Study Note on Microstructure and Hardness of Plasma Hardfacing Different Powders on Duplex Stainless Steel

Literature Overview

This 2023 paper by He Tao, Ma Jun, Wang Jianlong, Chen Jiaqi, Meng Fanmin, and Deng Dewei, published in Physical Testing and Chemical Analysis (Vol. 59, Issue 5, pp. 9-15), investigates the microstructure and hardness of plasma hardfacing deposits on F51 duplex stainless steel using three different alloy powders: Co106F, Co112F, and Ni55. The work was conducted at Wuzhong Instrument Co., Ltd. and Dalian University of Technology, supported by funding from the High-End Control Valve Industry Technology Collaborative Innovation Center and the Liaoning Major Equipment Manufacturing Collaborative Innovation Center. The study is particularly relevant to the control valve industry, where duplex stainless steel components require enhanced surface properties for erosion-corrosion resistance in demanding process environments.

Core Technical Points

Plasma Hardfacing Process Characteristics

Plasma hardfacing (plasma transferred arc overlay welding, PTAW) offers several advantages over conventional arc hardfacing for this application. The high energy density of the plasma arc (typically 1-10 MW/m²) produces a narrow, deep weld pool with minimal dilution of the base metal. This is critical when hardfacing duplex stainless steel, as excessive dilution can disrupt the austenite-ferrite balance and compromise the corrosion resistance of the base material. The powder feeding system allows for precise control of the alloy composition of the deposited layer, enabling tailored microstructures for specific service conditions.

Microstructure Analysis of Deposited Layers

The study reveals distinct microstructural differences among the three hardfacing alloys:

• Co106F and Co112F deposits exhibit similar microstructures composed primarily of dendritic grains. The cobalt-chromium matrix with carbide precipitates provides good erosion resistance and moderate corrosion resistance.
• Ni55 deposits show a more compact grain structure with columnar grains appearing at the interface between the first and second deposited layers. The columnar grain formation at the layer boundary is attributed to supercooling effects during the second pass, where the partially solidified first layer acts as a heat sink, promoting directional solidification.

Element Diffusion and Interface Behavior

The line scan analysis reveals the diffusion of elements between the hardfacing layer and the duplex stainless steel substrate. Chromium and nickel diffusion across the interface is expected, but the extent of dilution is limited by the low dilution rate inherent to plasma hardfacing. This is a significant advantage for maintaining the corrosion resistance of the F51 substrate, which relies on a balanced austenite-ferrite microstructure (typically 40-60% ferrite). Excessive dilution could shift this balance and reduce pitting resistance.

Process and Standards Analysis

The plasma hardfacing process parameters are critical to achieving the desired microstructure and properties. Key parameters include:

• Plasma arc current: Typically 100-200 A for overlay applications
• Powder feed rate: 5-15 g/min, depending on desired layer thickness
• Travel speed: 10-30 cm/min
• Shielding gas: Argon, with possible addition of nitrogen or helium
• Layer thickness: Typically 0.5-1.5 mm per pass

The two-layer deposition strategy used in this study is a common practice to ensure adequate thickness and uniformity. The first layer provides initial coverage and bonding, while the second layer builds thickness and improves surface quality. The columnar grain formation observed at the first-second layer interface is a known phenomenon in multi-pass overlay welding and can be mitigated by varying the deposition direction between passes or by interpass cooling.

Integration with Engineering Practice

For control valve manufacturers working with duplex stainless steel, this study provides valuable guidance on selecting the appropriate hardfacing alloy for specific service conditions:

• Co106F is suitable for moderate erosion-corrosion environments where cost is a primary concern. Its lower hardness and cost make it appropriate for valves in less aggressive service.
• Co112F offers slightly higher hardness and is a good choice for applications requiring enhanced erosion resistance, such as slurry service or high-velocity flow conditions.
• Ni55 provides the highest hardness and is ideal for severe erosion-corrosion environments, such as those found in oil and gas production systems with high-velocity sand-laden fluids.

The hardness improvement of 1.5-1.62 times the base material is substantial and directly translates to extended component life in erosive service. However, engineers should be aware that increased hardness can sometimes correlate with reduced toughness, which may be a concern in applications subject to thermal cycling or impact loading.

Key Questions and Reflections

An important question arising from this study is the long-term corrosion resistance of the hardfacing deposits in chloride-containing environments. While the hardness improvement is clearly documented, the study does not address electrochemical corrosion behavior. For duplex stainless steel applications in the oil and gas industry, pitting and crevice corrosion in chloride environments remain critical concerns. I would recommend complementary studies on the electrochemical behavior of these hardfacing layers, including potentiodynamic polarization tests and accelerated corrosion testing in simulated process fluids.

Another consideration is the effect of the hardfacing process on the duplex balance of the F51 substrate in the heat-affected zone (HAZ). The plasma arc's high energy density can potentially shift the austenite-ferrite ratio in the HAZ, which could affect the corrosion resistance of the base material near the deposit. This should be evaluated through metallographic examination of the HAZ region, using ferrite number measurements or quantitative image analysis.

Study Insights and Implications

This study demonstrates the versatility of plasma hardfacing as a surface engineering tool for enhancing the performance of duplex stainless steel components. The ability to tailor the deposit composition and microstructure through powder selection provides a flexible approach to addressing specific service challenges. For engineers in the control valve industry, this work highlights the importance of matching the hardfacing alloy to the specific erosion-corrosion conditions of the service environment. The Ni55 powder's superior hardness makes it the preferred choice for severe service, while Co106F offers a cost-effective solution for moderate conditions. Future work should focus on comprehensive performance evaluation including corrosion resistance, thermal cycling behavior, and long-term durability in representative process environments.