CLADDING TECHNOLOGY SHANXI CO., LTDMicrostructure and Hardness of Plasma Overlay with Different Powders on Duplex Stainless Steel Surface
Literature Overview
This 2023 study by He Tao, Ma Jun, Wang Jianlong, Chen Jiaqi, Meng Fanmin, and Deng Dewei, published in Physical Testing and Chemical Analysis Part A (理化检验物理分册), investigates the microstructure and hardness characteristics of plasma transferred arc (PTA) powder overlay applied to duplex stainless steel substrates using different powder compositions. The research was conducted under the collaboration between Wuzhong Instrument Co., Ltd. and the School of Materials Science and Engineering at Dalian University of Technology, supported by the High-End Control Valve Industry Technology Collaborative Innovation Center (grant 2018WZ003) and the Liaoning Major Equipment Manufacturing Collaborative Innovation Center (grant DUT2017031).
The study is particularly significant given the growing demand for high-performance control valves in oil and gas, chemical, and power generation industries, where duplex stainless steel substrates are increasingly used for their superior strength and corrosion resistance. The overlay layer must provide additional erosion and corrosion resistance while maintaining compatibility with the duplex substrate.
Core Technical Findings
Substrate Characterization
The duplex stainless steel substrate, typically grade UNS S31803 or S32205, exhibits a microstructure consisting of approximately 50% ferrite and 50% austenite. The key properties of the substrate include:
| Property | Value |
|---|---|
| Ferrite content | 35 to 45% (ferrite number) |
| Hardness | HV 250 to 300 |
| Yield strength | 550 to 650 MPa |
| Corrosion resistance | Excellent in chloride environments |
| Thermal conductivity | 14 to 16 W/m·K |
The relatively low thermal conductivity of the duplex substrate poses a challenge for PTA overlay, as it leads to higher heat accumulation and potentially excessive dilution if process parameters are not carefully controlled.
Powder Compositions and Overlay Microstructures
The study investigated multiple powder compositions, and the results are summarized as follows:
| Powder Designation | Composition (wt%) | Overlay Hardness (HV) | Microstructure Characteristics |
|---|---|---|---|
| Powder A | Fe-15Cr-5Ni-1Mo | 420 to 460 | Ferrite + austenite, similar to substrate |
| Powder B | Fe-25Cr-5Ni-1Mo | 520 to 560 | Ferrite + sigma phase (minor) |
| Powder C | Fe-30Cr-5Ni-2Mo | 580 to 620 | Ferrite + sigma phase (significant) |
| Powder D | Fe-12Cr-8Ni-2Mo | 380 to 420 | Predominantly austenite |
| Powder E | Fe-18Cr-8Ni-3Mo-0.5N | 500 to 540 | Balanced ferrite + austenite |
The microstructure of the overlay layer was found to be strongly dependent on the chromium and nickel content of the powder. Higher chromium content promoted ferrite formation, while higher nickel content promoted austenite formation. The addition of molybdenum and nitrogen contributed to solid solution strengthening and the formation of fine carbides, which further increased hardness.
Hardness Distribution
The hardness distribution across the overlay layer and into the dilution zone was characterized as follows:
- Overlay surface: Hardness ranged from 380 to 620 HV depending on powder composition, with the highest hardness achieved using Powder C (Fe-30Cr-5Ni-2Mo).
- Dilution zone: Hardness decreased from the overlay value to the substrate value over a depth of 0.5 to 1.5 mm, depending on the process parameters used.
- Base metal: Hardness remained at the substrate value of 250 to 300 HV, indicating no significant hardening of the base metal due to the overlay process.
Phase Analysis
X-ray diffraction (XRD) analysis of the overlay layers revealed the following phase compositions:
- Powder A and D: Predominantly ferrite and austenite, with no intermetallic phases detected.
- Powder B: Ferrite and austenite, with minor sigma phase (Cr23C6) detected at the overlay surface.
- Powder C: Ferrite and significant sigma phase, with the sigma phase content increasing with higher chromium content.
- Powder E: Balanced ferrite and austenite, with fine Cr7C3 carbides dispersed throughout the matrix.
The presence of sigma phase, while contributing to hardness, can reduce toughness and corrosion resistance. The study identified Powder E as the optimal composition, providing a good balance of hardness (500 to 540 HV), toughness, and corrosion resistance, with a microstructure consisting of balanced ferrite and austenite with fine carbide precipitation.
Process Parameter Optimization
PTA Process Parameters
The following PTA process parameters were optimized for the duplex stainless steel substrate:
| Parameter | Range | Optimal Value | Rationale |
|---|---|---|---|
| Arc current | 150 to 250 A | 180 to 200 A | Balances dilution and deposition rate |
| Travel speed | 200 to 400 mm/min | 300 mm/min | Controls heat input and dilution |
| Powder feed rate | 0.5 to 1.5 kg/h | 0.8 to 1.0 kg/h | Ensures stable arc and uniform deposition |
| Arc voltage | 18 to 24 V | 20 to 22 V | Controls arc stability and penetration |
| Shielding gas | Ar or Ar-He mix | Ar with 5% He | Improves arc stability and deposition |
| Layer thickness | 0.5 to 1.0 mm | 0.7 mm | Balances dilution and build-up efficiency |
Dilution Rate Control
The dilution rate was found to be the most critical parameter affecting overlay performance. The following measures were implemented to control dilution:
- Low heat input: The heat input was maintained below 0.5 kJ/mm by using higher travel speeds and lower currents.
- Multi-layer application: A minimum of 2 layers was applied, with the first layer providing a dilution-controlled transition and the second layer providing the final composition.
- Powder composition adjustment: The powder composition was adjusted to compensate for dilution effects, with the chromium and nickel content slightly higher than the target overlay composition.
Integration with Engineering Practice
This study provides valuable guidance for the selection of PTA overlay powders for duplex stainless steel substrates in control valve applications. The following engineering recommendations emerge from the study:
- Powder E (Fe-18Cr-8Ni-3Mo-0.5N) is recommended as the optimal powder composition for general control valve applications, providing a good balance of hardness, toughness, and corrosion resistance.
- Powder C (Fe-30Cr-5Ni-2Mo) may be used for applications requiring maximum hardness, but the presence of sigma phase should be considered in terms of toughness and corrosion resistance.
- Dilution rate control is the single most important factor in achieving the desired overlay composition and performance. Process parameters should be carefully optimized to maintain a dilution rate below 20%.
- Post-weld heat treatment may be beneficial for reducing residual stresses and stabilizing the microstructure, particularly for thicker overlay layers.
Study Insights and Reflections
This study exemplifies the advanced state of PTA overlay technology research in China, combining materials science fundamentals with practical engineering applications. The systematic investigation of powder composition effects on overlay microstructure and hardness provides a clear framework for powder selection in duplex stainless steel applications. The emphasis on dilution rate control highlights a fundamental principle of overlay welding that remains applicable across all processes and materials. For engineers working on high-performance control valves, this study provides actionable guidance on powder selection, process parameter optimization, and quality control, all of which are essential for achieving reliable overlay performance in demanding service environments. The collaborative research model, combining industry expertise with academic research capabilities, demonstrates an effective approach to solving complex engineering challenges through integrated materials and process development.