MIG Welding Process for Blast Furnace Through-Flow Pure Copper Tuyere

Literature Overview

This paper by Ma Changxuan and Sun Zhenping from Yantai Engineering Vocational and Technical College was published in Welding Technology in 2010. The research focuses on the MIG welding process for through-flow pure copper tuyeres used in blast furnaces. Blast furnace tuyeres are critical components that inject oxygen-rich air or oxygen into the furnace at high temperatures and pressures, and they are subjected to extreme thermal cycling, erosive gas flow, and mechanical stress. The use of pure copper for the tuyere body provides excellent thermal conductivity and resistance to thermal fatigue, but the welding of pure copper presents significant technical challenges due to its high thermal conductivity, low melting point, and susceptibility to hot cracking.

Core Technical Content

The through-flow tuyere design features a continuous flow path for the blast air, which requires a seamless internal surface to minimize pressure drop and erosion. The MIG welding process was selected for its high deposition rate and the ability to produce smooth, continuous weld beads suitable for the through-flow configuration. The welding was performed on pure copper (Cu-ETP or equivalent), which has a thermal conductivity of approximately 390 W/(m·K) at room temperature, more than three times that of carbon steel.

The authors addressed several key challenges in the MIG welding of pure copper tuyeres. The high thermal conductivity of copper causes rapid heat dissipation from the weld zone, requiring high heat inputs to achieve complete penetration. However, excessive heat input leads to large weld beads, excessive distortion, and hot cracking due to the limited solid solubility of impurities in copper. The authors optimized the welding parameters including current, voltage, travel speed, and wire feed speed to balance these competing requirements.

Interpretation of Key Technical Points

The most critical challenge in welding pure copper tuyeres is the prevention of hot cracking. Pure copper has a very narrow solidification range, which means that the weld metal solidifies almost entirely by the dendritic growth mechanism. This creates a microstructure that is highly susceptible to hot cracking, particularly transverse cracks that form perpendicular to the direction of solidification. The authors addressed this challenge through several measures: preheating the base metal to 150–250°C to reduce the cooling rate, using a multi-pass welding strategy with interpass temperature control, and selecting a wire composition that contains small amounts of silver or tellurium to refine the grain structure and reduce hot cracking susceptibility.

The through-flow design of the tuyere also presents a geometric challenge for welding. The internal flow path must be accessible for welding, and the weld beads must be smooth to minimize turbulence and erosion. The authors describe a welding technique that involves rotating the tuyere to maintain a horizontal welding position throughout the weld, which ensures consistent weld geometry and minimizes the risk of slag inclusion and porosity. The use of a horizontal position also allows gravity to assist in the flow of the molten metal, promoting a smoother weld surface.

The shielding gas selection is another critical aspect of the welding process. Pure argon is preferred for copper welding because it provides excellent shielding without introducing oxygen or other reactive gases that could oxidize the copper melt. However, the high thermal conductivity of copper requires a larger gas flow rate than would be needed for steel welding to ensure adequate shielding coverage. The authors recommend a gas flow rate of 20–25 L/min, which is higher than the typical 15–20 L/min used for steel MIG welding.

Engineering Practice Integration

In blast furnace operations, tuyeres are among the most heavily consumed components, typically requiring replacement every 100–200 days depending on the operating conditions. The welding quality of the tuyere directly affects its service life, as any defect in the weld zone can serve as a crack initiation site under the cyclic thermal and mechanical loading experienced during blast furnace operation. The authors report that the MIG-welded tuyeres achieved a service life comparable to that of continuously cast tuyeres, which is a significant achievement given the cost advantage of the welding approach.

The welding procedure developed in this study has been implemented in industrial production at several blast furnace operators in China. The key to successful implementation was the development of a detailed welding procedure specification that includes preheating requirements, interpass temperature limits, and post-weld heat treatment. The post-weld heat treatment involves annealing at 600–650°C for 1–2 hours, which relieves residual stresses and improves the ductility of the weld metal.

From a quality control perspective, the welded tuyeres are inspected using radiographic testing (RT) to detect internal defects such as porosity and hot cracks, and ultrasonic testing (UT) to detect subsurface cracks. The acceptance criteria are based on JB/T 4730 and are stricter than those for general structural welds, reflecting the critical role of the tuyere in blast furnace operation. The authors report that the defect rate in the welded tuyeres was below 2%, which is acceptable for industrial production.

Key Questions and Reflections

One question that arises from this study is the long-term durability of the MIG-welded tuyeres under the extreme conditions of blast furnace operation. The thermal cycling experienced by the tuyere can reach 1500–1600°C at the tuyere tip, with rapid cooling during water cooling cycles. The weld zone, being a region of compositional and microstructural heterogeneity, may be more susceptible to thermal fatigue than the base metal. Long-term monitoring of the welded tuyeres in service would provide valuable data on their durability and could inform future process improvements.

Another reflection concerns the potential for applying the MIG welding process to other copper components in metallurgical applications. The techniques developed for tuyere welding could be adapted for welding copper cooling plates in continuous casting machines, copper heat exchangers in chemical processing plants, and copper electrical busbars in power generation facilities. The key to successful adaptation would be the adjustment of welding parameters to account for differences in geometry, thickness, and service conditions.

Study Insights and Implications

This research demonstrates that the MIG welding process is a viable and cost-effective method for manufacturing through-flow pure copper tuyeres for blast furnace applications. The systematic approach to process development, including parameter optimization, defect prevention, and quality control, provides a template for the welding of other copper components in demanding industrial applications. The practical significance of this work is substantial, as it enables the production of high-quality copper tuyeres at a fraction of the cost of continuous casting, while maintaining comparable service life and reliability. For engineers working on metallurgical equipment manufacturing, this study provides valuable insights into the challenges and solutions associated with the welding of pure copper components.