Optimization Design of Iron-Based High-Temperature Wear-Resistant Plasma Arc Overlay Alloy Powder

Literature Overview

The paper by Ji Jie, Miao Hui, Liu Zhengjun, and Zhang Shusheng from Tianjin University and Shenyang University of Technology, published in Welding Technology in 1998, presents a systematic approach to the design and optimization of iron-based alloy powders for plasma transferred arc (PTA) overlay welding applications targeting high-temperature wear resistance. This early but foundational work addresses the critical challenge of developing overlay materials that maintain wear resistance at elevated operating temperatures, a requirement that is increasingly important in applications such as power generation equipment, cement kiln components, and metallurgical machinery. The research employs a combination of thermodynamic calculations, experimental trial, and microstructural analysis to identify optimal powder compositions.

Core Technical Content

The design methodology employed in this study is based on the principle of carbide type selection and matrix hardening optimization for high-temperature service. At elevated temperatures, the conventional carbide types found in iron-based overlay alloys (such as M23C6 and M7C3) may undergo coarsening, dissolution, or phase transformation, leading to a significant degradation of wear resistance. The researchers therefore focused on identifying carbide types that retain their hardness and stability at temperatures up to 600 degrees Celsius.

Thermodynamic Design Approach

The powder composition design was guided by phase equilibrium calculations using the Thermo-Calc software, which allowed the prediction of stable phases and their compositions at various temperatures. The key design parameters included the carbon equivalent, the Cr/Fe ratio, the addition of refractory carbide formers (W, Mo, V), and the selection of matrix alloying elements for solid solution strengthening.

The researchers identified that the optimal high-temperature wear-resistant powder composition should contain:

The inclusion of cobalt is particularly noteworthy as it significantly improves the red hardness (retained hardness at elevated temperatures) of the overlay matrix. Cobalt-based solid solution strengthening is less susceptible to thermal degradation compared to carbon-based strengthening, making it essential for high-temperature applications.

Microstructural Characterization

The resulting overlay deposits exhibited a complex microstructure consisting of a martensitic matrix with retained austenite, embedded with a mixture of M7C3 (Fe,Cr,V) carbides, WC-type carbides, and minor amounts of CrB and Mo2C. The M7C3 carbides appeared as irregular blocky particles with sizes ranging from 5 to 25 micrometers, while the WC-type carbides were finer (1 to 5 micrometers) and more uniformly distributed.

The volume fraction of carbides in the optimized composition reached approximately 35 to 45 percent, which is higher than typical values for conventional iron-based overlay alloys. This high carbide volume fraction is achievable with PTA welding due to the controlled dilution and the ability to use pre-blended powder compositions with precise chemical control.

High-Temperature Wear Performance

The wear resistance was evaluated using a high-temperature pin-on-disk tribometer at temperatures of 25, 300, 400, 500, and 600 degrees Celsius. The results demonstrated that the optimized alloy maintained 75 to 85 percent of its room-temperature wear resistance at 500 degrees Celsius, compared to only 40 to 50 percent retention for conventional Cr-C overlay alloys. This superior thermal stability is attributed to the combined effects of refractory carbides (WC, Mo2C) that resist dissolution at elevated temperatures, the cobalt-strengthened matrix that maintains solid solution strengthening, and the fine boride particles that pin grain boundaries and inhibit creep.

Process Parameters for PTA Overlay

The plasma transferred arc welding process parameters used in the study are summarized below:

The relatively low travel speed and high powder feed rate result in a high dilution ratio of approximately 10 to 15 percent, which is acceptable given the high alloy content of the powder. The addition of 2 to 5 percent hydrogen in the shielding gas serves to clean the arc and improve wetting, but care must be taken to avoid hydrogen-induced cracking in thick sections.

Defect Analysis and Countermeasures

During the PTA overlay process, several common defects were identified and their causes analyzed:

The most critical defect to control is cracking, which can occur in the overlay layer or at the overlay-base metal interface. The high carbon and alloy content of the powder increases the hardenability of the deposit, making it susceptible to cold cracking. The recommended countermeasures include preheating the base metal to 200 to 250 degrees Celsius, using a multi-pass sequence with reduced first-pass current, and applying a post-weld tempering treatment at 550 to 600 degrees Celsius for 2 hours.

Engineering Application Considerations

For engineering applications, the optimized powder composition is particularly suitable for components subjected to combined abrasive and adhesive wear at elevated temperatures, such as:

• Cement kiln wear plates and liners
• Coal mill classifier blades
• Furnace component linings
• Metallurgical equipment wear parts
• Power plant boiler tube components

The PTA process offers several advantages for these applications, including precise composition control, low dilution, high deposition efficiency, and the ability to build up thick overlay layers with uniform microstructure. However, the process requires specialized equipment and trained operators, which can increase the cost of application. For large-scale production, the powder should be pre-blended and certified to ensure consistent chemical composition and particle size distribution.

Key Questions and Reflections

A significant question that arises from this research is the scalability of the optimized composition to different welding processes. While PTA offers excellent control, many industrial applications require higher deposition rates that can only be achieved with processes such as submerged arc welding or flux-cored arc welding. The challenge lies in maintaining the precise composition and microstructure when transitioning from PTA to higher-deposition-rate processes, where dilution and cooling rate differences can significantly alter the final overlay properties.

Another important consideration is the cost-benefit analysis of using cobalt-containing alloys. While cobalt significantly improves high-temperature wear resistance, it also substantially increases the material cost. For applications where the operating temperature is below 400 degrees Celsius, a cobalt-free composition with higher vanadium and tungsten content may provide adequate performance at a lower cost. This trade-off must be carefully evaluated for each specific application.

Study Insights and Conclusions

The research by Ji Jie and colleagues represents an early but significant contribution to the field of high-temperature wear-resistant overlay alloy design. The systematic approach combining thermodynamic calculations with experimental validation provides a methodological framework that remains relevant for modern alloy development. The key insight that cobalt-strengthened matrices combined with refractory carbides (WC, Mo2C) can maintain wear resistance at elevated temperatures has been widely adopted in subsequent alloy development programs. For engineering practice, the recommended powder composition and PTA process parameters provide a reliable starting point for developing custom overlay solutions for high-temperature wear applications, while the defect analysis and countermeasures offer practical guidance for achieving consistent quality in production environments.