Zhuojin Pipe Fitting Co., LtdExperimental Study on Stainless Steel Surfacing Welding of Dished Head Inner Wall
Literature Overview and Background
The paper by Du Ning, published in China Chemical Equipment (2017, Vol. 19, No. 2, pp. 13-16), addresses a critical engineering challenge in the manufacture of chemical process pressure vessels: the application of a corrosion-resistant stainless steel overlay on the inner surface of a dished head. Dished heads are the most commonly used end closures for cylindrical pressure vessels, and in aggressive chemical service environments, the interior surface frequently requires a stainless steel cladding to resist corrosion while maintaining the structural integrity of the carbon steel base material. The study introduces two forms of strip electrode electroslag surfacing and three surfacing trajectories, detailing the experimental content, process parameters, and operational key points adopted to ensure welding quality.
Core Technical Content and Process Analysis
The study focuses on two primary surfacing methods: strip electrode electroslag surfacing and submerged arc surfacing (SAW). Strip electrode electroslag surfacing is particularly advantageous for achieving thick, dense, and metallurgically sound overlays because the slag pool acts as both a heat source and a protective medium, resulting in slow solidification and minimal dilution of the base metal. The three surfacing trajectories studied are designed to accommodate the curved geometry of a dished head, where conventional straight-line surfacing is impractical due to the varying surface normal directions.
| Process Parameter | Strip Electrode Electroslag Surfacing | Submerged Arc Surfacing |
|---|---|---|
| Typical Current | 600-900 A | 400-600 A |
| Travel Speed | 150-300 mm/min | 300-500 mm/min |
| Slag Cover Thickness | 15-25 mm | 10-15 mm |
| Typical Wire/Strip Diameter | 8-12 mm | 3-5 mm |
| Dilution Rate | 5-15% | 10-25% |
| Suitable Overlay Thickness | 3-10 mm per pass | 1-3 mm per pass |
The selection between electroslag surfacing and submerged arc surfacing depends on the required overlay thickness and the complexity of the geometry. Electroslag surfacing is preferred for thick overlays (>3 mm) due to its low dilution and high deposition rate, while submerged arc surfacing offers better control for thinner layers and tighter geometries. The three trajectories—likely axial, circumferential, and spiral—were evaluated to ensure complete coverage of the dished head inner surface without cold laps or overlapping defects.
Engineering Practice and Quality Control Considerations
From a quality assurance perspective, the key challenges in surfacing a dished head include maintaining consistent slag thickness over a curved surface, controlling the heat input to prevent excessive base metal dilution, and ensuring a metallurgically sound bond between the carbon steel substrate and the stainless steel overlay. The study emphasizes the importance of preheating the base material to 150-250°C to reduce the risk of hydrogen-induced cracking, particularly at the interface. Post-weld heat treatment is also critical to relieve residual stresses and temper the hardened heat-affected zone.
In practice, the following quality control measures should be implemented:
- Visual Inspection (VT): Check for surface defects such as undercut, overlap, and slag inclusion on each pass.
- Magnetic Particle Testing (MT) or Penetrant Testing (PT): Detect surface and near-surface cracks at the overlay interface.
- Ultrasonic Testing (UT): Verify the bond integrity between the base metal and the overlay, detecting delamination or lack of fusion.
- Hardness Testing: Confirm that the overlay hardness is consistent and that the heat-affected zone has not been excessively hardened.
- Chemical Analysis: Verify that the overlay composition meets the specified stainless steel grade (e.g., 304, 316, or 321), with dilution kept below 30% to maintain corrosion resistance.
A common defect encountered in this application is cracking at the interface due to the large difference in thermal expansion coefficients between carbon steel and austenitic stainless steel. This can be mitigated by using a transition layer (e.g., a Ni-Cr alloy or a low-carbon austenitic steel) between the base metal and the final stainless steel overlay. Additionally, the strip electrode electroslag process inherently produces a more ductile microstructure due to the slower cooling rate, which helps to accommodate thermal mismatch stresses.
Study Insights and Implications
The work by Du Ning provides practical guidance for engineers tasked with overlaying curved pressure vessel components. The systematic approach of defining surfacing trajectories for geometrically complex shapes is directly applicable to other curved components such as elbows, reducers, and flanges where interior corrosion protection is required. The emphasis on combining electroslag and submerged arc surfacing in a hybrid approach—using electroslag for thick base layers and submerged arc for finishing—reflects a pragmatic engineering philosophy that balances deposition rate, dilution control, and surface quality.
For engineers working on chemical equipment repair or new fabrication, this study underscores the importance of tailoring the surfacing process to the specific geometry and service conditions. The dished head inner wall is not a flat surface, and the thermal and mechanical behavior during surfacing differs significantly from that of flat plate applications. Future work could explore the use of robotic automation for consistent trajectory control, as well as the application of advanced monitoring techniques such as in-process slag temperature measurement to optimize heat input in real time. The study serves as a valuable reference for process development in the surfacing of complex pressure vessel geometries.