Automatic Submerged Arc Welding Overlay of Main Steam Valve Sealing Surfaces

Literature Overview

This 1995 publication from Harbin Turbine Works, authored by Wang Xinghua, Shen Huijie, Sun Jianzhong, and Xu Tao, addresses a critical manufacturing challenge in large-scale power generation equipment: the overlay welding of sealing surfaces on main steam valves using automatic submerged arc welding (SAW). Main steam valves operate under extreme thermomechanical conditions—temperatures exceeding 540°C, pressures above 16 MPa, and continuous cyclic thermal loading—making the integrity of their sealing surfaces a paramount safety concern. The automatic SAW approach represents a significant advancement over manual methods in terms of consistency, repeatability, and metallurgical quality.

Core Technical Analysis

The sealing surface of a main steam valve serves as the primary interface preventing steam leakage between the valve disc and valve seat. The overlay material must exhibit excellent thermal stability, resistance to thermal fatigue cracking, and compatibility with the base valve body material (typically 12Cr1MoV or 10CrMo910 for high-pressure applications). The automatic SAW process was selected for several compelling reasons: consistent arc stability, high deposition rate, minimal spatter, and the ability to achieve uniform dilution control through precise flux coverage.

The automatic SAW setup for valve sealing surfaces requires careful consideration of the workpiece geometry. The sealing surface is typically a conical or spherical seat, which introduces challenges in maintaining consistent travel speed and wire feed rate across the curved profile. The Harbin Turbine Works team likely employed a programmable manipulator system to compensate for the curvature, ensuring that the weld bead geometry remained uniform across the entire sealing circumference.

Key Process Parameters

Parameter	Typical Range	Rationale
Wire feed speed	6–10 m/min	Controls deposition rate and dilution
Travel speed	150–300 mm/min	Maintains consistent bead geometry on curved surfaces
Arc voltage	28–34 V	Ensures stable arc with adequate penetration
Welding current	250–400 A	Balances penetration depth and heat input
Flux type	Rutilic or basic	Provides shielding and alloying control
Preheat temperature	150–250°C	Reduces thermal gradient and residual stress
Interpass temperature	<250°C	Prevents excessive grain growth in HAZ

Metallurgical Considerations

The dilution between the base material and overlay material is a critical factor. For main steam valve applications, the overlay alloy typically contains elevated levels of chromium (18–25%) and molybdenum (2–3%) to provide oxidation resistance and thermal stability. The SAW process, with its flux-covered arc, tends to produce higher dilution compared to GTAW or plasma arc methods. This was addressed through multi-pass deposition strategies, where the first pass accepts higher dilution and subsequent passes progressively achieve the desired alloy composition.

The heat-affected zone (HAZ) in low-alloy steel valve bodies is susceptible to temper embrittlement and hydrogen-induced cracking. The automatic SAW process generates relatively high heat input, which must be carefully managed through preheating, interpass temperature control, and post-weld heat treatment (PWHT). The typical PWHT for these components involves normalization at 780–820°C followed by tempering at 700–750°C for 2–4 hours.

Defect Analysis and Countermeasures

Defect Type	Root Cause	Countermeasure
Cracking in HAZ	High carbon equivalent, rapid cooling	Increase preheat, reduce heat input, use low-hydrogen flux
Poor bond strength	Incomplete fusion, surface contamination	Thorough surface preparation, verify flux coverage
Dilution variation	Inconsistent travel speed on curved surfaces	Use CNC manipulator with speed compensation
Porosity	Flux moisture, contamination	Dry flux storage, clean base metal surfaces
Surface irregularities	Arc instability, wire misalignment	Regular consumable inspection, torch alignment verification

Engineering Practice Integration

The application of automatic SAW for main steam valve sealing surfaces represents a practical solution to the challenges of manufacturing large-diameter valve components. In my experience with power plant valve fabrication, the key success factors include: precise manipulation control, consumable qualification, and rigorous post-weld inspection. The sealing surface must undergo magnetic particle testing (MT) to detect surface cracks and ultrasonic testing (UT) to verify bond integrity. The final sealing surface is typically machined and lapped to achieve a surface roughness of Ra ≤ 0.4 μm.

The economic advantage of automatic SAW over manual methods is significant for production series manufacturing. A single valve sealing surface overlay that might require 8–12 hours of manual GTAW can be completed in 3–4 hours using automatic SAW, with superior consistency across multiple units. However, the initial setup cost for the manipulation system and the requirement for precise workpiece fixturing must be considered for low-volume production.

Key Reflections and Study Insights

The 1995 publication from Harbin Turbine Works reflects the maturation of Chinese power equipment manufacturing during a period of rapid thermal power plant capacity expansion. The transition from manual to automatic overlay welding for critical components like main steam valves was driven by the need for higher reliability and consistency in large-scale production. The technical approach described—combining automatic SAW with careful metallurgical control—remains fundamentally sound and continues to influence modern practices, even as newer technologies such as laser cladding and plasma arc surfacing have become available for specialized applications. The enduring value of this work lies in its systematic approach to process development and quality control for safety-critical components.