CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Repair of Converter Support Rings
Literature Overview
This 2000 study by Qiu Fuxiang and Xu Kejian, published in the journal of Welding Technology, addresses the weld overlay repair of converter support rings in steelmaking operations. The authors are affiliated with the Construction Engineering Department of Xiangtan Iron and Steel Group Co., Ltd., indicating that the work was driven by practical needs in the steel industry. Converter support rings are critical components in basic oxygen furnace (BOF) converters, which are used in steelmaking to refine molten iron into steel. These rings support the converter shell and are subjected to extreme thermal and mechanical loading during the steelmaking process.
Core Technical Content
Converter support rings are subjected to a combination of severe service conditions, including high temperatures from the molten steel and slag, thermal cycling from repeated heating and cooling during each heat, mechanical loading from the weight of the converter and its contents, and chemical attack from the basic slag. These conditions lead to wear, corrosion, and thermal fatigue cracking of the support ring, necessitating periodic repair or replacement. Weld overlay repair offers an economical alternative to replacement, particularly for large support rings where replacement would require significant downtime.
The study investigates the weld overlay repair process for converter support rings, focusing on the selection of appropriate filler metals, process parameters, and quality control measures. The key technical challenges include achieving adequate bond strength between the overlay and the support ring base metal, controlling distortion to maintain the ring's geometric accuracy, and ensuring the overlay material has the required thermal shock resistance and wear resistance.
Service Conditions and Failure Modes
The following table summarizes the service conditions and typical failure modes of converter support rings:
| Service Condition | Typical Values | Failure Mode | Repair Requirement |
|---|---|---|---|
| Operating temperature | 1200-1600°C (transient) | Thermal fatigue cracking | Overlay with thermal shock resistant material |
| Thermal cycling frequency | 2-4 heats per day | Fatigue cracking | Overlay with high fatigue strength material |
| Mechanical loading | 50-200 tons (static) | Wear, deformation | Overlay with high wear resistance material |
| Chemical attack | Basic slag (CaO, MgO) | Corrosion, spalling | Overlay with slag-resistant material |
| Thermal gradient | Up to 500°C across section | Thermal stress cracking | Controlled preheat and cooling |
The study identified several common failure modes of converter support rings:
- Wear at the support surface: The area where the support ring contacts the converter shell wears down over time, reducing the effective support area and increasing stress concentrations.
- Thermal fatigue cracking: Repeated thermal cycling causes cracking at the support ring's outer surface, particularly at stress concentration points such as bolt holes and keyways.
- Corrosion from slag attack: The basic slag attacks the support ring's surface, causing material loss and pitting.
- Deformation from thermal stress: Uneven heating and cooling can cause the support ring to deform, affecting the converter's alignment and operation.
Process Selection and Filler Metal Selection
The selection of the weld overlay process and filler metal depends on the specific service conditions and the required properties of the overlay. The following table summarizes the common process options and filler metals for converter support ring repair:
| Process | Filler Metal | Key Properties | Applicable Failure Mode |
|---|---|---|---|
| Submerged Arc Welding (SAW) | H13, H19, Stellite 6 | High wear resistance, good toughness | Wear, mechanical loading |
| Gas Metal Arc Welding (GMAW) | Stellite 6, Stellite 21 | Thermal shock resistance, slag resistance | Thermal fatigue, slag attack |
| Gas Tungsten Arc Welding (GTAW) | Stellite 6, H13 | Precise control, low dilution | Thin sections, critical areas |
| Plasma Transferred Arc (PTA) | Stellite 6, carbide | Excellent dilution control, high hardness | High wear areas |
| Flame Welding | High-carbon steel | Simple equipment, moderate hardness | General wear areas |
For converter support rings, the authors recommended a multi-pass approach using a combination of processes. The first pass was typically performed using GTAW to ensure good fusion with the base metal and to provide a clean, defect-free interface. Subsequent passes were performed using GMAW or SAW to build up the required thickness efficiently. The filler metal selection was based on the specific failure mode and service conditions. For areas subjected to thermal fatigue and slag attack, Stellite 6 (a cobalt-based alloy) was recommended due to its excellent thermal shock resistance and slag resistance. For areas subjected to mechanical wear, high-carbon steel filler metals (such as H13 or H19) were used to achieve high surface hardness after proper heat treatment.
Process Parameters and Quality Control
The following table summarizes the key process parameters and quality control measures for converter support ring weld overlay repair:
| Parameter | Recommended Value | Quality Control Method |
|---|---|---|
| Preheat temperature | 200-300°C | Infrared thermometer monitoring |
| Interpass temperature | 200-300°C | Infrared thermometer monitoring |
| Heat input | 1.0-2.0 kJ/mm | Process monitoring |
| Travel speed | 100-200 mm/min | Process monitoring |
| Weld bead width | 15-25 mm | Visual inspection |
| Weld bead height | 3-5 mm per pass | Visual inspection |
| Surface hardness | 40-55 HRC (H13/H19) or 400-500 HV (Stellite) | Hardness testing |
| Bond strength | Minimum 250 MPa | Peel test or bond strength test |
| Surface defects | No cracks, no porosity > 1 mm | MT, PT |
The authors emphasized the importance of preheating and controlled cooling in preventing cracking during the weld overlay repair of converter support rings. The base metal of converter support rings is typically a low-alloy steel with moderate carbon equivalent, which is susceptible to cold cracking if the cooling rate is too high. Preheating to 200-300°C and maintaining the interpass temperature within the same range ensures that the cooling rate is sufficiently slow to prevent martensitic transformation and cold cracking. After welding, the support ring should be cooled slowly, either by allowing it to cool in still air or by applying insulating blankets to retard cooling.
Engineering Practice and Economic Considerations
The economic benefits of weld overlay repair over replacement for converter support rings are substantial. The following table summarizes the cost comparison:
| Cost Item | Weld Overlay Repair | Full Replacement |
|---|---|---|
| Material cost | Filler metal: 50-100 kg | New support ring: 5000-10000 kg |
| Labor cost | 20-40 man-hours | Fabrication and installation: 200-500 man-hours |
| Downtime | 2-3 days | 10-20 days (fabrication + shipping + installation) |
| Total cost | 50,000-150,000 CNY | 500,000-2,000,000 CNY |
| Service life extension | 1-2 years | Full service life |
The study demonstrates that weld overlay repair can extend the service life of converter support rings by 1-2 years, at a fraction of the cost of full replacement. This makes weld overlay repair an economically attractive option for steel mills that operate converters on a continuous basis and cannot afford extended downtime for replacement.
Key Questions and Reflections
The study raises several important questions for engineers involved in converter support ring repair. First, how does the weld overlay repair affect the residual stress state of the support ring, and what are the implications for fatigue life under thermal cycling? The welding process introduces significant residual stresses, which can either improve or degrade the support ring's fatigue performance depending on their magnitude and distribution. Second, how does the thermal shock resistance of the overlay material compare to the base metal under repeated thermal cycling? The overlay material must maintain its integrity under repeated heating and cooling cycles, and any cracking or spalling of the overlay would compromise the repair. Third, what is the optimal overlay thickness for maximizing service life while minimizing distortion and cost? Thicker overlays provide more material for wear and can compensate for more severe damage, but they also introduce more distortion and require more heat input.
This study provides valuable practical guidance for the weld overlay repair of converter support rings. The combination of process selection, filler metal choice, process parameter optimization, and quality control presented in the study offers a comprehensive framework for successful repair. The economic analysis demonstrates the significant cost savings achievable through weld overlay repair compared to full replacement, making it an attractive option for steel mills seeking to optimize maintenance costs. The study also highlights the importance of proper process qualification and quality control in ensuring the reliability of repaired support rings, which is essential for maintaining the safety and efficiency of steelmaking operations.