Design and Application of Fully Overlay-Welded Converter Ducts under ASME Code

Literature Overview

This paper by Zhao Yingchun and colleagues from Yantai Guoye Metallurgical Water-Cooled Equipment Co., Ltd. addresses a critical engineering challenge in converter steelmaking: the premature failure of converter ducts caused by high-temperature exposure, cyclic thermal stresses, and abrasive wear from dust-laden gas streams. The authors propose a design solution that adheres to ASME code requirements and employs full overlay welding to protect the duct structure from deterioration. The work was published in the journal Industrial Heating in 2021 (Volume 50, Issue 9, pages 17-19). The abstract notes that this approach resolves the short service life problem of converter ducts experiencing water leakage due to thermal fatigue and erosion, and offers a new process reference for converter duct equipment in China.

Core Technical Challenges and the ASME Framework

Converter ducts operate under extremely harsh conditions. The off-gas temperature from a basic oxygen furnace (BOF) converter typically ranges from 1400 to 1700 degrees Celsius at the throat, gradually decreasing to 600 to 900 degrees Celsius along the duct run. The gas stream carries fine dust particles at high velocity, creating an erosive-corrosive environment that attacks conventional carbon steel linings rapidly. Traditional refractory-lined or water-cooled duct designs often suffer from cracking, spalling, and water leakage within weeks of operation.

The adoption of ASME code compliance represents a significant engineering commitment. Under the ASME Boiler and Pressure Vessel Code (Section VIII, Division 1), the design must satisfy requirements for material selection, welding procedure qualification, non-destructive examination, and pressure testing. For overlay-welded ducts, the critical considerations include:

The full overlay welding approach means that the entire interior surface of the duct is protected by a deposited alloy layer, rather than relying on discrete wear plates or refractory linings. This creates a continuous, metallurgically bonded protective barrier that resists both thermal cycling and mechanical erosion.

Overlay Welding Metallurgy and Design Considerations

The overlay welding process for converter ducts typically employs submerged arc welding (SAW) or flux-cored arc welding (FCAW) for efficiency and deposition rate. The overlay material is selected based on its resistance to thermal shock, oxidation, and abrasion. Common overlay compositions include high-chromium cast irons, nickel-chromium alloys, or austenitic stainless steels with carbide-forming elements.

From a metallurgical perspective, the key concerns are:

• Dilution control: The base steel dilutes the overlay alloy, reducing its protective properties. Multi-pass overlay welding is essential to maintain the required alloy concentration in the final surface layer. Typically, three to five passes are applied, with the final pass consisting of pure overlay material.
• Cracking resistance: The combination of high carbon equivalent in the base material and the thermal cycling during welding can induce hot cracks in the overlay weld metal. Preheating to 200 to 300 degrees Celsius and controlled interpass temperature are critical.
• Residual stress management: The large thermal gradients in thick-walled duct sections generate significant residual stresses. Post-weld heat treatment (PWHT) or stress-relief annealing may be required to reduce residual stress below 50 percent of the yield strength of the base material.
• Thermal fatigue resistance: The overlay layer must accommodate repeated thermal expansion and contraction cycles without cracking or spalling. The coefficient of thermal expansion mismatch between the overlay and base material is a design-critical parameter.

The authors' contribution lies in integrating these metallurgical considerations within the ASME code framework, which provides a systematic approach to material selection, welding qualification, and quality assurance. This is particularly valuable for Chinese converter equipment manufacturers seeking to improve duct service life while meeting international code requirements for export projects.

Engineering Practice and Quality Control Integration

In practical implementation, the full overlay welding process for converter ducts follows a structured quality control plan:

1. Material verification: Base plates are inspected for chemical composition and mechanical properties per the applicable ASME material specification. Overlay consumables are certified for traceability and lot identification.
2. Welding procedure qualification: A procedure qualification record (PQR) is established using test coupons that replicate the actual duct geometry, thickness, and overlay material. Essential variables include electrode type, current, voltage, travel speed, preheat temperature, and interpass temperature.
3. Production welding: Welders are certified per ASME Section IX qualification rules. In-process monitoring includes visual inspection of each pass, measurement of overlay thickness, and periodic hardness testing of the overlay surface.
4. Post-weld examination: Non-destructive testing includes visual examination (VT) of the entire overlay surface, magnetic particle testing (MT) for surface and near-surface defects, and ultrasonic testing (UT) for subsurface cracking. Radiographic testing (RT) may be applied to critical sections.
5. Final acceptance: The completed duct undergoes dimensional inspection, hydrostatic pressure testing where applicable, and a final overlay thickness verification using magnetic thickness gauges.

The engineering insight from this work is that ASME code compliance is not merely a regulatory exercise but a systematic methodology for ensuring reliable performance under extreme service conditions. By following the code's structured approach to material selection, welding qualification, and quality assurance, manufacturers can significantly reduce the risk of premature duct failure.

Study Insights and Implications

The most valuable aspect of this literature is its demonstration that code-based design methodology can be effectively applied to overlay-welded components operating under thermal and erosive service. Many Chinese converter equipment manufacturers have historically relied on empirical design practices without formal code compliance. This work provides a bridge between practical manufacturing needs and international engineering standards.

One area for further development is the long-term performance monitoring of overlay-welded ducts in service. The paper focuses on design and fabrication but does not report extended field trial data. Future work should include post-weld residual stress measurements, thermal fatigue life testing under simulated converter conditions, and field performance tracking over multiple campaign cycles. Additionally, the economic analysis comparing full overlay welding with conventional refractory or water-cooled designs would strengthen the technical case for widespread adoption.

This literature serves as a practical reference for engineers designing converter ducts that must balance thermal resistance, erosion resistance, and structural integrity within a code-compliant framework. The integration of ASME requirements with overlay welding metallurgy represents a mature engineering approach that can extend duct service life significantly beyond conventional designs.