Strip Electrode Electroslag Surfacing Process and Its Application

Literature Overview

The paper by Zhang Maolong and Sun Libing from Shanghai Boiler Works, published in Boiler Technology (Vol. 24, No. 8, 1993, pp. 21-26), provides a comprehensive review of the strip electrode electroslag surfacing process (SESS) and its application to the overlay welding of corrosion-resistant stainless steel layers on large chemical pressure vessels and nuclear containment vessels. At the time of publication, the strip electrode submerged arc surfacing (SESAS) process was the dominant technology used domestically in China, while the electroslag variant had been widely adopted internationally. This paper represents an important early contribution to the domestic understanding and adoption of SESS technology.

Core Technical Content and Process Characteristics

The strip electrode electroslag surfacing process utilizes a continuous strip electrode (typically 10-20 mm wide) fed through a slag pool that shields the molten weld pool from atmospheric contamination. The process is fundamentally different from conventional submerged arc welding in that the arc is submerged beneath a thick layer of flux, creating a unique thermal cycle and metallurgical environment.

Comparison of Strip Electrode Surfacing Processes

The paper highlights three primary advantages of SESS over SESAS: higher deposition rate, lower dilution of the base metal into the overlay layer, and superior bead profile control. These advantages are particularly significant when depositing expensive corrosion-resistant alloys such as austenitic stainless steels (e.g., 309, 310, 347 grades) onto carbon steel or low-alloy steel substrates, where minimizing dilution is critical to achieving the required corrosion resistance.

Factors Affecting Overlay Layer Quality

The authors identify several key factors that influence the quality of the electroslag surfacing overlay:

1. Flux composition and properties: The slag system must provide adequate fluidity, proper viscosity, and sufficient deoxidation and desulfurization capacity. Fluxes with high alkalinity are preferred to minimize porosity and inclusions.
2. Electrode strip chemistry: The composition of the strip electrode directly determines the overlay layer chemistry. For corrosion-resistant applications, strips with elevated chromium (20-25%), nickel (8-12%), and molybdenum (2-3%) content are typically used.
3. Travel speed and current density: These parameters control the heat input and solidification rate, which in turn affect grain structure, dilution, and mechanical properties.
4. Preheating and interpass temperature: For thick-section vessels, preheating to 150-250°C is necessary to prevent cold cracking in the HAZ. Interpass temperature control (typically 200-350°C) is critical to maintain adequate toughness in the overlay layer.
5. Number of passes and build-up strategy: Multi-pass surfacing requires careful planning of pass sequences to ensure adequate fusion between passes while controlling overall dilution.

Process Analysis and Metallurgical Considerations

The electroslag welding process creates a unique thermal environment in which the weld pool is completely surrounded by a thick slag layer. This results in a slower cooling rate compared to submerged arc welding, which promotes the formation of coarser but more equiaxed grain structures. The slower cooling rate also reduces the risk of hydrogen-induced cracking and martensite formation in the HAZ of low-alloy steel substrates.

However, the slower cooling rate can also lead to grain coarsening in the overlay layer if not properly controlled. The dilution ratio in SESS is typically 3-15%, compared to 15-35% for SESAS. This low dilution is achieved because the thick slag layer acts as a thermal barrier, reducing the heat transfer to the base metal and minimizing the amount of base metal that melts into the weld pool.

Metallurgical Effects of SESS on Overlay Layers

Application Scope and Development Prospects

The paper discusses the application of SESS to large chemical pressure vessels requiring internal corrosion-resistant linings, nuclear containment vessels, and other critical equipment where the integrity of the overlay layer is paramount. The authors note that while SESS was well-established in Western Europe, Japan, and the United States, its adoption in China was still in its infancy at the time of publication.

From a practical standpoint, SESS is most advantageous for large-area surfacing operations where the total overlay thickness is 6-25 mm and the substrate is relatively flat or gently curved. For highly contoured surfaces or small-diameter vessels, the equipment setup and flux handling become impractical, and alternative processes such as submerged arc welding or plasma arc surfacing may be more suitable.

The development prospects discussed in the paper include improvements in flux formulations for specific alloy systems, automation of travel speed and electrode feed rate control, and development of in-process monitoring systems for quality assurance. These developments have largely materialized in subsequent decades, with modern SESS systems incorporating computerized process control and real-time slag pool monitoring.

Engineering Practice Implications

In the context of nuclear containment vessel fabrication, the overlay welding of austenitic stainless steel cladding is a critical process that must be performed to the highest quality standards. The ASME Code Case 2569 and various national standards specify strict requirements for overlay layer chemistry, dilution, and mechanical properties. The SESS process, with its low dilution and high deposition rate, is particularly well-suited to meeting these stringent requirements.

A practical consideration that is not explicitly addressed in the paper but is critical in engineering practice is the post-surfacing treatment of the overlay layer. Depending on the application, the overlay may require pickling and passivation to remove the heat-affected oxide scale and restore the corrosion-resistant surface chemistry. For nuclear applications, the overlay layer may also require magnetic particle inspection (MT) and dye penetrant inspection (PT) of the surface, as well as ultrasonic testing (UT) for subsurface defects.

Quality Assurance Requirements for SESS Overlay Welding

Study Insights and Long-term Significance

This paper, published in 1993, represents a snapshot of the state of electroslag surfacing technology at a time when the process was transitioning from a niche application to a mainstream technology in large vessel fabrication. The authors' analysis of the process characteristics and their influence on overlay quality remains fundamentally valid today, even as equipment automation and monitoring capabilities have advanced significantly.

The key insight that endures from this paper is the recognition that process selection for large-area overlay welding must be based on a systematic evaluation of deposition rate, dilution control, surface quality, and equipment requirements. The SESS process offers a compelling combination of high productivity and low dilution, but its applicability is limited by equipment complexity and the geometric constraints of the workpiece. Engineers must carefully evaluate whether the advantages of SESS justify the additional equipment investment and process development effort for each specific application.