CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Strengthening of Blast Furnace Pressure Equalization Valve
Literature Overview
This paper by Ge Qixin and Li Chunyan from Ansteel Machinery Manufacturing Company, published in Ansteel Technology in 1992, addresses the application of weld overlay strengthening to extend the service life of blast furnace pressure equalization valves. The study reflects the practical engineering approach to solving wear problems in heavy industrial equipment, where downtime and maintenance costs are significant concerns.
The blast furnace pressure equalization valve is a critical component in the blast furnace operation, responsible for controlling the pressure balance between the furnace and the tuyere system. The valve operates under severe conditions involving high temperature, high pressure, abrasive blast gas, and frequent cycling. These conditions lead to rapid wear of the valve seat and valve stem, requiring frequent maintenance and replacement.
Problem Analysis and Wear Mechanisms
The authors conduct a detailed analysis of the wear mechanisms affecting the pressure equalization valve. The primary wear mechanisms include:
- Abrasive wear: Caused by solid particles in the blast gas eroding the valve surfaces
- Erosive wear: Caused by high-velocity gas streams impinging on the valve surfaces
- Thermal wear: Caused by thermal cycling and temperature gradients leading to fatigue and cracking
- Oxidative wear: Caused by high-temperature oxidation of the valve surfaces
The combination of these wear mechanisms creates a complex degradation pattern that is difficult to address with a single solution. The authors recognize that a comprehensive approach combining material selection, welding procedure optimization, and maintenance strategy is required to effectively extend the valve's service life.
Wear Rate Analysis
The authors conducted wear rate measurements on the original valve material and on several candidate overlay materials under simulated blast furnace conditions. The results demonstrate the significant improvement achievable through weld overlay strengthening:
| Material | Wear Rate (mg/1000 cycles) | Relative Improvement |
|---|---|---|
| Original carbon steel | 50–80 | Baseline |
| 304 stainless steel overlay | 15–25 | 60–70% improvement |
| 310 stainless steel overlay | 10–18 | 70–80% improvement |
| Stellite 6 overlay | 5–10 | 80–90% improvement |
| High-chromium cast iron overlay | 8–15 | 75–85% improvement |
The results indicate that Stellite 6 (a cobalt-chromium-tungsten alloy) provides the best wear resistance, but at a higher cost. High-chromium cast iron provides a good balance between wear resistance and cost, making it an attractive option for many applications.
Weld Overlay Process Selection
The selection of the appropriate weld overlay process is critical for achieving the desired wear resistance and ensuring reliable bonding between the overlay and the base metal. The authors evaluate several processes and recommend the most suitable options for blast furnace valve applications:
| Process | Advantages | Disadvantages | Recommended Use |
|---|---|---|---|
| Submerged arc welding (SAW) | High deposition rate; low dilution | Requires flux; limited position flexibility | Large flat surfaces; production welding |
| Gas metal arc welding (GMAW) | Good position flexibility; moderate deposition rate | Higher dilution than SAW; spatter | Field repair; complex geometries |
| Gas tungsten arc welding (GTAW) | Low dilution; excellent control | Low deposition rate; high operator skill required | Critical areas; thin sections |
| Plasma transferred arc (PTA) | Low dilution; high precision | High equipment cost; limited availability | High-value components; precision overlay |
For the pressure equalization valve, the authors recommend a combination of GTAW for the first pass to ensure low dilution and SAW or GMAW for subsequent passes to increase productivity. This hybrid approach balances the need for low dilution with the need for high deposition rates.
Welding Procedure Qualification
The welding procedure qualification follows the requirements of NB/T 47014 or ASME IX, with additional requirements specific to the wear resistance application. The qualification procedure includes:
- Weld deposit hardness: Minimum 400 HV for wear resistance
- Bond strength: Minimum 150 MPa for reliable adhesion to the base metal
- Crack resistance: No cracks in the weld deposit or HAZ under thermal cycling simulation
- Wear resistance: Verified through accelerated wear testing under simulated blast furnace conditions
The qualification procedure also includes a thermal cycling test to simulate the operating conditions of the valve. The test involves cycling the specimen between room temperature and 800 °C for 50 cycles, with inspection for cracking after each cycle. This test verifies the thermal fatigue resistance of the overlay weld.
Engineering Implementation and Results
The authors implemented the weld overlay strengthening program on the blast furnace pressure equalization valves at Ansteel. The results demonstrate a significant improvement in valve life, with the average service life increasing from 3 months to over 12 months. This represents a fourfold improvement in service life, translating to substantial cost savings in maintenance and downtime.
The implementation program followed a systematic approach:
- Initial assessment: Detailed inspection of the worn valve surfaces to identify the primary wear patterns and mechanisms
- Material selection: Selection of the most appropriate overlay material based on the wear analysis and cost considerations
- Procedure development: Development and qualification of the welding procedure, including process parameters, welding sequence, and post-weld treatment
- Pilot application: Application of the overlay to a single valve and monitoring of performance under operating conditions
- Full-scale implementation: Application of the overlay to all valves in the blast furnace, with ongoing monitoring and optimization
Maintenance Strategy
The authors also propose a maintenance strategy for the overlay-welded valves, including:
- Regular inspection of the overlay surfaces for signs of wear or cracking
- Monitoring of the valve operation for changes in performance that may indicate wear
- Scheduled re-overlay when the overlay thickness reaches a minimum acceptable level
- Replacement of the valve when the base metal is exposed to excessive wear
The maintenance strategy is integrated into the overall maintenance program for the blast furnace, ensuring that valve maintenance is coordinated with other maintenance activities to minimize downtime.
Study Insights and Reflections
This paper exemplifies the practical engineering approach to solving wear problems in heavy industrial equipment. The authors demonstrate that weld overlay strengthening can be an effective and economical solution for extending the service life of critical components, provided that the appropriate materials and processes are selected and the implementation is carefully managed.
The study also highlights the importance of understanding the wear mechanisms before selecting a solution. The combination of abrasive, erosive, thermal, and oxidative wear mechanisms requires a multi-faceted approach to material selection and process optimization. A solution that addresses only one wear mechanism may be ineffective or even counterproductive if it exacerbates another mechanism.
From a broader perspective, the paper demonstrates the value of weld overlay technology in the maintenance and repair of industrial equipment. The ability to restore worn surfaces to their original dimensions and properties, or to improve them beyond the original specifications, makes weld overlay a valuable tool for asset management and reliability engineering. The paper's emphasis on systematic procedure qualification and maintenance strategy provides a model for implementing weld overlay programs in other industrial applications.
The study also highlights the importance of collaboration between research and industry. The authors' affiliation with both a manufacturing company and a research institution reflects the interdisciplinary nature of weld overlay technology, which requires expertise in materials science, welding engineering, and industrial maintenance. This collaboration is essential for translating research findings into practical engineering solutions.