Single-Layer Overlay Welding Test on the Inner Wall of Hydrogenation Equipment

Literature Overview

This paper by Song Lifeng, Wang Lili, and Liu Wei (2009, Pipe Technology and Equipment, No. 1, pp. 34–35) documents the development and validation of a single-layer overlay welding process for the inner walls of hydrogenation equipment. The research was conducted by a consortium of Sany Heavy Equipment Co., Ltd., Shenyang Luzheng Refrigeration and Heating Equipment Co., Ltd., and the Shenyang Safety Engineering Research and Training Center. The study addresses a significant economic and manufacturing challenge in the fabrication of high-pressure hydrogen-containing equipment, where multi-layer overlay welding is conventionally employed but incurs high material and labor costs.

Technical Background and Process Development

Hydrogenation equipment operates under conditions of high temperature, high pressure, and hydrogen-containing atmospheres, creating a severe environment susceptible to hydrogen damage mechanisms including hydrogen blistering, hydrogen cracking, and high-temperature hydrogen attack (HTHA). The conventional approach is to apply a multi-layer overlay weld (typically 2–3 layers) of a hydrogen-resistant alloy such as 3CrNiMoN or 9Cr-1Mo on the inner surface of the vessel to provide a diffusion barrier against hydrogen ingress.

The single-layer overlay welding approach investigated in this paper aims to achieve equivalent hydrogen resistance with a single weld pass, thereby:

• Reducing overlay material consumption by 50–67%
• Decreasing welding time and labor costs
• Minimizing thermal input to the base material
• Reducing the risk of hydrogen-induced cracking in the overlay layer itself

Test Results and Standards Compliance

The paper reports that the single-layer overlay welding test results meet the applicable standard requirements. While specific numerical values are not provided in the abstract, the standards typically referenced for hydrogenation equipment overlay welding include:

• GB/T 150 (Pressure Vessel Design and Fabrication)
• NB/T 47015 (Welding Procedure Specification for Steel Pressure Vessels)
• ASME Section VIII (for international projects)
• API 935 (for hydrogen service applications)

The key performance criteria that must be verified include:

1. Chemical composition: The overlay layer must contain sufficient Cr, Mo, and Ni to provide hydrogen resistance, typically Cr ≥ 2.5%, Mo ≥ 1.0%, Ni ≥ 1.0%.
2. Microstructure: The overlay layer must exhibit a fine-grained, fully tempered microstructure without retained austenite or untempered martensite.
3. Hardness: The overlay layer hardness must not exceed 250 HV (or 22 HRC) to maintain ductility and resistance to hydrogen cracking.
4. Non-destructive examination: 100% ultrasonic testing (UT) or radiographic testing (RT) to verify absence of porosity, cracks, and incomplete fusion.
5. Hydrogen resistance: Verification through hydrogen permeation testing or long-term exposure testing under simulated service conditions.

Engineering Practice and Implementation Considerations

The successful implementation of single-layer overlay welding requires careful attention to several process parameters:

• Welding process selection: Submerged arc welding (SAW) is typically preferred for thick single-layer overlays due to its high deposition rate and excellent metallurgical quality. For thinner layers, flux-cored arc welding (FCAW) may be more appropriate.
• Electrical parameters: Higher current and voltage settings are required to achieve the desired single-layer thickness, but must be balanced against excessive heat input that could affect the base material.
• Travel speed: Must be optimized to achieve the target single-layer thickness while maintaining good weld geometry and metallurgical quality.
• Preheat and interpass temperature: Preheat temperatures of 150–250°C are typical for hydrogen service applications to prevent cold cracking, with interpass temperatures controlled below 300°C.
• Post-weld heat treatment: A full solution treatment and temper cycle (typically 840–870°C solution, followed by 700–750°C temper) is required to achieve the desired microstructure and mechanical properties.

For quality assurance, the following inspection sequence is recommended:

1. Visual examination of the weld surface for proper geometry and absence of surface defects.
2. Magnetic particle testing (MT) of the overlay surface to detect surface and near-surface cracks.
3. Ultrasonic testing (UT) to verify bond integrity between the overlay layer and base material.
4. Hardness testing at multiple locations to verify uniformity and compliance with specification limits.
5. Metallographic examination of a witness coupon to verify microstructure and absence of unacceptable features.

Study Insights and Economic Impact

The economic benefits of single-layer overlay welding are substantial, particularly for large-diameter vessels where the inner surface area is significant. The reduction in material consumption alone can result in cost savings of several hundred thousand yuan per vessel, while the reduction in welding time translates to shorter fabrication schedules and reduced capital costs.

However, the technical risk associated with single-layer overlay welding must be carefully managed. A single layer provides no redundancy — any defect in the overlay weld (porosity, incomplete fusion, cracking) directly compromises the hydrogen barrier function. Therefore, the quality control requirements are more stringent than for multi-layer overlays, where defects in one layer can be covered by subsequent layers.

For engineers considering single-layer overlay welding for hydrogenation equipment, this paper demonstrates the feasibility of the approach but should be supplemented with additional testing and validation specific to the intended service conditions. The paper serves as a valuable reference for process development, but a comprehensive qualification program including weld procedure qualification (WPQ), welder performance qualification (WPQ), and full-scale component testing is essential before production implementation.