Development and Application of High-Speed Electroslag Welding Overlay in Boiler Manufacturing

Literature Overview

This study, published in the journal Boiler Technology in 2003 by Zhang Liangcheng and Ding Bixue from Shanghai Boiler Works Co., Ltd., addresses the development and industrial application of high-speed electroslag welding (ESW) overlay processes. The paper is particularly significant because Shanghai Boiler Works was one of China's leading boiler manufacturers at the time, and the research directly addressed practical demands in high-pressure and ultra-high-pressure boiler fabrication, where corrosion-resistant overlay layers on carbon steel or low-alloy steel substrates are essential for extending component life in aggressive service environments.

Core Technical Content

High-speed electroslag welding overlay represents a significant advancement over conventional ESW overlay techniques. The fundamental principle relies on the high thermal input and stable slag pool characteristic of the electroslag process, which produces a dilution ratio between the overlay material and the base metal that can be carefully controlled by adjusting the welding parameters. The "high-speed" variant achieves deposition rates significantly higher than standard ESW overlay, making it economically attractive for large-area cladding applications on boiler components such as headers, drums, and pressure parts.

The key technical challenge in ESW overlay lies in controlling dilution. Because the slag pool temperature is very high (typically 1800–2100 °C), substantial melting of the base metal occurs, which can compromise the corrosion resistance of the overlay layer. The authors investigated how welding current, voltage, travel speed, electrode composition, and flux formulation interact to minimize dilution while maintaining sound metallurgical bonding.

Process Analysis and Engineering Practice

The high-speed ESW overlay process was developed specifically for the demanding requirements of boiler manufacturing, where components such as main steam headers, feedwater pipes, and waterwall headers require corrosion-resistant and erosion-resistant overlay layers. The process was applied to overlay materials including austenitic stainless steels (304, 316), nickel-based alloys, and high-silicon cast irons, depending on the service environment.

One critical aspect discussed in the literature is the multi-pass overlay strategy. To achieve acceptable corrosion resistance at the overlay-base metal interface, a transition layer is often deposited first using a filler material with intermediate composition, followed by the final overlay passes with the target alloy composition. This approach ensures that even if dilution reaches 10–15% in the first pass, subsequent passes progressively enrich the surface composition to meet the required corrosion resistance specifications.

The process parameters for high-speed ESW overlay were optimized to achieve deposition rates of 5–10 kg/h, which is 2–3 times higher than conventional ESW overlay and significantly higher than submerged arc welding (SAW) overlay. This productivity advantage makes the process particularly suitable for large-diameter components where the overlay area is extensive.

From a quality control perspective, the authors emphasized the importance of interpass temperature control, preheating of the base metal to prevent cracking, and post-weld heat treatment to relieve residual stresses. The typical preheat temperature for low-alloy steel substrates is 150–250 °C, and the interpass temperature should not exceed 350 °C to avoid grain coarsening in the heat-affected zone.

Common Defects and Countermeasures

Based on the engineering experience documented in the paper, the following defects and countermeasures were identified:

Study Insights and Engineering Implications

The development of high-speed ESW overlay by Shanghai Boiler Works represents an important milestone in China's boiler manufacturing capability. The process bridges the gap between productivity and quality, offering deposition rates that rival SAW overlay while maintaining the superior dilution control of conventional ESW. The key insight from this work is that process optimization must be holistic — welding parameters, filler material selection, flux formulation, and heat treatment must be considered as an integrated system rather than independent variables.

In modern engineering practice, high-speed ESW overlay has found continued application in the fabrication of hydrogenation reactors, ammonia synthesis converters, and other high-pressure vessels where large-area cladding is required. The process remains one of the most cost-effective methods for depositing thick overlay layers (up to 10–15 mm) on large-diameter components. However, the process is limited to flat or large-radius geometries and cannot be applied to complex shapes, which has led to the development of complementary processes such as SAW overlay and PTA cladding for more geometrically complex components.

This literature remains highly relevant for engineers involved in pressure vessel fabrication and repair, as it provides practical guidance on process parameter selection, defect prevention, and quality assurance for one of the most productive overlay welding methods available in industrial practice.