Surface Overlay Welding Process for Blast Furnace Tuyere Sleeve Life Extension

Literature Overview and Application Context

The paper published in Special Casting and Nonferrous Alloys (2016, Vol. 36, No. 5) by Lu Longwen and colleagues from Wuhan Iron and Steel Group and Huazhong University of Science and Technology addresses a critical industrial challenge: extending the service life of blast furnace tuyere sleeves through surface overlay welding of high-temperature wear-resistant alloys. This research was supported by a technical cooperation project between Wuhan Iron and Steel Co., Ltd. and the research institutions (Project No. 13-060). Blast furnace tuyere sleeves are among the most severely consumed components in ironmaking operations, subjected to simultaneous erosion by high-velocity blast gas, chemical attack from hot gases, and thermal cycling, making their failure a major contributor to unplanned furnace downtime and production losses.

Failure Analysis and Process Development

The study begins with a systematic analysis of the usage conditions and primary failure modes of tuyere sleeves in blast furnaces. The main failure mechanisms include erosion by refractory particles carried in the blast stream, oxidation and decarburization at elevated temperatures, and thermal fatigue cracking due to repeated heating and cooling cycles. Based on this failure analysis, the authors proposed an arc cladding process to deposit high-temperature wear-resistant alloy material on the inner surface of the tuyere sleeve.

The material architecture consists of a two-layer structure:

• Transition layer: JSTNi6382 alloy, designed to provide good metallurgical bonding with the steel substrate while accommodating thermal expansion mismatch.
• Surface wear layer: A high-temperature wear-resistant alloy designated as D450 or D600, providing the primary erosion and oxidation resistance.

Performance Characterization Results

The cladding layers were characterized through metallographic examination, hardness testing, wear resistance testing, and thermal shock resistance evaluation. Hardness measurements were conducted along the fusion direction at equidistant intervals, revealing a gradient increase in hardness from the substrate interface toward the surface. This gradient is metallurgically expected and beneficial, as it provides a smooth transition in mechanical properties that reduces the risk of interfacial cracking under thermal and mechanical loading.

The wear resistance results are particularly striking: at room temperature, the D450 alloy exhibited 5.8 times the wear resistance of red copper, while the D600 alloy achieved 17 times the wear resistance of red copper. More importantly, the D450 sample demonstrated thermal shock resistance significantly superior to that achieved by plasma spraying processes. This comparison is critical because plasma spraying is a commonly used alternative for surface protection of tuyere sleeves, and the superior thermal shock performance of the arc cladding approach suggests better metallurgical bonding and reduced spallation risk under thermal cycling conditions.

Process Analysis and Standards Considerations

The arc cladding process offers several advantages over alternative surface modification techniques for this application:

1. Metallurgical bonding: Unlike thermal spray processes that rely on mechanical adhesion, arc cladding creates a true metallurgical bond between the overlay and substrate, which is essential for withstanding the thermal cycling and erosive conditions in blast furnaces.
2. Thermal shock resistance: The metallurgical bond and the gradient hardness profile contribute to superior thermal shock resistance, reducing the likelihood of spalling during furnace campaigns.
3. Scalability: Arc cladding is well-suited to the relatively large dimensions of tuyere sleeves and can be performed using standard industrial welding equipment.

However, engineers must be aware of potential challenges including:

• Dilution control: Ensuring adequate dilution between the transition layer and substrate to prevent cracking while maintaining sufficient alloy content in the surface layer.
• Residual stress management: The significant thermal gradients during cladding can induce residual stresses that may compromise service life. Post-weld stress relief heat treatment should be considered.
• Uniformity: Achieving consistent cladding thickness and composition across the entire tuyere sleeve surface requires careful process parameter control.

Integration with Engineering Practice

For steelmaking engineers responsible for blast furnace maintenance, this research provides a validated approach to extending tuyere sleeve life. The key implementation considerations include:

1. Pre-weld preparation: Thorough cleaning of the tuyere sleeve surface to remove scale, oxide, and contaminants to ensure good bonding.
2. Process parameter optimization: Arc current, voltage, travel speed, and wire feed rate must be optimized to achieve the desired dilution ratio and microstructure.
3. Quality control: Non-destructive testing (MT or PT) of the cladding layer surface to detect cracks and porosity, combined with hardness profiling to verify the gradient structure.
4. Service monitoring: Regular inspection of cladding layer thickness during furnace campaigns to predict remaining service life and schedule maintenance.

The selection between D450 and D600 surface layers should be based on the specific operating conditions of the blast furnace. D450 offers a balanced combination of wear resistance and thermal shock resistance suitable for moderate erosion conditions, while D600 provides superior wear resistance for severe erosion environments but may require additional consideration of thermal shock susceptibility.

Key Questions and Study Reflections

A significant question is the long-term performance of the cladding layer under actual blast furnace operating conditions, which involve temperatures exceeding 1000°C, high gas velocities, and continuous chemical attack. Room temperature wear resistance data, while informative, may not fully represent the behavior at elevated temperatures where oxidation, sintering, and phase transformations can alter the microstructure and properties.

Additionally, the cost-effectiveness of the two-layer cladding approach compared to alternative solutions such as copper-lined tuyeres or advanced refractory materials should be evaluated in the context of total cost of ownership, including downtime costs, material costs, and labor costs for application and maintenance.

Study Insights and Implications

This research exemplifies the integration of failure analysis, materials selection, and process engineering in solving a real-world industrial problem. The two-layer design philosophy—transition layer for bonding compatibility and surface layer for performance—is a principle that can be extended to other overlay applications involving dissimilar materials or extreme service conditions. The superiority of arc cladding over plasma spraying in thermal shock resistance highlights the fundamental advantage of metallurgical bonding over mechanical adhesion in thermally severe environments. For practitioners, the key takeaway is that surface protection solutions must be evaluated not only on their room temperature properties but also on their behavior under the actual thermal and mechanical conditions of service, and that the bonding mechanism between overlay and substrate is a critical determinant of long-term performance.