Single-Layer Electroslag Surfacing for Hydrogenation Equipment Shell Inner Wall

Literature Overview

This 2023 paper published in Welding (No. 9, pp. 59-64) represents the most recent and technically advanced of the five studies reviewed. Authored by Li Ling, Guo Xiaoxiao, Xin Hongbo, and Yu Shijun from Zhenhai Petrochemical Construction Engineering Co., Ltd., the study investigates the application of single-layer strip electrode electroslag surfacing technology for the inner wall overlay of hydrogenation equipment shells. This is a significant advancement over conventional multi-pass overlay welding methods, offering thicker overlay layers with better metallurgical quality in a single pass.

Core Technical Content

Hydrogenation equipment shells require a corrosion-resistant overlay layer on the inner surface to protect against high-temperature hydrogen attack and corrosive environments. The overlay layer is typically an austenitic stainless steel or nickel-based alloy applied to the Cr-Mo low-alloy steel shell. The conventional approach uses multi-pass SMAW or GMAW overlay welding, which is time-consuming and may result in incomplete fusion between passes.

The single-layer strip electrode electroslag surfacing technology uses a continuous strip electrode to deposit a thick overlay layer in a single pass through electroslag welding. The key results are:

Process Technology Analysis

Single-layer strip electrode electroslag surfacing is a specialized welding process that differs significantly from conventional arc welding overlay methods:

Process Mechanism

The strip electrode is fed continuously into the molten slag pool, and the molten metal is deposited on the workpiece surface. The slag pool provides:

• Excellent heat insulation, resulting in slow cooling rates
• Protection of the molten metal from atmospheric contamination
• Refining of the weld metal through slag-metal reactions
• Uniform heat input distribution

Process Parameters

The study establishes optimal process parameters for the application:

Metallurgical Advantages

The electroslag process offers several metallurgical advantages over conventional arc welding:

1. Slow cooling rate: The slag pool provides excellent thermal insulation, resulting in cooling rates of 1-10°C/s compared to 50-200°C/s in conventional arc welding. This slow cooling rate promotes:

• Complete austenitization and grain growth
• Carbide precipitation and spheroidization
• Reduced residual stress
• Elimination of untempered martensite

1. Single-pass deposition: The ability to deposit 5-7 mm in a single pass eliminates:

• Incomplete fusion between passes
• Composition variation between passes
• Multiple heat input cycles on the same area
• Potential interpass contamination

1. Uniform composition: The continuous feeding of the strip electrode and the thorough mixing in the slag pool result in a uniform composition throughout the overlay layer.

Quality Verification and Standards Compliance

The study provides comprehensive quality verification data:

Ferrite Content

The ferrite content of 5-10 FN is within the acceptable range for austenitic stainless steel overlay welds. The WRC-1992 diagram is used to calculate the expected ferrite content based on the chemical composition, and the magnetic method provides experimental verification. The agreement between calculated and measured values confirms the accuracy of the composition analysis.

Hardness Requirements

The hardness requirement of ≤235 HV10 for the overlay layer, heat-affected zone (HAZ), and base material is consistent with the requirements for hydrogen service equipment. This hardness limit ensures:

• Adequate resistance to hydrogen-induced cracking
• Good toughness and ductility
• Resistance to stress corrosion cracking
• Compatibility with post-weld heat treatment requirements

Microstructural Examination

The absence of untempered martensite and microcracks in the overlay layer, HAZ, and base material is a critical finding. Untempered martensite is a major concern in hydrogen service because it is:

• Brittle and prone to cracking
• Permeable to hydrogen due to its high dislocation density
• Susceptible to hydrogen-induced cracking
• Difficult to eliminate without post-weld heat treatment

The electroslag process eliminates untempered martensite through the slow cooling rate, which allows for complete transformation of austenite to ferrite and pearlite (or tempered martensite if the cooling rate is still too fast).

Engineering Practice Integration

The adoption of single-layer strip electrode electroslag surfacing for hydrogenation equipment offers several practical benefits:

1. Increased overlay thickness: The 5-7 mm overlay thickness exceeds the conventional 4.0-4.5 mm requirement, providing additional protection against corrosion and hydrogen damage.
2. Reduced production time: Single-pass deposition eliminates the time required for multiple passes and interpass preparation, potentially reducing overlay welding time by 50-70%.
3. Improved quality consistency: The elimination of interpass effects and the uniformity of the electroslag process result in more consistent quality compared to multi-pass arc welding.
4. Reduced operator skill requirement: The electroslag process is more automated and less sensitive to operator technique than manual arc welding, reducing the impact of operator variability on quality.
5. Scalability: The process is well-suited for large-scale production of hydrogenation equipment shells, which are typically large diameter vessels requiring extensive overlay coverage.

Key Questions and Reflections

• The study focuses on flat test specimens and does not address the challenges of applying the process to curved surfaces, nozzles, or complex geometries typical of hydrogenation equipment.
• The long-term performance of the electroslag overlay layer under high-temperature hydrogen service is not addressed. The study focuses on as-welded properties, and the effect of prolonged exposure to hydrogen at 400-450°C remains uncertain.
• The dilution between the strip electrode and the base material is not explicitly quantified. For hydrogenation equipment, the dilution ratio affects the corrosion resistance and hydrogen barrier properties of the overlay layer.
• The study does not compare the electroslag process with alternative overlay methods such as laser cladding, plasma spray, or multi-pass GMAW with fill wire. A comparative analysis would provide a more complete evaluation of the process advantages.
• The economic analysis of the electroslag process, including equipment costs, consumable costs, and labor costs, is not provided. The process may offer technical advantages but may not be economically competitive for all applications.

Study Insights and Implications

This study represents a significant advancement in overlay welding technology for hydrogenation equipment. The single-layer strip electrode electroslag surfacing process offers the potential to produce thicker, more uniform, and higher quality overlay layers compared to conventional multi-pass arc welding methods.

The key insight is that the slow cooling rate and single-pass nature of the electroslag process eliminate several quality concerns associated with conventional overlay welding, including untempered martensite, microcracks, and interpass incomplete fusion. These metallurgical advantages translate directly to improved reliability and potentially extended service life of hydrogenation equipment.

For engineers involved in hydrogenation equipment design and fabrication, this study demonstrates that process innovation can address long-standing quality challenges. The adoption of electroslag surfacing for hydrogenation equipment shells represents a practical step toward improved equipment reliability and reduced maintenance costs. The technology is particularly promising for large-scale production environments where process consistency and quality control are critical requirements.