Plasma Arc Surfacing of Wear-Resistant Powder Coatings on Low Carbon Steel

Literature Overview

The study by Shi Duanhu, Lu Mengyun, Sha Jing, and Yang Feng from Xuzhou Institute of Technology, published in the Journal of Xuzhou Institute of Technology (Natural Science Edition) (Vol. 32, Issue 4, 2017, pp. 13–17), investigates the plasma arc surfacing process for depositing Fe90 iron-based wear-resistant powder coatings on low carbon steel substrates. Funded by the Jiangsu Provincial Natural Science Foundation (BK20141143) and the Jiangsu Provincial University Natural Science Research Key Project (16KJA430003), this research systematically examines how welding process parameters influence the hardness and wear resistance of the deposited coatings.

Core Technical Findings

Process Parameter Optimization

The authors conducted a systematic parameter study, varying powder feeding rate, welding current, shielding gas flow rate, ion gas flow rate, and powder carrier gas flow rate. The optimization was performed under the constraint that other parameters remained constant while one parameter was varied at a time, following a controlled experimental design approach.

The optimal welding parameters determined through this study are summarized below:

Powder Feeding Rate Effects

Increasing the powder feeding rate leads to higher coating hardness and reduced wear volume. This relationship can be explained by the increased dilution ratio of the alloy powder relative to the base metal. At higher feeding rates, a greater proportion of the deposited material originates from the alloy powder rather than the melted base metal, resulting in a coating composition that more closely matches the designed alloy chemistry. The Fe90 powder contains alloying elements and hard-forming constituents that contribute to elevated hardness when their concentration in the coating is maximized.

However, the powder feeding rate cannot be increased indefinitely. Excessive feeding rates can lead to incomplete melting of the powder particles, resulting in unmelted inclusions, poor metallurgical bonding, and reduced coating integrity. The optimal value of 25 g/min represents a balance between maximizing alloy content and maintaining adequate melting and fusion.

Welding Current Effects

Increasing the welding current also improves both hardness and wear resistance. A higher current produces a more intense heat input, which promotes more complete melting of both the powder particles and the underlying base metal. This enhanced melting leads to better metallurgical bonding between the coating and the substrate, reducing the risk of delamination during service. Additionally, the higher current increases the dilution rate, which, while generally considered undesirable for maintaining coating composition, in this case contributes to a more homogeneous microstructure with fewer unmelted particles.

The bonding quality between the coating and substrate is a critical factor in coating life. The study confirms that higher welding currents improve this bonding, which directly translates to longer service life. This finding is particularly relevant for applications where the coating is subjected to impact loading or thermal cycling, conditions that can cause poorly bonded coatings to spall.

Microstructural Analysis

The Fe90 iron-based powder typically contains iron as the base element with additions of chromium, molybdenum, carbon, and other alloying elements. Upon melting and solidification during plasma arc surfacing, the coating microstructure develops a matrix of martensite and bainite with dispersed carbide phases. The hardness of the coating, which is the primary indicator of wear resistance, is governed by the volume fraction, size, and distribution of these hard carbide phases within the matrix.

The plasma arc surfacing process offers several advantages over other surfacing methods for this application. The focused plasma jet provides a highly concentrated heat source with a narrow heat-affected zone, which minimizes thermal distortion of the base component. The process also allows for precise control of the heat input, enabling the operator to optimize the solidification rate and, consequently, the microstructure of the coating.

Engineering Practice Applications

Plasma arc surfacing of iron-based wear-resistant coatings is widely applied in the steel pipe and pipe fitting industry for several critical applications:

• Repair of worn mandrel dies and expanding mandrels in pipe production lines
• Protection of pipe cutting tools and shearing blades
• Restoration of worn coupling surfaces on pipe handling equipment
• Surface hardening of low carbon steel components that require localized wear resistance

The parameters identified in this study are directly applicable to production environments. The shielding gas flow rate of 9 L/min ensures adequate protection against atmospheric contamination, while the ion gas flow rate of 0.6 L/min maintains a stable, focused plasma arc. Engineers should note that these values may need adjustment depending on the specific equipment, ambient conditions, and coating thickness requirements.

The wear testing methodology used in this study provides a quantitative basis for comparing different coating systems. The wear volume measurement allows for direct comparison of coating performance, which is essential for materials selection decisions. Engineers should ensure that wear testing is conducted under conditions that replicate the actual service environment, including load magnitude, sliding speed, temperature, and the presence of abrasive particles.

Study Insights and Reflections

This paper exemplifies the practical engineering approach to surfacing process development. Rather than pursuing fundamental metallurgical understanding alone, the authors focused on establishing a clear relationship between process parameters and coating performance. The resulting parameter window is immediately applicable in production settings, which is the primary objective of applied welding research.

One limitation of the study is the single-parameter variation approach. In practice, process parameters interact with each other, and the optimal setting for one parameter may depend on the values of others. A full factorial or response surface methodology approach would provide a more comprehensive understanding of the parameter space. However, for practical process development, the approach used here is efficient and sufficient to establish a workable process window.

The finding that higher powder feeding rates improve hardness and wear resistance has important economic implications. Fe90 iron-based powder is significantly less expensive than nickel-based or cobalt-based surfacing alloys. The ability to achieve good wear resistance with an iron-based powder, using parameters that maximize the alloy content in the coating, represents a cost-effective solution for many industrial applications. This is particularly relevant for large components where extensive surfacing areas would make nickel-based coatings prohibitively expensive.

The study also highlights the importance of substrate preparation in surfacing applications. The bonding quality between the coating and the low carbon steel substrate is directly related to the welding current and heat input. In practice, this means that thorough surface cleaning, proper fit-up, and possibly preheating are essential steps that should not be neglected. Any contamination or oxide on the substrate surface will compromise the metallurgical bond, regardless of how well the welding parameters are optimized.

In conclusion, this research provides a solid foundation for the plasma arc surfacing of iron-based wear-resistant coatings on low carbon steel components. The optimized parameters offer a practical starting point for production implementation, and the underlying principles regarding powder feeding rate and welding current effects can be extended to other iron-based powder compositions and substrate materials. Engineers working on pipe, fitting, and heavy equipment maintenance should consider this approach as a cost-effective alternative to more expensive coating systems when the service conditions are compatible with iron-based alloy performance.