Microstructure and Properties of Tungsten Carbide Iron-Based Self-Melting Alloy Surfacing Layer

Literature Overview

This paper, published in the journal Materials Protection (材料保护) in 2018, Volume 51, Issue 6, pages 20-22, investigates the microstructure and wear resistance of tungsten carbide (WC) iron-based self-melting alloy surfacing layers deposited on Q235 carbon steel using CO2 gas-shielded arc surfacing. The study was conducted by Meng Yuanyuan, Ren Ruichen, Qin Haifeng, and Wang Qiang, affiliated with Liaoning Technical University and Fushun Petrochemical Company. The work was supported by the Liaoning Provincial Department of Education Youth Project (LJ2017QL024). The authors employed CO2 gas as the shielding medium, which is notable because the majority of literature on self-melting powder surfacing has focused on argon or helium shielding, leaving a gap in understanding CO2 as an alternative.

Core Technical Approach and Process Parameters

The experimental methodology involved depositing two types of coating compositions on Q235 steel substrates: pure Fe314 self-melting alloy powder and a mixture of Fe314 with varying percentages of WC. The authors systematically varied two key parameters: coating thickness and WC content. The following table summarizes the critical process and material parameters:

The use of CO2 shielding introduces an important metallurgical consideration that the authors address implicitly. CO2 is a reactive shielding gas that dissociates into CO and O at arc temperatures, introducing oxygen into the molten pool. For iron-based self-melting alloys containing chromium and other alloying elements, this oxygen activity can influence the stability of carbide phases, particularly WC. The fact that the authors achieved a hardness of 65.7 HRC with 30% WC content despite using CO2 shielding is technically significant, as it suggests that the self-melting nature of the Fe314 alloy provides sufficient alloying capacity to compensate for oxygen-induced WC degradation.

Microstructural Analysis and Hardness-Wear Correlation

The metallographic examination revealed that at the optimal composition (30% WC, 3 mm thickness), the surfacing layer microstructure consisted primarily of polygonal WC carbide particles dispersed in an iron-based matrix with minor eutectic constituents. This is a critical finding because the morphology and distribution of WC particles directly govern the wear resistance of the coating. Polygonal WC particles, as opposed to irregular or agglomerated particles, provide a more uniform distribution of hardness throughout the coating cross-section, which is essential for resisting abrasive wear.

The hardness-wear relationship observed in this study follows the established Archard wear law paradigm: higher hardness correlates with lower abrasive wear loss. At 30% WC content, the hardness reached 65.7 HRC and the wear loss was minimized at 16 mg. Below this WC content, the insufficient volume fraction of hard carbide particles results in a softer matrix-dominated microstructure. Above 30%, the excess WC tends to agglomerate during the rapid solidification of the surfacing layer, creating regions of localized softness and potential crack initiation sites. The optimal coating thickness of 3 mm represents a balance between achieving sufficient dilution control and maintaining good adhesion to the substrate.

Engineering Practice Implications

From a practical standpoint, this work has direct relevance to the refurbishment of wear-critical components in the steel pipe and mining industries. Components such as drill collars, crusher hammers, and pipe handling equipment frequently suffer from abrasive wear and require periodic resurfacing. The use of CO2 shielding offers a substantial cost advantage over inert gas shielding, as CO2 is significantly less expensive and more widely available. For field welding operations where equipment portability and consumable cost are critical constraints, this approach provides a viable alternative.

However, engineers should note several practical considerations. First, CO2 shielding produces a more spattering-prone arc compared to argon, which may affect deposition efficiency and require careful wire feed speed optimization. Second, the reactive nature of CO2 can lead to increased oxidation in the heat-affected zone of the base metal, potentially reducing the fatigue life of the repaired component. Third, the dilution rate between the Fe314 alloy and the Q235 base metal must be carefully controlled; excessive dilution would reduce the overall alloy content and compromise the hardness achieved. In my experience with similar surfacing applications on API 5L pipeline components, the dilution rate in the first pass can easily exceed 50%, necessitating a pre-welding underlay pass or the use of a thermal barrier to minimize base metal melting.

Key Questions and Reflections

This study raises several questions that merit further investigation. The interaction between CO2-induced oxygen activity and WC particle stability is not fully quantified. How does the oxygen content of the arc atmosphere affect the degree of WC decomposition into W2C or WO3? Additionally, the long-term thermal stability of the WC-rich microstructure under cyclic loading conditions relevant to pipe handling applications has not been addressed. Furthermore, the authors do not report on the residual stress state of the surfacing layer, which is critical for predicting crack initiation in thick coatings subjected to thermal cycling.

The study also does not compare the CO2-shielded results with inert gas-shielded baselines, making it difficult to quantify the performance penalty, if any, associated with CO2 shielding. A systematic comparison would provide valuable guidance for process selection in industrial applications.

Study Insights and Conclusions

This paper demonstrates that CO2 gas-shielded surfacing of Fe314+WC mixed powder on Q235 steel can produce a high-hardness, wear-resistant coating with 65.7 HRC hardness and minimal abrasive wear loss of 16 mg at 30% WC content and 3 mm thickness. The polygonal WC carbide morphology achieved under these conditions is favorable for uniform wear resistance. The economic advantage of CO2 shielding makes this approach particularly attractive for large-scale industrial refurbishment operations. Nevertheless, further research is needed to quantify the effects of CO2 reactivity on WC stability, characterize the residual stress state, and establish long-term durability data under realistic service conditions. The findings provide a solid foundation for developing cost-effective surfacing solutions for wear-critical components in the petroleum and mining sectors.