Zhuojin Pipe Fitting Co., LtdAutomatic Submerged Arc Surfacing of Main Steam Valve Sealing Surfaces
Literature Overview and Technical Background
The paper published in the journal Welding (1995, Vol. 7, pp. 19–20) by Wang Xinghua and colleagues from Harbin Turbine Works describes the development and implementation of automatic submerged arc surfacing (SAW) for the sealing surfaces of main steam valves in a subcritical 300/600 MW thermal power turbine unit. The technology was originally introduced from Westinghouse Electric Company (WH) and subsequently optimized for domestic production conditions. The core requirement is the deposition of 12% Cr steel overlay onto the valve sealing surfaces, a critical quality characteristic that directly governs the operational safety and long-term reliability of the turbine unit.
This work represents a significant milestone in China's power generation equipment manufacturing history, as it documents the transition from imported technology to domestic capability in a safety-critical application. The authors emphasize that the surfacing quality of the main steam valve is not merely a surface treatment concern but a fundamental quality feature that determines the integrity of the turbine's steam admission system.
Core Technical Analysis
The selection of 12% Cr steel for the sealing surface overlay is dictated by the demanding service conditions of the main steam valve, which operates at temperatures typically in the range of 540–565 °C and pressures up to 16.7 MPa in subcritical units. The 12% Cr grade provides adequate oxidation resistance, thermal fatigue resistance, and dimensional stability under cyclic thermal loading. The key metallurgical challenge lies in achieving proper dilution control between the overlay layer and the base material, which is typically a low-alloy steel forging such as 12Cr1MoV or a similar grade.
The automatic submerged arc surfacing process was selected over manual methods for several compelling reasons:
- Consistent deposition rate and geometry control across multiple passes
- Reduced risk of operator-induced defects such as porosity and incomplete fusion
- Improved productivity for repetitive, large-scale production runs
- Better gas shielding quality due to the flux blanket, minimizing atmospheric contamination
| Parameter | Typical Specification |
|---|---|
| Base material | 12Cr1MoV or equivalent low-alloy forging steel |
| Overlay material | 12% Cr martensitic stainless steel |
| Process | Automatic submerged arc surfacing (SAW) |
| Flux type | Low-hydrogen, rutile-basic composite |
| Wire diameter | 1.6–2.0 mm |
| Current range | 300–450 A (DC) |
| Travel speed | 150–350 mm/min |
| Preheating temperature | 200–300 °C |
| Interpass temperature | ≤ 250 °C |
| Post-weld heat treatment | 650–700 °C, 2–4 h, furnace cool |
The optimization work conducted by the authors involved systematic welding trials to determine the optimal combination of current, voltage, travel speed, and flux composition. They also addressed the critical issue of dilution control, which directly affects the final chromium content and microstructure of the overlay layer. A dilution rate exceeding 30% can significantly reduce the corrosion and oxidation resistance of the deposited layer, shifting the microstructure away from the desired tempered martensite toward a softer ferrite-pearlite structure.
Process Development and Quality Assurance
The authors describe a comprehensive approach that encompassed welding procedure qualification, welder training, and product trial production. This methodology aligns well with modern quality management frameworks such as PDCA (Plan-Do-Check-Act). The qualification process involved:
- Plan phase: Analysis of Westinghouse's original welding procedures, identification of applicable Chinese standards (GB/T and DL/T), and determination of required overlay properties including hardness (typically 200–280 HBW for 12% Cr steel), tensile strength (≥ 620 MPa), and impact toughness.
- Do phase: Execution of welding trials with varying parameter combinations, metallographic examination of cross-sections, and mechanical property testing of test coupons.
- Check phase: Comparison of trial results against technical specifications, identification of process windows that consistently meet requirements, and documentation of any deviations.
- Act phase: Standardization of the optimal procedure, development of work instructions for production welders, and establishment of in-process quality checkpoints.
A particularly noteworthy aspect of this work is the emphasis on welder training. Even with automatic equipment, the setup, flux handling, and monitoring of the surfacing operation require skilled operators. The authors recognized that the consistency of automatic surfacing is only as good as the operator's ability to maintain proper wire feed, travel speed, and flux coverage throughout the operation.
Engineering Practice Implications and Reflections
From a modern engineering perspective, this 1995 publication remains highly relevant for several reasons. First, the fundamental metallurgical and process challenges of 12% Cr steel surfacing on main steam valves have not changed; the same dilution concerns, microstructural control requirements, and post-weld heat treatment protocols apply to current subcritical and supercritical units. Second, the systematic approach to technology transfer and process qualification documented here provides a model for similar endeavors in other equipment categories.
In contemporary practice, the automatic SAW surfacing of valve sealing surfaces is often supplemented or replaced by robotic gas metal arc welding (GMAW) or plasma arc surfacing, which offer greater flexibility in geometry handling and reduced heat input. However, the SAW approach retains advantages in terms of cost-effectiveness for high-volume production and superior shielding quality. The key lesson from this literature is that process selection should be driven by the specific geometry, production volume, and quality requirements of the application, rather than by a default preference for newer technologies.
The authors' work also underscores the importance of understanding the interaction between the base material microstructure and the overlay. In the case of 12Cr1MoV base material, the prior austenite grain size and the distribution of precipitates in the heat-affected zone (HAZ) significantly influence the weldability and the final properties of the surfacing weld. Modern practice would additionally incorporate residual stress measurement (using X-ray diffraction or neutron diffraction) and distortion prediction through finite element analysis to further refine the process.
This literature serves as a valuable historical reference and a practical guide for engineers involved in power plant valve manufacturing. Its systematic treatment of technology adaptation, process qualification, and quality assurance remains a model for engineering excellence in safety-critical welding applications.