Effect of Plasma Surfacing Current on Microstructure and Properties of Fe90 Alloy Overlay

Literature Overview and Research Significance

This comprehensive study by Deng Dewei, Liu Haiying, Zhang Lin, Sun Qi, Yang Shuhua, Song Maofu, Zheng Haitong, and Zhang Min, conducted at Dalian University of Technology and Shenyang Blower Works Group Research Institute, was supported by multiple prestigious funding sources including the National 973 Program (2011CB013402) and Liaoning Provincial Natural Science Foundation (2014028002). Published in the Journal of Heat Treatment of Materials in 2017, this research investigates the influence of plasma transferred arc (PTA) current parameters on the microstructure and mechanical properties of Fe90 surfacing alloy, a widely used material for enhancing the wear and corrosion resistance of centrifugal compressor components.

Core Technical Content and Experimental Design

The Fe90 alloy (Fe-90 type) is a high-alloy austenitic surfacing material containing significant amounts of chromium, nickel, and molybdenum, specifically designed for applications requiring simultaneous resistance to abrasive wear, cavitation erosion, and corrosive environments. In centrifugal compressor impellers and diffusers, the high-velocity gas flow containing particulate matter creates severe erosive wear conditions that demand specialized overlay protection.

The research systematically varied the plasma current from 100 A to 300 A while maintaining other parameters (travel speed, powder feed rate, arc distance) at constant values. This controlled experimental approach allows isolation of the current effect on solidification behavior, dilution, and microstructural development. The plasma surfacing process offers advantages over conventional arc welding methods, including lower dilution rates (typically 5-15%), reduced thermal input to the base material, and excellent process controllability.

Process Parameters and Their Effects

At lower current levels (100-150 A), the heat input is insufficient to fully melt the powder particles, resulting in incomplete fusion and porosity defects. The dilution rate remains low (5-10%), but the deposit quality suffers from lack of proper metallurgical bonding. At higher current levels (250-300 A), excessive heat input leads to increased dilution (15-25%), coarser microstructure, and potential base material damage due to thermal distortion.

Microstructural Evolution with Current Variation

The optimal current range of 180-220 A produces the most favorable microstructure for Fe90 alloy plasma surfacing. At these parameters, the cooling rate is approximately 50-200 K/s, resulting in a fine austenitic matrix with dispersed carbide particles (primarily M23C6 and M6C). The grain size is refined to 20-50 μm, providing good balance between hardness (35-45 HRC) and toughness.

At lower currents, incomplete melting creates unmelted powder particles embedded in the matrix, acting as stress concentrators and reducing fatigue resistance. At higher currents, the increased heat input promotes grain coarsening and carbide coarsening, reducing hardness but potentially improving toughness. The phase composition shifts from predominantly austenite with carbides at optimal parameters to martensite formation at excessive currents due to increased dilution from the base material.

Mechanical Properties and Performance Evaluation

The wear resistance, evaluated through dry sliding wear tests against alumina counterfaces, follows a similar trend to hardness, with the optimal current range providing the best performance. The cavitation erosion resistance, particularly important for compressor applications, also peaks in the optimal parameter window where the microstructure offers the best combination of hardness and toughness.

Engineering Practice and Application to Compressor Components

For centrifugal compressor impellers and diffusers, the Fe90 plasma surfacing overlay provides protection against erosive wear from high-velocity gas streams containing particulate matter. The typical overlay thickness is 1-3 mm, applied in multiple passes to ensure uniform coverage and minimize residual stresses. The process parameters must be carefully optimized for each component geometry to ensure complete coverage of complex surfaces while maintaining the desired microstructure.

The research findings have direct implications for manufacturing practice at Shenyang Blower Works, where the developed parameter windows have been implemented in production to extend component service life by 3-5 times compared to uncoated components. The standardized procedures derived from this research have been incorporated into the company's welding procedure specifications (WPS) for compressor component refurbishment.

Key Reflections and Study Insights

This research exemplifies the systematic approach required for optimizing plasma surfacing parameters for specific alloy systems. The comprehensive investigation of current effects provides practical guidance for engineers setting up PTA operations for Fe90 alloy applications. The findings emphasize that parameter optimization must consider not only mechanical properties but also process quality indicators such as porosity, dilution control, and surface quality. For the broader plasma surfacing community, this work contributes to the growing body of knowledge on processing-structure-property relationships in high-alloy overlay systems, enabling more informed decision-making in industrial applications.