Stainless Steel Strip Electroslag Welding Overlay on the Inner Wall of Spherical Heads

Technical Background and Engineering Context

Spherical pressure vessels and spherical heads are extensively used in the chemical, petrochemical, and pharmaceutical industries for storing and processing corrosive media such as sulfuric acid, hydrochloric acid, and various organic solvents. The structural requirement for pressure containment is typically met by a carbon steel or low-alloy steel shell, while the corrosion resistance requirement is met by applying a stainless steel overlay to the internal surface. Strip electroslag welding (strip ESW) has emerged as the most efficient and reliable process for applying thick stainless steel overlays to large spherical heads, offering deposition rates of 5 to 15 kg/h with minimal dilution and excellent bond strength. This study note examines the technical details, process parameters, and quality control practices for this application.

Process Principles and Equipment Requirements

Strip electroslag welding operates on the principle of electroslag formation between a strip electrode and the base metal, with the molten slag pool providing both heat generation and shielding. The process is characterized by a high current density (typically 150 to 300 A/cm²) and a relatively low travel speed (100 to 400 mm/min), resulting in a deep, narrow weld with minimal dilution of the base metal.

Key Equipment Components

Process Sequence

The strip ESW overlay process for spherical heads follows a systematic sequence:

1. Surface preparation: The internal surface of the spherical head is ground to remove mill scale, rust, and contaminants. A backing strip is welded to the surface using GTAW to create a starting trough.
2. Melt-back pass: The backing strip is melted back into the base metal using a high current (2000-2500 A) to create a shallow trough (1-2 mm deep). This ensures metallurgical bonding between the overlay and the base metal.
3. Overlay deposition: The strip electrode is fed into the slag pool while the travel mechanism advances at a controlled speed. The process is repeated in multiple passes to achieve the required overlay thickness.
4. Post-overlay treatment: The overlay surface is ground smooth, and stress-relief heat treatment is performed if required by the applicable code.

Process Parameter Optimization

The optimization of strip ESW parameters is critical for achieving a defect-free overlay with the correct composition and bond strength. The following table presents the recommended parameter ranges for overlaying 304L stainless steel on a Q345R carbon steel spherical head:

Parameter Interactions

The literature emphasizes that the current, voltage, and travel speed are interdependent and must be optimized as a group. A key relationship is the current density at the strip-electrode interface, which should be maintained between 150 and 250 A/cm². Below this range, the electroslag pool is unstable and produces irregular weld profiles. Above this range, excessive heat input leads to high dilution and potential cracking.

The dilution rate in strip ESW is typically 5 to 15%, which is significantly lower than that of conventional welding processes (SAW: 15-30%, GTAW: 10-20%). The literature reports that the dilution rate can be further reduced by using a higher current density and a slower travel speed, which increases the ratio of filler metal to base metal in the weld cross-section.

Microstructural Analysis and Bond Strength

The quality of the strip ESW overlay is evaluated through microstructural examination, hardness testing, and bond strength testing. The literature provides the following typical results:

The bond strength between the overlay and the base metal is a critical quality parameter. According to ASTM A368 and NB/T 47014, the minimum bond strength for stainless steel overlays on carbon steel is 200 MPa (peel test) or 150 MPa (shear test). The literature reports bond strengths of 250 to 350 MPa for properly executed strip ESW overlays, well above the minimum requirements.

Metallographic Examination Findings

Microstructural examination of the overlay-base metal interface reveals a characteristic dilution zone with a thickness of 0.5 to 2.0 mm. This zone exhibits a gradient in composition from the carbon steel side (ferrite + pearlite) to the overlay side (austenite). The transition is gradual and does not exhibit any sharp compositional boundary, which is favorable for bond strength and cracking resistance.

The literature also notes that the presence of 5 to 10% δ-ferrite in the overlay is beneficial, as it prevents solidification cracking and improves resistance to intergranular corrosion. However, excessive δ-ferrite (>15%) can reduce ductility and corrosion resistance, so the composition should be controlled through appropriate selection of the strip electrode grade and dilution management.

Quality Control and Inspection

The quality control program for strip ESW overlays on spherical heads includes the following inspection methods:

Common Defects and Remediation

The literature identifies the following common defects in strip ESW overlays on spherical heads:

1. Bonding failure (delamination): Caused by inadequate melt-back or surface contamination. Remediation involves re-grinding the affected area and re-applying the overlay with improved surface preparation.
2. Cracking in the HAZ: Caused by excessive thermal gradient due to insufficient preheating or high travel speed. Remediation includes stress-relief heat treatment and, in severe cases, re-welding with a lower heat input.
3. Porosity in the overlay: Caused by moisture contamination of the flux or inadequate shielding gas coverage. Remediation involves drying the flux and improving the gas shielding arrangement.
4. Excessive dilution: Results in overlay composition deviating from the specified grade, potentially compromising corrosion resistance. Remediation involves adding additional passes with the correct strip electrode composition.

Engineering Practice and Code Compliance

The application of strip ESW overlay to spherical heads must comply with the relevant pressure vessel codes and standards. The following table summarizes the key code requirements:

The literature emphasizes that the Welding Procedure Specification (WPS) for strip ESW overlay must be qualified through a Performance Qualification Record (PQR) that demonstrates the process parameters, consumables, and inspection results meet the specified requirements. The PQR should include tests for bond strength, hardness, chemical composition, and intergranular corrosion resistance.

Reflections and Study Insights

This literature review highlights the remarkable efficiency and reliability of strip ESW for applying thick stainless steel overlays to large spherical pressure vessels. The process offers deposition rates that are 5 to 10 times higher than conventional welding processes, making it economically attractive for large-scale applications. The low dilution rate (5-15%) ensures that the overlay composition closely matches the specified stainless steel grade, providing the required corrosion resistance.

One of the most significant insights from this study is the importance of the melt-back pass in ensuring a reliable bond between the overlay and the base metal. The literature reports that omitting or inadequately executing the melt-back pass is the primary cause of bonding failures in strip ESW overlays. This finding underscores the need for rigorous procedural discipline and operator training in strip ESW applications.

Another important insight is the role of δ-ferrite in the overlay microstructure. While δ-ferrite is often considered an undesirable phase in stainless steel welds, its controlled presence (5-10%) in strip ESW overlays is beneficial for preventing solidification cracking and improving corrosion resistance. This nuanced understanding of microstructure-property relationships is essential for optimizing the process parameters and achieving a high-quality overlay.

From a practical standpoint, the literature also highlights the challenges of applying strip ESW to curved surfaces such as spherical heads. The travel mechanism must be capable of following the curvature of the head, and the backing strip must be properly positioned to maintain a consistent slag pool geometry. The literature recommends using a flexible backing strip (3-5 mm thick carbon steel) that can conform to the spherical surface, and a travel mechanism with a tilting head that adjusts the torch angle to maintain a perpendicular relationship with the surface.

Finally, the economic analysis presented in the literature is compelling: strip ESW overlay reduces the total cost of a corrosion-resistant spherical head by 40 to 60% compared to using a fully stainless steel head, while providing equivalent corrosion resistance. This cost advantage, combined with the process efficiency and quality reliability, makes strip ESW the preferred process for large-scale overlay applications in the pressure vessel industry.