Zhuojin Pipe Fitting Co., LtdDevelopment and Abrasive Wear Performance of High-Hardness CO2-Shielded Surfacing Flux-Cored Wires with Tungsten
Literature Overview
This paper, published in China Surface Engineering in 2009 by Li Guodong and colleagues from Beijing University of Technology, reports on the development of a high-hardness flux-cored wire for CO2 gas-shielded surfacing welding. The research systematically investigates the effect of tungsten content on the microstructure, hardness, and abrasive wear resistance of the surfacing layer. Four compositions with tungsten contents of 3.26%, 4.13%, 6.15%, and 8.26% were studied. The findings are highly relevant to engineers working on wear-resistant overlay applications in pipe, fitting, and heavy-duty mechanical components where abrasive service conditions prevail.
Core Technical Findings
The surfacing layer microstructure consists primarily of martensite with retained austenite. Carbides appear as spherical particles dispersed throughout the matrix, a result of non-uniform solidification during the surfacing process. The tungsten content significantly influences both hardness and wear resistance, and the study identifies an optimal composition window.
| Tungsten Content | Average Hardness (HRC) | Wear Mass Loss (g) | Notes |
|---|---|---|---|
| 3.26% | Below 59 | Higher than 0.097 | Baseline composition |
| 4.13% | Below 59 | Higher than 0.097 | Intermediate composition |
| 6.15% | 59 HRC | 0.097 | Optimal composition identified |
| 8.26% | Slightly below 59 | Slightly lower than 0.097 | Diminishing returns on hardness |
The optimal tungsten content of 6.15% yields a post-heat-treatment hardness of 59 HRC with a wear mass loss of 0.097 g under wet abrasive testing. Increasing tungsten beyond this level results in a slight decrease in hardness but a marginal improvement in wear resistance, suggesting a complex interaction between carbide formation, retained austenite content, and the work-hardening capacity of the martensitic matrix.
Process and Metallurgical Analysis
The CO2 gas-shielded flux-cored wire surfacing process was selected for its combination of high deposition efficiency, good dilution control, and field applicability. The flux core contributes alloying elements and stabilizes the arc, while the CO2 shield provides economic protection against atmospheric contamination. The formation of spherical carbides indicates that the solidification rate during single-pass surfacing allows for sufficient diffusion, producing relatively coarse but well-dispersed hard phases. This is characteristic of rapid cooling in a thick deposit where the heat input is relatively high compared to the wire feed rate.
The martensitic microstructure with retained austenite is critical for wear performance. The retained austenite serves as a reservoir that can transform to martensite under impact or abrasion loading, providing a built-in work-hardening mechanism. This is particularly valuable in applications involving sliding contact with hard particles, such as pipe elbows, reducer cones, and valve seats in slurry-handling systems.
The tungsten carbide (WC) formed in the deposit is a key wear-resisting phase. Tungsten's high melting point and strong carbon affinity ensure that WC remains stable even at elevated service temperatures. However, excessive tungsten can lead to the formation of brittle intermetallics or promote microcracking due to thermal expansion mismatch with the iron matrix. This explains why hardness peaks at 6.15% W and slightly declines at higher contents.
Engineering Practice Integration
For pipe and fitting manufacturers, this research provides a practical foundation for selecting surfacing consumables for wear-critical components. In oil and gas pipelines, slurry transport pipes, and mining equipment, elbows and tees subjected to abrasive flow often fail prematurely. Applying a tungsten-bearing flux-cored wire overlay can extend service life significantly. The 59 HRC hardness level achieved is comparable to or exceeds that of many through-hardened alloy steels, while retaining sufficient toughness to resist spalling under impact loading.
When implementing this wire in production, several process parameters must be controlled:
- Preheat temperature should be maintained between 100 and 150°C to reduce the risk of hydrogen-induced cracking in the heat-affected zone.
- Interpass temperature should not exceed 250°C to ensure adequate martensite formation and avoid excessive grain growth.
- Post-weld heat treatment, such as tempering at 500 to 600°C, should be applied to relieve residual stresses while maintaining hardness above 55 HRC.
- Wire feed speed and travel speed must be optimized to achieve a single-pass deposit thickness of 3 to 5 mm, which balances dilution control with mechanical performance.
Key Questions and Reflections
A critical question arising from this study is the long-term stability of the retained austenite under cyclic loading. While retained austenite contributes to work-hardening during initial abrasion, repeated loading may lead to progressive transformation and eventual loss of this beneficial phase. Engineers should consider this when designing surfacing layers for components experiencing sustained cyclic contact stress, such as pump impellers or crusher hammers.
Another consideration is the dilution effect. In multi-pass surfacing, the first pass experiences the highest dilution from the base metal, which can reduce the effective tungsten content in the deposit and lower the achieved hardness. Subsequent passes experience lower dilution and may produce harder layers. This layer-to-layer variability must be accounted for in quality control, particularly when specifying minimum hardness requirements for critical surfaces.
Study Insights and Implications
This study demonstrates that tungsten is an effective alloying element for enhancing the abrasive wear resistance of CO2-shielded flux-cored wire surfacing deposits. The identified optimal composition of 6.15% W provides a practical design target for consumable manufacturers. The combination of high hardness (59 HRC) with low wear mass loss (0.097 g) indicates excellent performance for applications involving sliding abrasion with hard particles.
The research also highlights the importance of understanding the relationship between microstructure and wear mechanism. The spherical carbides dispersed in a martensitic matrix provide a synergistic combination of hardness and toughness. This insight can be extended to other alloying systems, such as chromium carbide or boride-based surfacing alloys, where similar microstructural optimization can be pursued.
For the pipeline and fitting industry, this work supports the development of cost-effective repair and enhancement strategies for wear-critical components. Rather than replacing entire elbows or tees, surfacing with tungsten-bearing flux-cored wire can restore or exceed original performance at a fraction of the cost. This aligns with sustainability goals and reduces downtime in critical process plants.