CLADDING TECHNOLOGY SHANXI CO., LTDStudy Note on Stainless Steel Strip Electroslag Welding Overlay of Spherical Heads Inner Surfaces
Technical Background and Engineering Challenges
Spherical heads, or dished heads, are critical pressure-containing components used in pressure vessels, reactors, and storage tanks. When these components require corrosion-resistant inner surfaces, the application of stainless steel overlay through strip electroslag welding (ESW) provides a cost-effective and reliable solution. The literature examined addresses the technical challenges and optimization of stainless steel strip ESW overlay welding on the inner surfaces of spherical heads, which presents unique geometric and thermal challenges due to the curved surface configuration and the large thickness variations inherent to spherical head geometries.
The strip ESW process offers several advantages for this application including high deposition rates, low dilution rates, and the ability to achieve thick overlay layers in a single pass. However, the curved geometry of spherical heads introduces challenges related to electrode positioning, slag pool stability, and uniform heat distribution that must be carefully managed to achieve quality overlay results.
Process Parameters and Geometric Considerations
The strip ESW overlay welding process on spherical head inner surfaces requires careful parameter selection and equipment adaptation to accommodate the curved geometry. The study investigated the welding of 304L and 316L stainless steel strips onto carbon steel spherical heads with thicknesses ranging from 20 to 80 millimeters.
| Parameter | Typical Range | Notes |
|---|---|---|
| Strip Size | 20x3 mm or 25x3 mm | 304L or 316L stainless steel |
| Welding Current | 600-1200 A | Depends on strip size and thickness |
| Arc Voltage | 35-55 V | Maintains stable slag pool |
| Travel Speed | 50-150 mm/min | Slower for thicker sections |
| Slag Pool Depth | 15-25 mm | Controlled by current and speed |
| Preheat Temperature | 100-200°C | Prevents cracking in low-alloy steel |
| Interpass Temperature | ≤300°C | For multi-pass applications |
| Overlay Thickness | 3-6 mm per pass | Multiple passes for thicker overlays |
| Shielding Gas | Argon (backing) | Prevents oxidation of root side |
The geometric challenges of spherical head overlay welding require specialized electrode holders and traverse mechanisms that can maintain consistent electrode position relative to the curved surface. The study found that the use of a pendulum-type electrode holder with automatic height control was essential for maintaining consistent slag pool depth and overlay thickness across the curved surface. Without such equipment, variations in electrode height lead to fluctuations in the slag pool, resulting in uneven overlay thickness and potential defects such as lack of fusion or excessive dilution.
Microstructural Analysis and Dilution Control
The microstructural examination of the ESW overlay deposits revealed a characteristic columnar grain structure with the grain direction aligned with the heat flow direction, which is perpendicular to the weld surface in this application. The dilution rate from the carbon steel substrate was found to be in the range of 5 to 15 percent, which is significantly lower than that achieved with other overlay welding processes such as SAW or GMAW. This low dilution rate is one of the primary advantages of the strip ESW process for stainless steel overlay applications.
The corrosion resistance of the overlay was evaluated through electrochemical testing and immersion corrosion tests in 3.5 percent NaCl solution and dilute sulfuric acid solutions. The results confirmed that the low dilution rate achieved with strip ESW produces overlay deposits with corrosion resistance approaching that of pure stainless steel, making this process particularly suitable for applications requiring high corrosion resistance with thick overlay layers.
Quality Control and Inspection Methods
The quality assurance of strip ESW overlay welding on spherical heads requires a comprehensive inspection program that addresses the specific challenges of this process. The following inspection methods and acceptance criteria were evaluated in the study:
| Inspection Method | Application | Acceptance Criteria | Standard Reference |
|---|---|---|---|
| Visual Testing (VT) | Surface condition, porosity, undercut | No visible defects | GB/T 3323 / ASME IX |
| Magnetic Particle Testing (MT) | Surface and near-surface cracks | No linear indications | JB/T 4730.4 |
| Ultrasonic Testing (UT) | Bond strength, lack of fusion, laminations | Full bond, no defects | JB/T 4730.2 |
| Radiographic Testing (RT) | Internal porosity, inclusions | Per ASME Section V | ASME V T-200 |
| Hardness Testing | Overlay and heat-affected zone | 150-250 HV for 304L/316L | ASTM E92 |
| Impact Testing | HAZ toughness | ≥27 J at -20°C (if required) | ASTM E23 |
| Intergranular Corrosion | Overlay corrosion resistance | Pass ASTM A262 Practice E | ASTM A262 |
The ultrasonic testing for bond strength is particularly important for strip ESW overlay applications, as incomplete bonding between the overlay and substrate is a potential failure mode that can lead to overlay delamination in service. The study found that proper parameter control, particularly the maintenance of adequate slag pool depth and consistent travel speed, is the most effective means of preventing bonding defects.
Engineering Practice and Defect Prevention
The field application of strip ESW overlay welding on spherical heads requires careful planning and execution to achieve reliable results. Several critical factors must be addressed in the engineering practice:
- Surface preparation: The inner surface of the spherical head must be thoroughly cleaned to remove all mill scale, rust, and contamination. Grinding to a clean, bright surface with Ra values between 6.3 and 12.5 micrometers is recommended, followed by solvent cleaning.
- Equipment setup: The electrode holder must be capable of maintaining precise position control over the curved surface. A pendulum-type holder with automatic height compensation is strongly recommended for spherical head applications.
- Parameter optimization: A trial weld should be performed on a test coupon with similar geometry and thickness to the production spherical head to optimize parameters before production welding. The parameters should be adjusted based on the specific spherical head geometry and thickness.
- Thermal management: Preheating and interpass temperature control are essential for preventing cracking, particularly for low-alloy steel substrates with higher carbon equivalent values. Post-weld stress relief at 580 to 620 degrees Celsius for two hours is recommended for critical applications.
- Inspection protocol: A comprehensive inspection program should be implemented, including visual inspection, MT for surface defects, UT for bond strength verification, and hardness testing to confirm overlay composition.
Study Insights and Practical Recommendations
The literature provides valuable guidance for the successful application of strip ESW overlay welding on spherical head inner surfaces, highlighting the process's unique advantages in achieving thick, low-dilution stainless steel overlays with high deposition rates. The key insight from this study is that the curved geometry of spherical heads introduces specific challenges that require specialized equipment and careful parameter control, but that these challenges can be effectively managed with proper planning and execution.
Engineers should recognize that strip ESW represents a highly efficient solution for thick stainless steel overlay applications where deposition rate and dilution control are critical factors. The low dilution rates achieved with this process produce overlays with corrosion resistance approaching that of pure stainless steel, making it an ideal choice for pressure vessel applications requiring high corrosion resistance. The comprehensive quality assurance program outlined in the literature should be adapted and implemented in production environments to ensure consistent overlay quality and long-term service reliability. Future developments should focus on the automation of electrode positioning systems for complex geometries and on the development of qualified welding procedure specifications that incorporate the geometric considerations unique to spherical head overlay welding.