Automatic Submerged Arc Surfacing of Main Steam Valve Sealing Surface

Literature Overview

This paper by Wang Xinghua, Shen Huijie, Sun Jianzhong, and Xu Tao from Harbin Turbine Works, published in the journal "Welding" in 1995, addresses a critical engineering challenge in thermal power generation. The study focuses on the automatic submerged arc welding (SAW) surfacing of 12%Cr steel on the sealing surfaces of main steam valves for subcritical 300/600 MW turbine units. The technology was originally imported from Westinghouse Company of the United States and subsequently optimized through extensive welding trials, welder training, and product trial production. The valve sealing surface surfacing quality is identified as one of the most important quality characteristics of the main steam valve, directly affecting the operational safety of the entire turbine unit.

Core Technical Content

The main steam valve operates under extreme conditions of high temperature, high pressure, and continuous cyclic loading. The sealing surface must maintain dimensional accuracy, metallurgical integrity, and leak-tight performance throughout the service life of the turbine. The selection of 12%Cr steel for the surfacing layer is driven by its excellent resistance to thermal fatigue, oxidation, and creep at elevated temperatures typical of superheated steam environments. The surfacing layer must also exhibit compatible thermal expansion with the base material to avoid cracking during thermal cycling.

The authors describe a comprehensive approach that includes welding procedure qualification, welder certification, and production trial runs. The key technical challenge lies in achieving consistent surfacing quality on curved sealing surfaces while maintaining the required dilution rate between the surfacing layer and the base metal. Submerged arc welding offers the advantages of high deposition efficiency, deep penetration, and the ability to produce smooth, dense weld deposits suitable for subsequent machining to precise sealing tolerances.

Process Development and Welder Training

The optimization process involved systematic welding trials to determine the optimal combination of welding current, voltage, travel speed, flux composition, and preheating temperature. The authors emphasize that achieving consistent results requires not only the correct welding parameters but also rigorous welder training and qualification. The production environment must be controlled to prevent contamination of the flux and the weld pool, which could introduce porosity or inclusions into the surfacing layer.

The study demonstrates that the domestic optimization of the imported technology resulted in a welding procedure that satisfies both the product technical requirements and the practical constraints of workshop production. This reflects a mature engineering approach where imported technology is not simply replicated but adapted to local materials, equipment, and operational conditions. The emphasis on welder training underscores the understanding that human factors are critical in achieving repeatable welding quality, particularly for safety-critical components.

Engineering Practice Implications

For engineers working on power plant valve maintenance and manufacturing, this paper provides valuable insight into the systematic approach required for surfacing critical sealing surfaces. The methodology can be extended to other high-pressure valve applications where reliable sealing under extreme conditions is paramount. The paper also highlights the importance of integrating welding metallurgy knowledge with production engineering considerations, a principle that remains relevant in modern power plant maintenance operations.

The study reinforces the principle that surfacing welding is not merely a deposition process but a comprehensive engineering activity involving materials selection, process optimization, operator qualification, and quality assurance. The long-term reliability of the valve depends on the metallurgical compatibility of the surfacing layer, the absence of defects such as cracks and porosity, and the dimensional accuracy achievable after post-weld machining.

Summary

This paper represents a significant contribution to the domestic development of high-temperature valve surfacing technology in China. By systematically optimizing the imported Westinghouse technology through rigorous welding trials and operator training, the authors established a reliable production method for main steam valve sealing surfaces. The work underscores the critical role of surfacing quality in power plant safety and demonstrates the engineering maturity achieved through disciplined technology transfer and adaptation.