Single-Layer Strip Electrode Electroslag Overlay Welding for Hydrogenation Equipment Shell Inner Wall

Literature Overview

This paper by Li Ling, Guo Xiaoxiao, Xin Hongbo, and Yu Shijun from Zhenhai Petrochemical Construction Engineering Co., Ltd., published in Welding (2023, Issue 9, pp. 59-64), presents a significant advancement in overlay welding technology for hydrogenation equipment. The research focuses on single-layer strip electrode electroslag overlay welding (SSOES) applied to the inner shell wall of hydrogenation reactors, achieving overlay thicknesses of 5-7 mm compared to the conventional 4.0-4.5 mm standard.

Core Technical Content

Hydrogenation equipment in petroleum refining and petrochemical operations requires overlay welding of the inner shell surface to provide resistance against:

• High-temperature hydrogen attack (HTHA)
• Corrosion from acidic species in the feed stream
• Thermal cycling fatigue
• Mechanical erosion from catalyst particles

The conventional overlay welding methods for hydrogenation equipment shells include:

• Submerged arc welding (SAW) with multiple layers
• Manual metal arc welding (SMAW) with multi-pass deposition
• Electroslag welding (ESW) with multiple layers

The single-layer strip electrode electroslag overlay welding technique represents a process innovation that achieves greater deposition thickness in a single pass, reducing production time while maintaining quality.

Test Materials and Results

The overlay welding trials were conducted on two commonly used hydrogenation equipment shell materials:

Key Process Parameters

The optimal welding parameters determined through experimental trials included:

• Strip electrode composition: Carefully selected to achieve proper free ferrite content and corrosion resistance
• Slag system: Designed to provide stable arc and proper wetting characteristics
• Welding current and voltage: Optimized for single-layer deposition of 5-7 mm thickness
• Travel speed: Controlled to achieve uniform deposition and sound metallurgy
• Preheating temperature: Selected to prevent cracking while maintaining desired microstructure
• Interpass temperature: Controlled to prevent excessive grain growth and maintain hardness below 235 HV10

Technical Interpretation

The achievement of 5-7 mm single-layer deposition represents a significant advancement over the conventional 4.0-4.5 mm requirement for several reasons:

1. Production efficiency: Single-layer deposition eliminates the need for multiple passes, reducing welding time by approximately 50-60%
2. Quality consistency: A single deposition pass eliminates interpass defects and provides more uniform microstructure
3. Corrosion resistance: Greater thickness provides additional protection against HTHA and corrosion
4. Residual stress reduction: The electroslag welding process produces lower residual stresses compared to arc welding methods

Metallurgical Considerations

The free ferrite content of 5-10 FN is critically important for the overlay layer in hydrogenation service. This range provides:

• Sufficient austenite for ductility and crack resistance
• Adequate ferrite for resistance to intergranular corrosion
• Appropriate balance to prevent sigma phase formation during long-term high-temperature service

The WRC-1992 formula calculation confirms the magnetic measurement results, providing dual verification of the ferrite content. This is particularly important because the WRC-1992 formula accounts for the specific alloying elements present in the overlay composition.

Hardness Control

The requirement that hardness in the overlay layer, heat-affected zone, and base metal all remain at or below 235 HV10 is critical for hydrogenation service. This limit is established to prevent:

• Embrittlement due to excessive hardness
• Cracking during post-weld heat treatment
• Reduced resistance to high-temperature hydrogen attack

The electroslag welding process achieves this hardness control through:

• Controlled cooling rates inherent to the electroslag process
• Post-weld heat treatment (PWHT) to temper any hard phases
• Proper alloy composition of the strip electrode

Integration with Engineering Practice

The successful development of this single-layer strip electrode electroslag overlay welding process has direct implications for hydrogenation equipment fabrication and repair:

• Fabrication cost reduction: The process reduces overlay welding time and labor costs
• Quality improvement: Single-layer deposition provides more consistent metallurgical quality
• Equipment life extension: Greater overlay thickness provides longer service life before re-overlay is needed
• Applicability: The process is suitable for both new fabrication and field repair of hydrogenation equipment

Comparison with Conventional Methods

Study Insights and Implications

This research represents a meaningful process innovation in hydrogenation equipment overlay welding technology. The combination of experimental trials and theoretical analysis provides a rigorous foundation for process qualification. The achievement of 5-7 mm single-layer deposition while maintaining all quality parameters within specification demonstrates that the electroslag welding process can be successfully adapted for overlay welding applications that were previously limited to multi-pass arc welding.

The comprehensive verification of metallurgical quality — including the absence of unquenched martensite, microcracks, and excessive hardness — provides strong evidence that the process produces welds suitable for critical hydrogenation service. The dual verification of free ferrite content through both magnetic measurement and WRC-1992 calculation adds confidence in the composition control.

The practical significance of this work extends beyond the specific equipment studied. The process development methodology — combining experimental trials with metallurgical analysis and quality verification — provides a template for developing new overlay welding processes for other critical equipment applications. The finding that single-layer electroslag welding can achieve superior results compared to multi-pass arc welding challenges the conventional approach and opens new possibilities for process optimization in the industry.