Automatic Submerged Arc Weld Overlay of Main Steam Valve Sealing Surfaces

Literature Overview

This technical study focuses on the application of automatic submerged arc welding (SAW) for the weld overlay of sealing surfaces on main steam valves in power generation plants. Main steam valves are critical components in the steam turbine system, operating at high temperatures (500-600°C), high pressures (10-25 MPa), and subjected to cyclic loading during start-up and shutdown. The sealing surfaces must maintain tightness under these severe conditions, and any wear or erosion can lead to steam leakage, reduced efficiency, and potential safety hazards. The study addresses the process development, quality control, and performance evaluation of automatic SAW overlay for this specific application.

Core Technical Findings

The study developed a process for applying a multi-pass SAW overlay to the sealing surfaces of main steam valves, using a nickel-based alloy (Inconel 625) as the overlay material and a chromium-based alloy (Stellite 6) as the bonding layer. The multi-pass strategy ensures adequate bond strength and wear resistance while minimizing dilution and cracking susceptibility.

Process Parameter	Bonding Layer (Stellite 6)	Overlay Layer (Inconel 625)
Welding current	320-380 A	300-360 A
Arc voltage	26-30 V	24-28 V
Travel speed	200-280 mm/min	220-300 mm/min
Electrode diameter	φ2.4 mm	φ2.0 mm
Flux type	Basic (SJ101)	Basic (SJ101)
Number of passes	1	2-3
Preheat temperature	150-200°C	150-200°C
Interpass temperature	< 250°C	< 250°C
Overlay thickness	1.5-2.0 mm	2.0-3.0 mm

The Inconel 625 overlay provides excellent resistance to erosion-corrosion at high temperatures, while the Stellite 6 bonding layer ensures adequate metallurgical compatibility with the valve body material (typically 12Cr1MoV or similar low-alloy steel).

Process Development and Optimization

The process development followed a systematic approach using design of experiments (DOE) methodology to optimize the welding parameters. The key response variables were bond strength, overlay hardness, and absence of defects (cracks, porosity, lack of fusion). The optimization process identified the following critical factors:

Preheat temperature: Insufficient preheat leads to cracking at the fusion line due to拘束 stresses. Excessive preheat reduces hardness and may cause grain coarsening in the base metal. The optimal range of 150-200°C balances these competing requirements.
Heat input: The heat input must be sufficient to ensure complete fusion at the bonding layer/base metal interface but not so high as to cause excessive dilution or base metal melting. The optimal heat input range is 1.2-1.8 kJ/mm for the bonding layer and 1.0-1.5 kJ/mm for the overlay layer.
Travel speed consistency: Automatic SAW requires precise control of travel speed to ensure uniform overlay thickness and microstructure. Variations in travel speed of more than ±5% can lead to inconsistent overlay properties and potential defects.

Quality Control and Inspection

Comprehensive quality control is essential for main steam valve overlay, given the critical safety role of these components. The inspection regime includes:

Inspection Method	Purpose	Acceptance Criteria	Standard
Visual inspection (VT)	Surface defects, porosity	No cracks, no visible porosity > 1 mm	JB/T 4730
Magnetic particle testing (MT)	Surface/subsurface cracks	No linear indications > 1 mm	JB/T 4730
Ultrasonic testing (UT)	Bond strength, lack of fusion	No indications at fusion line	JB/T 4730
Hardness testing	Overlay hardness uniformity	HV30 300-400 for Inconel 625	ASTM B231
Metallographic examination	Microstructure, bond quality	No cracks, no lack of fusion	ASTM E3
Hydrostatic test	Leak tightness	No leakage at 1.5× design pressure	GB/T 150

The ultrasonic testing of the bonding layer/base metal interface is particularly critical. The bond strength between the Stellite 6 bonding layer and the 12Cr1MoV valve body is the primary determinant of the overlay's service life. UT in accordance with JB/T 4730 or equivalent standard is mandatory, with acceptance criteria requiring no indications at the fusion line.

Common Defects and Countermeasures

Defect	Root Cause	Countermeasure
Cracking at fusion line	High拘束 stress, hydrogen embrittlement	Increase preheat, use low-hydrogen flux, post-weld heat treatment
Lack of fusion	Insufficient heat input, poor base metal preparation	Increase current, ensure proper surface preparation
Porosity	Flux moisture, contamination	Dry flux at 300°C for 2h, clean base surface
Excessive dilution	Excessive heat input, wrong consumable	Optimize parameters, use appropriate consumable
Overlay thickness variation	Travel speed inconsistency, wire feed variation	Calibrate equipment, use consistent parameters
Hardness variation	Parameter variation, microstructure variation	Control parameters tightly, perform hardness mapping

Engineering Application and Performance

The developed process has been applied to main steam valves in multiple power generation plants. The results demonstrate significant improvements in service life and reliability:

Application	Before (Conventional)	After (SAW Overlay)	Improvement
Service life	8,000-12,000 hours	25,000-35,000 hours	2.5-3x
Leakage rate at 500°C	0.5-1.0%	< 0.1%	5-10x reduction
Maintenance frequency	Every 6 months	Every 24 months	4x reduction
Overlay thickness loss per 10,000h	0.8-1.2 mm	0.2-0.3 mm	3-4x reduction

The improved service life is attributed to the superior erosion-corrosion resistance of the Inconel 625 overlay at high temperatures, combined with the excellent bond strength provided by the Stellite 6 bonding layer. The overlay maintains its integrity under cyclic thermal loading, with no evidence of spalling or delamination after extended service.

Key Questions and Reflections

The study raises important considerations regarding the long-term performance of the overlay under cyclic thermal loading. While the overlay demonstrates excellent performance in initial service, the effect of repeated thermal cycling on the bond strength and overlay integrity over multiple years of operation warrants continued monitoring. Engineers should establish a monitoring program that includes periodic UT and VT inspections to detect any degradation in bond quality.

Another critical consideration is the effect of the overlay on the valve's thermal expansion behavior. The coefficient of thermal expansion of Inconel 625 is different from that of the 12Cr1MoV valve body, which may lead to differential thermal expansion stresses during start-up and shutdown. The study does not address this issue in detail, but it is an important consideration for the long-term reliability of the overlay.

The cost-effectiveness of the SAW overlay process should also be evaluated. While the initial cost of the overlay is higher than conventional machining or replacement, the extended service life and reduced maintenance frequency provide significant economic benefits. A life-cycle cost analysis should be conducted to quantify the economic advantages of the overlay process.

Study Insights and Implications

The application of automatic SAW overlay to main steam valve sealing surfaces represents a successful example of applying advanced welding technology to a critical power generation component. The process development, quality control, and performance evaluation demonstrate that weld overlay can significantly extend the service life and improve the reliability of critical components in severe service conditions.

For engineers involved in power plant maintenance and component improvement, the key takeaway is that weld overlay technology offers a viable alternative to component replacement, with significant cost and time savings. The process is particularly attractive for components that are difficult to replace or have long lead times, as the overlay can be performed in-situ or in a workshop setting with minimal downtime.

The broader implication is that weld overlay technology should be considered as a standard tool in the maintenance and improvement toolkit for power generation plants. As component life extension becomes increasingly important in the context of aging infrastructure and resource efficiency, weld overlay offers a practical and proven solution for extending the service life of critical components.