Single-Layer Strip Electrode Electroslag Welding Overlay Technology in Hydrocracking Reactor Manufacturing

Literature Overview

This study by Liu Yan, Ma Xiaobing, Wang Junjie, Li Fufu, An Tianyou, and Wang Xiping from Xinjiang Lanchi Heavy Energy Engineering Co., Ltd. (2020) addresses the critical challenge of producing large-diameter hydrocracking reactor shells with corrosion-resistant overlay layers using single-layer strip electrode electroslag welding (ESW). Published in the journal Chemical Machinery, this paper represents a significant engineering advancement in the fabrication of hydrogenation reactors used in petroleum refining and petrochemical processing. The authors demonstrate that single-layer strip electrode ESW can produce a uniform, defect-free overlay layer on large-diameter reactor shells, offering advantages in efficiency, quality control, and cost compared to multi-layer or multi-pass alternatives.

Core Technical Content

The hydrocracking reactor is one of the most demanding pressure vessels in the petroleum refining industry. It operates under high temperatures (350–450 °C), high hydrogen partial pressures (5–8 MPa), and aggressive sulfide environments, requiring a combination of high-strength base material (typically 2.25Cr-1Mo or 3.5Cr-1Mo steel) and a corrosion-resistant overlay layer (typically 309L or 309Mo austenitic stainless steel). The traditional approach involves multi-layer submerged arc welding (SAW) overlay or manual gas tungsten arc welding (GTAW) overlay, both of which suffer from low efficiency on large-diameter shells and difficulty in maintaining uniform dilution control.

The single-layer strip electrode ESW method proposed in this study utilizes a continuous strip electrode (typically 309L or 309Mo stainless steel strip, 12–15 mm wide, 0.8–1.2 mm thick) fed through a water-cooled electrode holder. The process exploits the deep penetration and high deposition rate inherent to electroslag welding, adapted for overlay application. Key process parameters include:

Process Analysis and Key Technical Points

The fundamental innovation lies in controlling the single-layer deposition to achieve adequate bond strength while minimizing dilution of the overlay material. The authors report dilution rates of 10–15% at the interface, which is acceptable for 309L overlay on 2.25Cr-1Mo base steel. The slag pool in ESW provides excellent protection against atmospheric contamination, and the high heat input produces a slow-cooling microstructure that reduces the risk of cracking in the overlay layer.

A critical aspect discussed is the control of the slag pool geometry. In conventional ESW for thick plate welding, the slag pool is designed for deep penetration into the base metal. For overlay application, the slag pool must be shallower to ensure that the deposited layer is predominantly overlay material rather than a heavily diluted mixture. The authors achieved this by optimizing the flux composition and the electrode holder geometry.

The single-layer approach eliminates the need for multiple passes, significantly reducing production time for large-diameter reactor shells (typically 3000–6000 mm OD). However, this also means that the single layer must be thick enough to provide adequate corrosion resistance (minimum 3 mm overlay thickness is required per ASME VIII Div.1 UHA-51 and NB/T 47015).

Engineering Practice and Quality Control

From an engineering practice perspective, the following quality control measures are essential:

1. Visual inspection: The overlay surface must be smooth and free from craters, undercut, and surface irregularities. The transition between the overlay and the base metal must be gradual and well-defined.
2. Bond strength testing: According to ASTM A263, the bond strength of the overlay layer must be verified through a tensile test of a coupon containing the overlay/base metal interface. The fracture must occur in the base metal or in the overlay material, not at the interface.
3. Intergranular corrosion testing: The overlay layer must pass the ASTM A263 intergranular corrosion test (acid copper test) to verify that sensitization during welding has not occurred.
4. Radiographic testing (RT): Per NB/T 47013 or ASME V, the overlay layer must be inspected for lack of fusion, porosity, and other volumetric defects. The single-layer ESW overlay typically shows a more uniform internal structure than multi-pass SAW overlay due to the continuous nature of the process.
5. Hardness testing: The overlay layer hardness should be measured at multiple locations to verify uniformity. Typical hardness values for 309L overlay range from 150–200 HV.

Study Insights and Implications

This paper highlights a practical solution to a long-standing challenge in pressure vessel fabrication. The single-layer strip electrode ESW method offers a compelling alternative for large-diameter reactor shells where traditional overlay methods are either too slow or produce inconsistent results. The key insight is that the inherent characteristics of ESW—high deposition rate, deep slag pool protection, and stable arc—can be harnessed for overlay application with appropriate process parameter optimization.

However, engineers must be aware of several limitations. The single-layer approach requires careful control of the heat input to prevent excessive grain growth in the overlay layer, which could compromise mechanical properties. Additionally, the method is most suitable for large-diameter shells where the geometry allows for stable ESW operation; it may not be practical for smaller diameters or complex geometries. The post-weld heat treatment is critical to relieve residual stresses and ensure the overlay layer meets the required intergranular corrosion resistance.

This technology represents a meaningful contribution to the field of bimetal pressure vessel fabrication, particularly for the Chinese petroleum refining industry where large-diameter hydrocracking reactors are in increasing demand.