Microstructure and Hardness of Plasma Hardfacing with Different Powders on Duplex Stainless Steel F51

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 (Physics Section) (Vol. 59, No. 5, pp. 9-15), investigates the microstructural evolution and hardness distribution of plasma hardfacing deposits applied to F51 duplex stainless steel substrate. The research was supported by the High-End Control Valve Industry Technology Collaborative Innovation Center and the Liaoning Major Equipment Manufacturing Collaborative Innovation Center. The study compares three overlay powder systems—Co106F, Co112F, and Ni55—applied in two layers using plasma transfer arc hardfacing, providing valuable insights into overlay selection for high-performance valve and pump components.

Technical Background and Material Selection

F51 duplex stainless steel (UNS S32750/S31803 grade) is widely used in control valves, pump impellers, and marine hardware due to its excellent combination of high strength (yield strength approximately 550 MPa), good corrosion resistance in chloride-containing environments, and resistance to stress corrosion cracking. However, the surface hardness of F51 in the as-delivered condition (typically 25-30 HRC) is often insufficient for applications involving severe wear, erosion, or cavitation. Plasma hardfacing provides a solution by depositing a hard, wear-resistant overlay layer while preserving the corrosion resistance and mechanical properties of the underlying duplex substrate.

The three overlay powders selected for this study represent different metallurgical systems:

Experimental Methodology

Plasma Hardfacing Process Parameters

Plasma transfer arc hardfacing was employed with a two-layer deposition strategy. The process parameters were carefully controlled to ensure good metallurgical bonding between layers and between the overlay and substrate:

Microstructural Analysis

The study employed multiple characterization techniques: optical microscopy (OM) for overall microstructure morphology, scanning electron microscopy (SEM) for fine-scale features, line scanning (EDS) for elemental distribution, and Vickers microhardness testing for hardness profiling across the deposit.

Key Findings and Interpretation

Microstructural Characteristics

The microstructural results reveal distinct differences among the three powder systems:

Co106F and Co112F deposits: Both cobalt-based powders produced microstructures composed primarily of dendritic crystals. The dendrite morphology indicates a columnar growth pattern from the substrate interface upward, which is typical of rapid solidification in plasma hardfacing. The similarity between Co106F and Co112F microstructures suggests that the minor compositional differences between these two grades do not significantly alter the solidification behavior under the same process conditions. The dendrite arm spacing in both deposits was in the range of 10-30 micrometers, indicating moderate cooling rates.

Ni55 deposit: The nickel-based deposit exhibited a more compact microstructure with a notable feature at the interface between Layer 1 and Layer 2—disordered columnar crystals were observed. This feature was attributed to supercooling during the interlayer cooling period. The columnar crystal growth at the interface suggests that the thermal gradient was oriented perpendicular to the interface, promoting directional solidification. The supercooling phenomenon occurs because the Layer 1 deposit, having already solidified, acts as a thermal sink during Layer 2 deposition, creating a steep thermal gradient at the interface.

Hardness Results

The hardness enhancement factors of 1.5-1.62 indicate that all three overlay systems provide substantial wear resistance improvement over the base F51 material. The Ni55 powder achieved the highest surface hardness, which can be attributed to the formation of hard carbide and silicide phases (such as Ni3Si, Ni3SiC, and Cr-rich carbides) within the nickel matrix. The Co106F powder showed the lowest surface hardness among the three, likely due to a more homogeneous solid solution microstructure with fewer hard precipitates compared to Ni55.

Elemental Diffusion Analysis

Line scanning analysis revealed elemental diffusion at the overlay-substrate interface and between the two overlay layers. Chromium and iron from the F51 substrate diffused into the first layer of the overlay, while cobalt or nickel from the overlay diffused into the substrate. This interdiffusion zone, typically 20-50 micrometers wide, creates a metallurgical bond that enhances adhesion strength. The diffusion of chromium from the substrate into the overlay layer can also contribute to the corrosion resistance of the overlay, which is particularly beneficial for valve applications exposed to corrosive media.

Engineering Application Analysis

Control Valve Applications

The F51 duplex stainless steel substrate is commonly used in control valve bodies and trim for oil and gas, chemical processing, and power generation applications. In these environments, valve components are subjected to combined wear, erosion, and corrosion—often referred to as erosion-corrosion or cavitation damage. The plasma hardfacing overlay provides a localized wear-resistant layer at critical surfaces (valve seats, plug faces, guide surfaces) without requiring the entire component to be manufactured from a more expensive wear-resistant material.

The hardness enhancement of 1.5-1.6 times the substrate hardness is generally sufficient to extend valve component life by 2-5 times in typical erosion-corrosion service. The selection between Co106F, Co112F, and Ni55 depends on the specific operating conditions:

• Co106F: Preferred for high-temperature applications (up to 600 degrees Celsius) where thermal stability is critical. The cobalt-chromium-tungsten system maintains hardness at elevated temperatures due to the high melting point of the cobalt matrix.
• Co112F: Similar to Co106F but with slightly enhanced high-temperature wear resistance, suitable for applications with moderate thermal cycling.
• Ni55: Preferred for applications requiring the highest room-temperature hardness and where moderate corrosion resistance is also needed. The nickel-silicon-cobalt system offers good resistance to both wear and corrosion in aqueous environments.

Process Quality Considerations

Plasma hardfacing of duplex stainless steel requires careful attention to several quality factors:

1. Dilution control: The dilution rate between the overlay and the F51 substrate should be kept below 15-20% to maintain the intended overlay composition. Excessive dilution introduces ferrite and austenite from the substrate into the overlay, potentially reducing hardness and altering corrosion behavior.
2. Heat input management: Excessive heat input can cause sensitization in the cobalt or nickel overlay, leading to carbide precipitation at grain boundaries and reduced corrosion resistance. The two-layer strategy with interlayer cooling helps manage cumulative heat input.
3. Post-weld inspection: Visual inspection, magnetic particle testing (MT), and hardness profiling are essential quality checks. The interface between overlay layers and between overlay and substrate should be examined for lack of fusion or cracking.
4. Surface finish: The as-deposited surface roughness of plasma hardfacing is typically Ra 3.2-6.3 micrometers. For valve seat applications requiring tight clearance, post-deposition grinding and polishing are required.

Critical Reflections

The observation that Co106F and Co112F produced nearly identical microstructures is an important finding that has practical implications for material selection. If the microstructural and hardness differences between these two grades are marginal under identical process conditions, the choice between them should be guided by cost, availability, and specific application requirements rather than by microstructural considerations alone. The Ni55 result, showing the highest hardness but also the most complex microstructure (including supercooled columnar crystals at the interlayer interface), suggests that process parameter optimization is particularly important for nickel-based overlays. The supercooling-induced columnar crystals at the Layer 1-Layer 2 interface could potentially create preferential paths for crack initiation under cyclic loading, which warrants further investigation for applications involving fatigue or thermal cycling.

The hardness ratios of 1.5-1.62 reported in this study are consistent with published literature on plasma hardfacing of cobalt and nickel alloys, confirming the reliability of the experimental methodology. However, the study would benefit from additional mechanical testing such as micro-indentation creep, erosion-corrosion testing, and cavitation erosion testing to provide a more comprehensive evaluation of overlay performance under realistic service conditions.

Study Insights and Implications

This study provides a systematic comparison of three commercially available overlay powders on a duplex stainless steel substrate, offering practical guidance for engineers selecting hardfacing materials for valve and pump components. The plasma hardfacing technique, with its high deposition efficiency, low dilution, and excellent metallurgical bonding, is well-suited for refurbishing worn valve components in the field. The finding that Ni55 provides the highest surface hardness while Co106F and Co112F offer comparable performance at potentially lower cost provides a clear decision framework for material selection. Future work should focus on long-term durability testing, including cyclic erosion-corrosion testing and thermal fatigue testing, to validate the laboratory findings under extended service conditions.