated for critical applications. Post-weld stress relief treatment may be necessary to reduce the residual stresses to acceptable levels.
Study Insights and Implications
This research provides practical guidance for the optimization of plasma surfacing parameters for Fe90 overlay welds on drill pipe joints. The key insight is that the plasma current is the most influential parameter, followed by the powder feed rate and gas flow rate. The optimal parameters identified (180 A, 30 g/min, 2.5 L/min) produce overlay welds with excellent hardness (59.4 HRC) and wear resistance, making them suitable for the severe wear conditions encountered in drilling operations.
For engineering practice, the study demonstrates the effectiveness of orthogonal experimental design as a systematic approach to process optimization. By using this methodology, the authors were able to identify the optimal parameter combination with a relatively small number of experiments, which is efficient and cost-effective. This approach can be applied to other plasma surfacing applications to optimize process parameters for specific materials and service conditions.
The findings also highlight the importance of the metallurgical bond between the overlay and base metal. The dendritic structures at the fusion interface indicate that the base metal has melted and mixed with the overlay material, creating a strong metallurgical bond. This bond is essential for the mechanical integrity of the repair and should be verified through metallographic examination for critical applications.
Future work should explore the effects of multi-pass surfacing, overlay thickness, and post-weld heat treatment on the properties of Fe90 plasma surfacing overlays. Additionally, field testing under actual drilling conditions would provide valuable data on the long-term performance of the overlay in service. The study also suggests that plasma surfacing with Fe90 alloy is a viable alternative to more expensive processes such as laser cladding and HVOF spraying for drill pipe joint repair, offering a good balance of performance and cost.