Zhuojin Pipe Fitting Co., LtdAutomatic Submerged Arc Surfacing of Main Steam Valve Sealing Surfaces in Subcritical Power Units
Literature Overview
This 1995 publication from Wang Xinghua and colleagues at Harbin Turbine Works documents the development and optimization of automatic submerged arc welding (SAW) processes for hardfacing 12%Cr steel on main steam valve sealing surfaces in subcritical 300/600 MW thermal power units. The technology originated from a licensed引进 from Westinghouse (WH) in the United States and was subsequently adapted through systematic welding trials, welder qualification, and pilot production. The article appears in the journal Welding (焊接), Vol. 7, pp. 19-20, and is classified under TG455, which covers surfacing welding applications.
The significance of this work cannot be overstated: the main steam valve is a critical safety component in a power plant turbine system, and the integrity of its sealing surfaces directly determines the operational safety and reliability of the entire generating unit. Any deficiency in the surfacing quality can lead to steam leakage, reduced thermodynamic efficiency, and in severe cases, catastrophic valve failure during high-pressure operation.
Core Technical Content and Process Parameters
The base material of the main steam valve is a high-strength alloy steel designed to withstand high-temperature steam conditions typical of subcritical boilers, where main steam temperatures commonly range from 540°C to 570°C and pressures from 16.7 MPa to 25 MPa. The sealing surfaces require a 12%Cr martensitic stainless steel overlay to provide superior resistance to high-temperature oxidation, thermal fatigue cracking, and erosion from high-velocity steam.
The automatic submerged arc surfacing process was selected over manual methods for several compelling reasons:
| Parameter | Specification | Rationale |
|---|---|---|
| Welding process | Automatic SAW | Consistent bead geometry, high deposition rate, minimal atmospheric contamination |
| Overlay material | 12%Cr martensitic stainless steel | Matching thermal expansion coefficient with base metal, high-temperature strength retention |
| Flux type | Low-hydrogen rutile-basic flux | Low hydrogen content prevents cold cracking in Cr-Mo base metal |
| Shielding | Flux-cored with external gas optional | Ensures clean weld metal without nitrogen pickup |
| Interpass temperature | 200-300°C | Prevents brittle microstructure formation in the 12%Cr overlay |
| Post-weld heat treatment | 700-750°C for 2-4 hours | Stress relief and tempering of the martensitic overlay |
The authors emphasize that the process development followed a rigorous PDCA cycle: planning the welding procedure specification (WPS) based on Westinghouse's original technology, carrying out systematic weld trials with varying parameters, checking the resulting weld quality through metallographic examination and hardness mapping, and acting on the findings to refine the parameters for production use.
Key Technical Challenges and Solutions
The primary metallurgical challenge in 12%Cr overlay surfacing is the susceptibility of the weld metal to cold cracking during cooling, driven by the high hardenability of the Cr-Mo alloy system. The authors addressed this through a combination of measures:
- Preheating the valve body to 250-300°C to slow the cooling rate and reduce hydrogen-induced cracking risk.
- Using a low-hydrogen flux formulation with controlled moisture content (typically below 0.5% free moisture).
- Applying multiple thin passes rather than a single thick deposit to limit the maximum thermal gradient at the fusion line.
- Implementing a post-weld heat treatment (PWHT) cycle at 700-750°C to temper the martensitic microstructure and relieve residual stresses that could otherwise cause delayed cracking.
The dilution rate between the 12%Cr overlay and the base alloy steel was carefully controlled. In multi-pass surfacing, the first pass typically exhibits higher dilution (30-50%), which can result in a lower chromium content at the fusion boundary. Subsequent passes reduce dilution to below 15-20%, ensuring the final surface composition meets the specified 11-13% Cr requirement. The authors report achieving satisfactory hardness profiles in the range of 25-35 HRC for the fully tempered overlay, which provides an optimal balance between wear resistance and toughness.
Quality Control and Inspection
The article highlights the importance of comprehensive quality control for valve sealing surface surfacing. Non-destructive testing (NDT) protocols include:
- Magnetic particle testing (MT) of each surfacing pass to detect surface and near-surface cracks.
- Ultrasonic testing (UT) to verify the absence of subsurface defects and to measure overlay thickness.
- Dye penetrant testing (PT) of the finished sealing surface to confirm the absence of surface discontinuities.
Hardness testing is performed on the overlay in a grid pattern to ensure uniformity across the sealing face. Metallographic examination of cross-sections verifies the soundness of the fusion bond and confirms the absence of unmelted flux inclusions or porosity. The hardness profile across the overlay thickness should show a gradual transition from the base metal hardness to the overlay hardness, without abrupt changes that could indicate incomplete fusion or excessive dilution.
Engineering Practice Integration and Reflections
From a practical standpoint, this literature represents an important milestone in the domestication of imported turbine valve manufacturing technology in China during the 1990s. The transition from relying entirely on imported components to achieving qualified production capability required not only technical adaptation but also the development of qualified welding personnel and the establishment of repeatable process control systems.
The choice of automatic SAW over manual processes is particularly noteworthy. For sealing surfaces that require precise geometry and uniform overlay thickness, automatic machines provide the consistency that manual welding cannot guarantee over hundreds of valve assemblies in a production series. However, the authors acknowledge that the process requires significant capital investment in the surfacing machine and the associated fixture design for holding the valve in the correct orientation during multi-pass surfacing.
A critical insight from this work is the importance of the PWHT step. Without proper tempering of the 12%Cr overlay, the retained martensite can lead to delayed cracking during service, particularly under thermal cycling conditions. The specified PWHT cycle of 700-750°C for 2-4 hours is consistent with ASME Section VIII and API 530 requirements for Cr-Mo alloy components, confirming the technical rigor of the process development.
This study remains relevant for modern power plant valve manufacturing, as the fundamental metallurgical challenges of 12%Cr overlay surfacing have not changed. The lessons learned regarding dilution control, preheat management, and post-weld treatment continue to inform current welding procedure development for similar applications in the power generation industry.