Zhuojin Pipe Fitting Co., LtdMIG Welding Process for Blast Furnace Through-Flow Pure Copper Tuyere
Literature Overview
This paper by Ma Changxuan and Sun Zhenping, published in Welding Technology (2010, Vol. 39, No. 3, pp. 67-68), addresses a practical production challenge in iron and steel enterprises: the repair and fabrication welding of through-flow pure copper tuyeres used in blast furnaces. The through-flow type tuyere is valued for its rapid water circulation, excellent heat conduction, and extended service life, making it a critical component in blast furnace operations. However, the authors identify that conventional welding methods frequently produce cracks, porosity, and incomplete fusion, resulting in unacceptably low product pass rates. The paper proposes a MIG (Metal Inert Gas) welding approach as a viable solution to these quality issues.
Core Technical Challenge Analysis
Pure copper tuyeres present unique welding difficulties that stem from the fundamental metallurgical and physical properties of copper and its alloys. Copper possesses extremely high thermal conductivity (approximately 398 W/(m·K) at room temperature), which causes rapid heat dissipation from the weld zone and makes it difficult to achieve sufficient melting and penetration. Additionally, copper has a low melting point relative to its solidus range, creating a narrow window between melting and overheating. The combination of these factors, together with the through-flow geometry that complicates water circuit integrity, makes welding quality control exceptionally demanding.
The authors categorize the principal defects as follows:
| Defect Type | Root Cause | Consequence |
|---|---|---|
| Cracking | High thermal stresses from rapid cooling; hydrogen embrittlement from moisture in flux or shielding gas | Loss of structural integrity; potential catastrophic failure under furnace operating conditions |
| Porosity | Gas entrapment due to insufficient shielding; oxidation of molten copper; hydrogen absorption from ambient moisture | Reduced effective cross-section; stress concentration sites; potential leak paths for cooling water |
| Incomplete Fusion | Insufficient heat input due to high thermal conductivity of copper; inadequate joint preparation | Weak bond interface; premature fatigue failure; water leakage |
MIG Welding Process Selection and Parameters
The selection of MIG welding (GMAW with solid wire) for this application reflects a deliberate engineering trade-off. Compared to SMAW (Shielded Metal Arc Welding), MIG offers higher deposition rates, better shielding gas coverage, and more consistent arc stability, all of which are critical for copper welding. The authors do not provide exhaustive parameter tables in the abstract, but the process logic can be reconstructed from standard copper welding practice:
| Parameter | Typical Range for Copper MIG | Rationale |
|---|---|---|
| Shielding Gas | Pure Ar (99.99%) or Ar-He mixtures | High ionization potential of He aids arc stability on high-conductivity metals |
| Wire Diameter | 1.0-1.6 mm solid copper wire | Balances deposition rate with arc control |
| Current Density | 400-600 A/mm² | High current density needed to overcome copper's thermal conductivity |
| Travel Speed | Moderate (100-200 mm/min depending on thickness) | Too fast: incomplete fusion; too slow: excessive distortion and cracking |
| Preheating | 200-400°C depending on joint thickness | Reduces thermal gradient and cooling rate; suppresses cracking |
| Interpass Temperature | Maintain above 200°C | Prevents excessive cooling between passes |
The through-flow geometry of the tuyere adds further complexity. The water passage must remain leak-proof, and the weld must withstand cyclic thermal loading from the blast furnace environment. This means that weld quality verification must include both mechanical testing and hydrostatic pressure testing.
Engineering Practice Insights
From a quality control perspective, this application demands a multi-layered inspection strategy. Visual inspection alone is insufficient for copper welds due to the tendency for subsurface porosity. Ultrasonic testing (UT) should be employed to detect internal voids, and hydrostatic testing at a pressure exceeding the maximum operating pressure by a safety factor of at least 1.5 is essential to verify water circuit integrity.
The authors' approach implicitly follows a PDCA (Plan-Do-Check-Act) cycle: identifying the quality problem (Plan), implementing MIG welding as the corrective measure (Do), verifying weld quality through testing (Check), and standardizing the process for production use (Act). This systematic approach is commendable and should be adopted by any enterprise facing similar copper welding challenges.
Key Reflections
This paper, while concise, highlights an important principle in welding engineering: the selection of welding process must be governed by the base material's physical properties rather than convention. Many foundries default to SMAW for copper repair work due to equipment availability, but the fundamental limitations of SMAW in achieving the heat input consistency required for copper are well documented. The MIG approach, with its continuous wire feed and controllable shielding, provides the process stability that copper demands.
For engineers involved in blast furnace maintenance, the key takeaway is that preheating strategy and interpass temperature control are non-negotiable for copper welding. The through-flow geometry further necessitates careful joint design to ensure full penetration and avoid water-side defects. I would recommend supplementing the authors' work with detailed microstructural analysis of the weld and HAZ to confirm that no deleterious phases have formed during the welding thermal cycle.