Application of Carbide Overlay Welding on Blast Furnace Bell and Hopper Components

Literature Overview

The study by Ma Lingyun, published in 2000 in the journal "Shandong Metallurgy" (山东冶金), reports on the application of carbide overlay welding for the repair of the bell and hopper components of a 125 m³ blast furnace at the Sintering Plant of Laiwu Steel Group Co., Ltd. Blast furnaces are the core equipment in the iron and steelmaking industry, and the bell and hopper assembly is a critical component of the charging system that controls the descent of raw materials (iron ore, sinter, and coke) into the furnace. These components are subjected to severe abrasive wear from the falling materials, thermal cycling from the hot furnace gases, and mechanical impact from the material flow. The application of carbide overlay welding represents a practical approach to extending the service life of these wear-prone components.

Core Technical Analysis

Operating Conditions and Wear Mechanisms

The bell and hopper of a blast furnace operate under the following conditions:

• Abrasive wear: The falling iron ore, sinter, and coke particles (typically 10-50 mm in size) impact the bell and hopper surfaces at velocities of 5-15 m/s, causing severe abrasive wear. The wear rate can be 5-15 mm per year on unprotected carbon steel surfaces.
• Thermal cycling: The components are exposed to furnace gas temperatures of 200-400°C, with periodic cooling during maintenance shutdowns. The thermal cycling causes thermal fatigue cracking, which accelerates the wear process.
• Mechanical impact: The continuous impact of falling materials causes plastic deformation and work hardening of the surface, which can lead to micro-cracking and spalling.
• Chemical erosion: The furnace gases contain CO, CO₂, SO₂, and H₂S, which can cause chemical attack on the steel surface, particularly at elevated temperatures.

The combined effect of these mechanisms results in significant material loss on the bell and hopper surfaces, particularly in the areas directly exposed to the falling material stream. The wear pattern is typically concentrated on the lower cone of the hopper and the inner surface of the bell, where the material impact is most intense.

Carbide Overlay Material Selection

The selection of carbide overlay material for blast furnace bell and hopper repair requires consideration of the following factors:

• Hardness: The overlay must have sufficient hardness to resist abrasive wear. Hardness values of 50-60 HRC are typically required for severe abrasion conditions.
• Toughness: The overlay must have adequate toughness to resist cracking and spalling under impact loading. Excessive hardness without sufficient toughness leads to brittle failure.
• Bond strength: The overlay must have strong bonding to the base metal to prevent delamination under the cyclic impact loading.
• Weldability: The overlay material must be weldable with the available equipment and consumables.

The following table presents the typical carbide overlay materials used for blast furnace applications:

For the blast furnace bell and hopper application, a multi-layer approach is often recommended, with a Cr-C layer providing the primary wear resistance and a Ni-Cr-C or Co-Cr-C layer providing the toughness and bond strength. The Cr-C layer, containing 1.5-3.0% carbon and 20-30% chromium, forms a hard chromium carbide matrix (Cr₇C₃ and Cr₂₃C₆) that provides excellent resistance to abrasive wear. The Ni-Cr-C layer, containing 5-10% chromium and 1.0-2.0% carbon, provides a tougher transition layer that improves the bond strength and resistance to spalling.

Welding Process and Parameters

The carbide overlay welding on the blast furnace bell and hopper is typically performed using submerged arc welding (SAW) or shielded metal arc welding (SMAW), depending on the accessibility of the component and the thickness of the required overlay.

The SAW process is preferred for large, flat surfaces due to its higher deposition rate and better weld quality. The SMAW process is used for areas that are difficult to access or for small repair areas. The preheating temperature is set to minimize the risk of cracking in the base metal and the overlay, particularly for thick sections or high-strength steels.

Engineering Practice and Quality Control

Surface Preparation and Fit-Up

The surface preparation of the bell and hopper components is critical to ensuring a sound overlay:

1. Inspection: The components are inspected for existing cracks, corrosion, and excessive wear. Any cracks are repaired by welding before the overlay is applied.
2. Machining: The worn surfaces are machined to remove the work-hardened layer and create a sound base for the overlay. The machined surface should be smooth and free of sharp edges.
3. Cleaning: The surface is cleaned to remove oil, grease, rust, and other contaminants. Solvent cleaning or flame cleaning is typically used.
4. Preheating: The component is preheated uniformly to the specified temperature using a gas flame or induction heater. Uniform preheating is essential to prevent differential thermal expansion.

Post-Weld Inspection

After the overlay is completed, the following inspection procedures are performed:

• Visual inspection: All weld seams are examined for surface defects such as cracks, porosity, undercut, and spatter. Any surface defects are ground smooth and re-inspected.
• Hardness testing: The hardness of the overlay is measured at multiple locations to verify that it meets the specified range (50-60 HRC for Cr-C, 45-55 HRC for Ni-Cr-C). The hardness profile across the overlay thickness is also checked to ensure uniformity.
• Bond strength testing: A coupon is prepared from the overlay and tested in tension or shear to verify the bond strength. The minimum bond strength should be 200 MPa for blast furnace applications.
• Dimensional verification: The restored bell and hopper are checked for dimensional accuracy against the original drawing, with particular attention to the geometry of the cone and the alignment of the bell with the hopper.

Performance Monitoring

After reinstallation, the repaired bell and hopper are monitored during the initial operating period:

• Wear rate measurement: The thickness of the overlay is measured at regular intervals (every 3-6 months) to determine the wear rate and predict the remaining service life.
• Vibration monitoring: The vibration levels of the bell assembly are monitored to detect any signs of loosening or imbalance.
• Visual inspection: The overlay surface is inspected during maintenance shutdowns to check for spalling, cracking, or excessive wear.

Key Questions and Reflections

Question 1: How does the carbide overlay compare with other wear protection methods? The carbide overlay provides the best combination of wear resistance and cost-effectiveness for blast furnace bell and hopper applications. Other methods, such as hardfacing with tungsten carbide particles or surface treatment (carburizing, nitriding), are either more expensive or not suitable for the thick sections and complex geometries of these components.

Question 2: What is the typical service life improvement? Field experience shows that the carbide overlay extends the service life of the bell and hopper by 3-5 times compared to the uncoated carbon steel, with the overlay thickness reducing from 8-10 mm to 3-5 mm over a 2-3 year period. This represents a significant improvement in the maintenance interval and a reduction in the total cost of ownership.

Question 3: What are the limitations of the carbide overlay? The carbide overlay, while providing excellent wear resistance, is susceptible to spalling under severe impact loading. The brittle nature of the chromium carbide matrix can lead to cracking and delamination if the overlay is too thick or if the bond strength is insufficient. The multi-layer approach, with a tough transition layer, mitigates this risk but adds complexity to the welding procedure.

Summary

The application of carbide overlay welding on blast furnace bell and hopper components represents a practical and effective approach to extending the service life of these wear-prone components in the iron and steelmaking industry. The selection of a multi-layer overlay, combining a hard Cr-C layer with a tough Ni-Cr-C or Co-Cr-C transition layer, provides the optimal balance of wear resistance and toughness. The use of SAW for large surfaces and SMAW for difficult-to-access areas ensures efficient and reliable repair. The post-weld inspection procedures, including hardness testing and bond strength testing, verify the quality of the overlay and ensure that it meets the specified performance requirements. The successful field application of the carbide overlay demonstrates its practical value in reducing maintenance costs and improving the availability of blast furnace charging systems. Future developments in overlay material composition and welding process technology will further enhance the performance and applicability of carbide overlay for blast furnace components.