CLADDING TECHNOLOGY SHANXI CO., LTDCold Welding Overlay of 100m³ Blast Furnace Bell Surface
Literature Overview
This 1997 technical paper by Yang Wenkui from Shandong Metallurgical Machinery Factory presents a cold welding process for surface overlay of a 100 m³ blast furnace bell. Blast furnace bells are critical components that regulate the charging of raw materials into the furnace. The bell surface endures severe abrasion from falling iron ore, coke, and flux materials, as well as thermal cycling from the hot furnace atmosphere. The term "cold welding" in this context likely refers to a low-heat-input welding process that minimizes thermal distortion of the large bell component, rather than true cold welding (solid-state bonding).
Core Technical Analysis
The Challenge of Large Component Overlay
The 100 m³ blast furnace bell is a large cast or fabricated component, typically weighing several tons. The primary challenges in overlaying its working surface include:
- Minimizing thermal distortion of the bell geometry
- Achieving uniform overlay thickness across a large curved surface
- Preventing cracking in the overlay layer due to residual stresses
- Maintaining the bell's mechanical integrity for pressure containment
Process Interpretation
The "cold welding" designation in the Chinese metallurgical context of the 1990s most likely refers to one of the following processes:
| Process | Heat Input | Suitability for Large Components |
|---|---|---|
| Flux-cored arc welding (FCAW) | Moderate | Good for field repair |
| Submerged arc welding (SAW) | High | Requires position control |
| Oxy-acetylene flame welding | Low | Suitable for curved surfaces |
| Plasma transferred arc (PTA) | Low to moderate | Excellent for large surfaces |
| Cold spray (if applicable) | Very low | Limited availability in 1997 |
Given the era and the industrial context, the most probable interpretation is that the author employed either oxy-acetylene flame welding or a low-heat-input arc welding process to minimize thermal effects on the bell's structural geometry.
Overlay Material Selection
For blast furnace bell surfaces, the overlay material must resist abrasion from angular ore particles and withstand intermittent thermal shock. Suitable materials include:
- High-carbon chromium cast iron (Fe3Cr2Mo, Fe3Cr2W) for abrasive wear resistance
- Stellite-type cobalt-based alloys (Stellite 6, Stellite 21) for combined wear and corrosion resistance
- Medium-carbon martensitic stainless steel (e.g., 410, 420) for moderate wear with good toughness
The hardness target for the overlay layer is typically 45–60 HRC, with a minimum thickness of 5–10 mm to ensure adequate wear life.
Process Control Measures
To achieve successful overlay on a large bell component, the following process controls are essential:
- Surface preparation: Thorough grinding of the substrate surface to a metallic finish, removal of scale, rust, and oil contamination
- Preheating: Localized preheating to 100–200°C to reduce thermal gradient, but avoiding global heating that would distort the bell
- Welding sequence: Systematic coverage pattern to ensure uniform heat distribution and minimize directional distortion
- Interpass temperature control: Monitoring and limiting interpass temperature to below 250°C to prevent softening of the previous pass
- Post-weld treatment: Controlled cooling (wrapping in insulation or furnace cooling) to minimize residual stress
Standards and Inspection Requirements
| Inspection Method | Purpose | Acceptance Criteria |
|---|---|---|
| Visual examination (VT) | Surface quality, undercut, porosity | No cracks, no undercut > 0.5 mm |
| Magnetic particle testing (MT) | Surface and near-surface cracks | No linear indications > 2 mm |
| Ultrasonic testing (UT) | Bond strength, internal defects | 100% bond across overlay/substrate interface |
| Hardness testing | Overlay hardness uniformity | 45–60 HRC across full thickness |
| Dilution measurement | Composition control | Base metal dilution < 30% |
Engineering Practice Implications
The blast furnace bell overlay work illustrates the intersection of metallurgical engineering and heavy equipment maintenance. The bell is a safety-critical component—failure could result in catastrophic furnace shutdown and potential environmental hazards. The overlay must therefore be performed to a high standard, with rigorous quality control at every stage.
The economic case for overlay repair is compelling: a new 100 m³ blast furnace bell can cost hundreds of thousands of yuan, while overlay restoration costs a fraction of that amount. However, the repair must be executed with precision, as any geometric distortion or bonding defect could compromise the bell's seal integrity.
Key Reflections
The use of "cold welding" terminology in this context highlights an important engineering communication issue. The term is ambiguous and could be misinterpreted by readers unfamiliar with the Chinese metallurgical literature of the period. Modern documentation would specify the exact welding process (e.g., "low-heat-input plasma arc overlay" or "oxy-acetylene flame hardfacing") to eliminate ambiguity. This case reinforces the importance of precise process nomenclature in technical documentation.