CLADDING TECHNOLOGY SHANXI CO., LTDWeld Overlay Process for Large Coke Oven Towers
Technical Context and Service Environment
Coke oven towers are large vertical structures used in the coking process, where coal is heated in the absence of air to produce coke and various by-products (coke oven gas, coal tar, ammonia, etc.). The towers are subjected to a highly corrosive environment containing sulfur compounds, hydrogen sulfide, ammonia, and water vapor at temperatures ranging from 200°C to 450°C. The internal walls of the tower are particularly vulnerable to corrosion, and weld overlay is the primary method of protecting these surfaces.
The scale of these towers is significant, with typical heights of 40–60 meters and internal diameters of 3–5 meters. The overlay area can exceed 200 square meters, making this one of the largest weld overlay applications in industrial practice.
Material Selection for Corrosion Protection
The overlay material must resist the combined effects of wet H2S corrosion, ammonia corrosion, and sulfuric acid dew point corrosion. The following materials are commonly used:
| Material | Application | Key Properties |
|---|---|---|
| 316L stainless steel | General corrosion protection | Good resistance to H2S, ammonia |
| 310S stainless steel | High-temperature oxidation resistance | Resistance up to 1100°C |
| Hastelloy C276 | Severe sulfuric acid environments | Excellent resistance to reducing acids |
| Alloy 625 (Inconel 625) | High-temperature, high-stress areas | Good strength at elevated temperatures |
| Ni-Cr-Mo alloy | Transition layer | Prevents carbon diffusion |
The selection depends on the specific location within the tower and the severity of the corrosive environment. In practice, a graded approach is often used, with more aggressive materials applied at the most critical locations.
Welding Process and Parameters
Given the large surface area and the need for uniform coverage, the welding process must be efficient and reliable.
Submerged Arc Welding (SAW) — Primary Process
| Parameter | Value |
|---|---|
| Current | 350–550 A |
| Voltage | 26–34 V |
| Travel speed | 200–400 mm/min |
| Wire diameter | 1.6–2.4 mm |
| Flux | Low-hydrogen basic flux |
| Preheat | 150–250°C |
| Interpass temperature | 150–300°C |
SAW is the preferred process for its high deposition rate and ability to produce uniform, multi-layer deposits over large areas. The process is well-suited to the vertical and overhead positions encountered in tower fabrication.
Gas Tungsten Arc Welding (GTAW) — For Critical Areas
GTAW is used for the first layer (root pass) and for areas where better control is required, such as near nozzles, manways, and other attachments.
| Parameter | Value |
|---|---|
| Current | 120–200 A |
| Voltage | 10–16 V |
| Travel speed | 100–250 mm/min |
| Wire diameter | 1.0–1.6 mm |
| Shielding gas | Argon (99.99%) |
Multi-Layer Overlay Strategy
The overlay is applied in a systematic sequence:
- Surface preparation: The base surface is ground to a uniform profile with a tolerance of ±1 mm. Any existing corrosion or coating is removed.
- Root pass (GTAW): A thin root layer is applied using GTAW to ensure good fusion with the base material.
- Transition layer (SAW, 1–2 passes): Ni-Cr or Ni-Fe alloy is applied to prevent carbon depletion and chromium carbide precipitation at the interface.
- Overlay layers (SAW, 3–5 passes): The functional stainless steel or nickel-based alloy is applied in multiple passes. The final pass should have a dilution ratio below 10%.
- Post-weld treatment: Stress relief at 450–550°C for 2–4 hours, depending on the base material and overlay thickness.
Quality Assurance and Inspection
The quality of the overlay is critical for the long-term performance of the tower. The following inspection methods are employed:
| Inspection Method | Purpose | Frequency |
|---|---|---|
| Visual inspection | Surface defects, geometry | Every pass |
| Magnetic particle testing (MT) | Surface cracks | Every 10 m² of overlay |
| Ultrasonic testing (UT) | Bond strength | Every 5 m² of overlay |
| Hardness testing | Mechanical properties | Every 10 m² of overlay |
| Intergranular corrosion test | Overlay material quality | Every batch of material |
| Metallographic examination | Microstructure | Representative samples |
The bond strength between the overlay and the base material is a critical parameter. The overlay must be fully bonded at all locations, with no lack of fusion or delamination. UT testing using the through-transmission method is the standard technique for verifying bond strength.
Engineering Challenges and Solutions
Several challenges arise from the scale and complexity of coke oven tower overlay:
- Vertical and overhead welding: A significant portion of the overlay is applied in vertical and overhead positions, which affects weld quality and deposition rate. SAW in the vertical position requires specialized equipment and careful parameter adjustment.
- Thermal distortion: The large surface area and thin base material (typically 8–16 mm) are susceptible to distortion. The welding sequence must be planned to minimize distortion, with symmetric welding patterns used wherever possible.
- Interpass temperature control: The large thermal mass of the tower means that the interpass temperature can drop significantly between passes. This requires careful monitoring and, in some cases, local preheating before each pass.
- Access and logistics: The height and internal geometry of the tower present significant access challenges. Specialized equipment and scaffolding are required, and the welding operation must be planned with logistics in mind.
- Corrosion of the overlay in service: Even with proper overlay, localized corrosion can occur at defects, scratches, or areas where the overlay has been damaged. Regular inspection and touch-up repair are essential for maintaining the integrity of the overlay.
Service Performance and Maintenance
The performance of the overlay in coke oven tower service is typically evaluated based on the following criteria:
- Corrosion rate: The overlay should reduce the corrosion rate by a factor of 10–100 compared to the bare base material.
- Service life: A well-applied overlay should provide 5–10 years of service before re-overlay is required.
- Failure modes: The most common failure modes are localized corrosion at defects, mechanical damage during maintenance, and thermal fatigue cracking at high-temperature locations.
Study Reflections
The weld overlay of large coke oven towers is a technically demanding task that requires careful planning, precise execution, and rigorous quality control. The scale of the operation, the severity of the corrosive environment, and the challenges of vertical and overhead welding all contribute to the complexity of the task.
The key insight from studying this topic is that the overlay is not a one-time application but a long-term protection strategy that requires ongoing maintenance and inspection. The quality of the overlay at the time of application is only the first step; regular monitoring and timely repair are essential for ensuring long-term performance.
In summary, the successful overlay of coke oven towers depends on a systematic approach that integrates material selection, process optimization, and quality assurance. Engineers must recognize the unique challenges of this application and plan accordingly, with a focus on long-term reliability rather than short-term cost savings.