Stainless Steel Strip Electrode Submerged Arc and Electroslag Overlay Welding for Hydrogenation Reactors

Literature Overview

The paper by Zhou Weiyu and Fan Zheng (1990), published in Boiler Technology (Vol. 21, No. 11, pp. 14-17), presents experimental results on single-layer and double-layer strip electrode submerged arc welding (SAW) and electroslag welding (ESW) overlay of stainless steel on 2¼Cr-1Mo steel for hydrogenation reactors in the petrochemical industry. The authors, affiliated with Sandvik, employed Sandvik 24.13.LNb (single layer) and Sandvik 24.13.LNb + 19.9.LNb (double layer) with sintered Sandvik flux 10SW. This work is significant because hydrogenation reactors operate under severe conditions of high temperature, high hydrogen partial pressure, and corrosive hydrocarbon environments, making the overlay layer critical for long-term structural integrity.

Core Technical Content

Hydrogenation reactors are pressure vessels that operate at temperatures up to 450–500 °C and pressures exceeding 10 MPa, with continuous exposure to atomic hydrogen and hydrocarbon corrosion. The 2¼Cr-1Mo steel base material provides the necessary strength and creep resistance for the vessel body, but it is susceptible to hydrogen attack and sulfidation corrosion at the inner surface. The overlay layer must therefore provide a barrier against hydrogen penetration and resist sulfidation in the presence of hydrogen sulfide.

Overlay Layer Specifications

The selection of Type 347L (24.13.LNb) as the primary overlay material reflects the standard practice in the petrochemical industry, where 347L is widely used for hydrogenation reactor overlays due to its excellent resistance to sulfidation and hydrogen attack. The addition of niobium provides carbide stabilization, preventing chromium depletion at grain boundaries and thereby maintaining corrosion resistance at elevated temperatures.

Process Comparison: SAW vs. ESW

The paper compares single-layer and double-layer approaches, noting that single-layer SAW provides a thinner but more precisely controlled overlay, while double-layer ESW provides greater thickness with lower cost per unit volume. The choice between these approaches depends on the specific requirements of the reactor: a single layer may suffice for moderate corrosion conditions, while a double layer is preferred for severe service where the first layer provides ductility and the second layer provides enhanced corrosion resistance.

Engineering Practice Insights

The overlay welding of hydrogenation reactors is subject to strict regulatory requirements, typically governed by ASME Section VIII Division 1 or Division 2, and in some cases by more stringent codes such as NB/T 47015 (Chinese national standard for pressure vessels). The overlay layer must be qualified through procedure qualification testing, including mechanical property testing, corrosion testing, and non-destructive examination.

The use of sintered flux 10SW is noteworthy because flux composition directly affects weld metal cleanliness. In hydrogenation service, even trace amounts of sulfur or phosphorus in the overlay layer can initiate sulfidation cracking or promote hydrogen permeation. The sintered flux provides better control of weld metal chemistry compared to granulated flux, as it can be formulated with precise alloy additions and lower impurity levels.

The double-layer approach using 24.13.LNb followed by 19.9.LNb is a sophisticated engineering solution. The first layer (24.13.LNb) provides good ductility and resistance to thermal fatigue cracking, which is critical because the reactor undergoes repeated start-up and shutdown cycles. The second layer (19.9.LNb) provides enhanced corrosion resistance through higher chromium content, creating a more effective barrier against sulfidation and hydrogen attack. This layered approach reflects a fundamental principle in overlay welding: the interface between layers must be designed to accommodate differential thermal expansion and to prevent delamination under cyclic loading.

Key Questions and Reflections

Several technical questions arise from this literature that deserve further consideration. First, the paper does not discuss the long-term performance of the overlay layers under actual hydrogenation conditions. Laboratory corrosion tests may not fully replicate the synergistic effects of high temperature, high hydrogen pressure, and hydrocarbon chemistry that occur in service. Second, the paper does not address the issue of overlay layer cracking during reactor start-up and shutdown, which is a known failure mode in hydrogenation reactors. The thermal cycling can generate stresses at the overlay-base interface that may exceed the yield strength of the overlay layer, leading to cracking and loss of protection.

Third, the paper does not discuss the inspection and maintenance of overlay layers during reactor operation. In practice, periodic inspection of the overlay layer is essential to detect thinning, cracking, or spalling before failure occurs. Techniques such as ultrasonic thickness measurement, magnetic particle testing, and eddy current testing are commonly used for this purpose, but the paper does not address these practical considerations.

Study Insights and Implications

This literature represents a significant contribution to the understanding of overlay welding for hydrogenation reactors, particularly in the context of the Chinese petrochemical industry. The comparison of single-layer and double-layer approaches, and the evaluation of SAW versus ESW processes, provides valuable guidance for engineers selecting overlay systems for hydrogenation service. The use of Sandvik materials and fluxes highlights the importance of material quality and process control in achieving reliable overlay performance. For engineers involved in pressure vessel design and maintenance, this paper underscores the critical role of overlay welding in extending the service life of hydrogenation reactors and ensuring safe operation under severe conditions.