Automatic Submerged Arc Weld Overlay of Main Steam Valve Sealing Surfaces

Literature Overview and Industrial Context

This 1995 study by Wang Xinghua, Shen Huijie, Sun Jianzhong, and Xu Tao from Harbin Turbine Works addresses a critical manufacturing challenge in the production of main steam valves for large power generation units. Main steam valves operate under extreme conditions — temperatures up to 570–620°C, pressures exceeding 16–25 MPa, and continuous cyclic loading — and their sealing surfaces must maintain tight contact and resistance to thermal fatigue, erosion, and galling throughout the valve's operational life. The study focuses on automatic submerged arc welding (SAW) as the overlay process for depositing hardfacing or corrosion-resistant alloy layers on the valve seat and plug sealing surfaces.

Technical Requirements for Valve Sealing Surface Overlay

The sealing surfaces of main steam valves are subjected to a unique combination of degradation mechanisms:

Thermal fatigue: Cyclic heating and cooling during valve operation causes thermal stress in the sealing surfaces, leading to microcracking and eventual failure
Erosion: High-velocity steam flow (typically 80–150 m/s at the valve seat) erodes the surface material, particularly at the sealing contact zone
Galling and seizure: Metal-to-metal contact between the plug and seat under high pressure and temperature can cause adhesive wear and seizure
Creep: Sustained high-temperature stress can cause creep deformation of the sealing surfaces, leading to loss of contact pressure

The overlay layer must therefore possess:

High hardness at elevated temperature (HV ≥ 350–450 at 550°C)
Excellent thermal fatigue resistance
Resistance to galling and seizure (low friction coefficient, high work-hardening rate)
Compatibility with the base material (similar thermal expansion coefficient)
Sufficient ductility to accommodate thermal cycling without cracking

Typical overlay materials for main steam valve sealing surfaces include:

Stellite 6 (Co-Cr-W alloy): Excellent thermal fatigue resistance, galling resistance, and hot hardness
Stellite 21 (Co-Cr-Mo alloy): Improved thermal fatigue resistance over Stellite 6
310 stainless steel (Ni-Cr austenitic): Good thermal expansion match, moderate wear resistance
Custom Ni-Cr-Mo alloys: Tailored for specific service conditions

Submerged Arc Welding Process for Valve Sealing Surfaces

SAW is the preferred overlay process for valve sealing surfaces due to several advantages:

High deposition rate (5–15 kg/h) suitable for thick overlay layers (typically 5–15 mm)
Deep penetration and good fusion with the base material
Excellent slag protection, reducing oxidation and nitrogen pickup
Suitable for automated, repeatable deposition on curved surfaces
Cost-effective for large production volumes

The automatic SAW process for valve sealing surfaces typically employs:

Flux: Rutile-type or basic-type flux (e.g., HJ431, HJ432, or equivalent)
Wire: Solid wire of the overlay alloy composition (e.g., Stellite 6 wire, 310SS wire)
Current: DC, typically 250–450 A
Voltage: 25–35 V
Travel speed: 50–150 mm/min
Preheat: 200–400°C depending on base material and overlay alloy

Process Parameter	Typical Value	Rationale
Current (DC)	250–450 A	Adequate penetration without excessive dilution
Voltage	25–35 V	Arc stability and wire feed control
Travel speed	50–150 mm/min	Controls heat input and dilution
Flux coverage	10–15 mm	Complete protection of weld pool
Preheat temperature	200–400°C	Reduces residual stress, prevents cracking
Interpass temperature	≤350°C	Controls cooling rate and phase formation
Post-weld heat treatment	700–850°C for 2–4 h	Stress relief, grain refinement

Quality Control and Inspection Requirements

The quality of the valve sealing surface overlay is critical to the safe and reliable operation of the power generation unit. The following quality control measures are essential:

Visual inspection (VT): All overlay surfaces must be free of cracks, porosity, undercut, and excessive spatter. The surface should be smooth and uniform, with no visible defects.
Magnetic particle testing (MT): Surface and near-surface defects (cracks, seams) must be detected. MT is particularly effective for detecting fine cracks in the overlay layer and at the overlay-base interface.
Ultrasonic testing (UT): Subsurface defects (porosity, inclusions, lack of fusion) must be detected. UT is also used to verify bond strength at the overlay-base interface.
Hardness testing: The overlay layer hardness must be verified after post-weld heat treatment. For Stellite 6, typical hardness is HV 350–450 after solution treatment and aging. Hardness uniformity across the overlay area must be verified.
Metallographic examination: Cross-sections of the overlay layer must be examined to verify:

No cracks at the overlay-base interface
No excessive dilution (typically <30% for critical applications)
Sound microstructure (martensitic for Stellite, austenitic for 310SS)
No porosity or inclusions

Sealing surface finish: After overlay and heat treatment, the sealing surface must be machined to the required surface finish (typically Ra 0.4–1.6 μm) and dimensional accuracy (typically ±0.05 mm for seat diameter, ±0.02 mm for surface flatness).

Engineering Challenges and Solutions

Several engineering challenges are specific to the overlay of valve sealing surfaces:

Geometric complexity: Valve sealing surfaces are curved (conical, spherical, or flat) and often have tight tolerances. The SAW process must be adapted to deposit uniform thickness on curved surfaces, which requires either multi-axis robotic manipulation or custom fixture design.
Dilution control: Excessive dilution of the overlay alloy with the base material (typically low-alloy steel or cast steel) can reduce the overlay layer's corrosion and wear resistance. Dilution is controlled by:
Using a low-heat-input welding process (lower current, higher travel speed)
Applying a "transition layer" of compatible material before the final overlay layer
Using a flux with low iron content to reduce iron pickup
Multiple thin passes rather than few thick passes
Residual stress management: The overlay process introduces significant residual stresses, which can lead to cracking or distortion of the valve body. Residual stresses are managed by:
Preheating the valve body to 200–400°C
Controlling interpass temperature to ≤350°C
Post-weld stress relief heat treatment (typically 700–850°C for 2–4 hours)
Sequential welding of symmetric areas to balance thermal effects
Post-weld machining: The overlay layer must be machined to final dimensions after welding and heat treatment. The machining process must be carefully controlled to avoid:
Work hardening of the overlay surface (use of appropriate cutting tools and speeds)
Thermal damage from excessive cutting heat (use of coolants, low cutting speeds)
Distortion of the valve geometry (use of precision fixtures, minimal clamping force)

Study Insights and Reflections

This study reflects the mature engineering practice of the 1990s in Chinese power generation equipment manufacturing. The use of automatic SAW for valve sealing surface overlay was a well-established technique at Harbin Turbine Works, one of China's leading manufacturers of large turbine components. The study likely documents the process parameters, quality control procedures, and performance data that were developed through years of industrial experience.

For modern engineering practice, the fundamental principles remain valid, but several advancements have been made:

Laser cladding: Offers lower dilution, higher precision, and better control over microstructure, but at higher cost and lower deposition rate
Hot-wire TIG (HW-TIG): Offers lower heat input and better control for thin overlay layers, suitable for repair applications
Plasma transferred arc (PTA) cladding: Offers excellent dilution control and microstructure, suitable for high-performance overlay applications
Advanced filler metals: Newer Co-Cr-W alloys (e.g., Stellite 71, Stellite 76) offer improved thermal fatigue resistance

The key engineering insight from this study is that the selection of overlay process and material must be driven by the specific service conditions of the valve — temperature, pressure, steam velocity, and cycle frequency. There is no single "best" solution; rather, the optimal combination of process, material, and quality control must be determined through careful analysis of the operating environment and failure modes.

Summary

The automatic submerged arc weld overlay of main steam valve sealing surfaces is a well-established industrial process that has been refined over decades of practice. The study by Wang Xinghua and colleagues from Harbin Turbine Works documents the process parameters, quality control procedures, and engineering considerations that are essential for producing reliable valve sealing surfaces. The key challenges — dilution control, residual stress management, geometric complexity, and post-weld machining — require careful process design and rigorous quality assurance. For modern engineers, the principles established in this study remain a solid foundation, complemented by newer technologies such as laser cladding and advanced filler metals that offer improved performance at higher cost. The ultimate goal remains unchanged: to produce valve sealing surfaces that maintain integrity and tightness throughout the operational life of the power generation unit.