Nickel-Based Alloy Automatic Overlay Welding Technology for Nuclear Steam Generator Tube Sheets

Literature Overview

This 2011 publication by Liu Mingyu, Xu Wenjing (CGN Engineering Co., Ltd.) and Zhang Maolong, Sun Zhiyuan (Shanghai Electric Nuclear Power Equipment Co., Ltd.) presents a comprehensive study of nickel-based alloy automatic overlay welding technology applied to nuclear power steam generator (SG) tube sheets. Steam generators are among the most critical components in pressurized water reactor (PWR) nuclear power plants, and their tube sheets serve as the structural interface between the primary coolant circuit and the secondary steam side. The tube sheet must withstand high-temperature water corrosion, chloride-induced stress corrosion cracking (Cl-SCC), and the mechanical loads imposed by the tube-to-tube-sheet weld joints. The application of nickel-based alloy overlay layers on the secondary side of the tube sheet is essential for achieving the required corrosion resistance and service life of 60 years or more.

Technical Background and Design Requirements

Nuclear steam generator tube sheets are typically fabricated from austenitic stainless steel (SA-270 or SA-270M) or nickel-based alloys (Inconel 690). The tube sheet thickness can range from 100 mm to over 200 mm depending on the reactor power rating and design pressure. The overlay welding requirement arises because the base material, while structurally adequate, may not provide sufficient resistance to chloride-induced stress corrosion cracking in the secondary side environment, particularly in boiling water regions where chloride concentrations can be elevated.

The overlay layer specifications typically require:

The challenge lies in achieving a defect-free overlay on a large-diameter, thick tube sheet while maintaining tight control over dilution, microstructure, and residual stresses. The tube sheet geometry, with its thousands of tube holes, creates significant challenges for welding sequence planning and distortion control.

Process Development and Key Technical Parameters

The authors developed an automatic submerged arc welding (SAW) process with powder addition for the overlay application. The process utilizes a multi-pass approach with careful control of each pass to minimize dilution and ensure uniform coverage across the large tube sheet surface area.

The welding sequence was carefully planned to minimize distortion and residual stress. The tube sheet was divided into concentric rings, with welding proceeding from the center outward or from the outer diameter inward, depending on the specific distortion behavior observed during qualification testing. The tube holes were protected with caps or plugs during the overlay welding operation to prevent weld spatter and slag ingress.

Metallurgical Analysis and Quality Control

The microstructure of the Inconel 625 overlay deposited by this process was characterized using optical microscopy, scanning electron microscopy (SEM), and electron backscatter diffraction (EBSD). The overlay microstructure consists of equiaxed austenitic grains with a grain size of approximately 30–50 μm, containing a small fraction of delta ferrite (2–5 vol%) distributed along grain boundaries. This delta ferrite content is beneficial for preventing hot cracking during welding but must be controlled to avoid sensitization and intergranular corrosion.

The dilution analysis was conducted by performing chemical analysis at multiple depths within the overlay. The results showed that the first pass exhibited dilution of approximately 15–20%, while subsequent passes reduced the dilution to 8–12%. This progressive reduction in dilution is attributed to the increasing thickness of the previously deposited overlay, which acts as a thermal mass and diluent buffer. The final dilution values were within the acceptable range specified by the design requirements.

The NDT results demonstrated that the process was capable of producing defect-free overlays when the parameters were properly controlled. The primary defect concerns were:

The authors emphasize that the post-weld solution heat treatment is critical for achieving the required corrosion resistance. The solution treatment at 1100 °C dissolves any precipitated carbides and homogenizes the microstructure, followed by rapid water quenching to prevent re-precipitation. This treatment step is mandatory for nuclear applications and must be performed in accordance with the applicable ASME code requirements.

Engineering Practice and Quality Assurance

The fabrication of nuclear steam generator tube sheets is subject to rigorous quality assurance requirements governed by ASME Section III, Class MC, and the applicable national nuclear regulatory standards. The overlay welding procedure must be qualified in accordance with ASME Section IX, and the welders must be certified for the specific process and material combination.

The inspection regime includes:

• Pre-weld inspection: Visual examination of the base surface for defects, cracks, or contamination; ultrasonic testing of the base plate for subsurface defects.
• In-process inspection: Monitoring of all welding parameters; visual inspection of each pass for surface defects; measurement of interpass temperature.
• Post-weld inspection: 100% ultrasonic testing (UT) for lack of fusion and volumetric defects; 100% penetrant testing (PT) for surface and near-surface defects; dimensional inspection of overlay thickness at specified intervals.
• Final inspection: Chemical analysis of the overlay at multiple locations; hardness testing to verify the overlay properties; intergranular corrosion testing (ASTM A923 Practice E) to verify corrosion resistance.

The authors report that the developed process achieved a first-pass acceptance rate exceeding 95% for the overlay welding operation, with the remaining rework attributed primarily to minor surface defects requiring local repair and re-inspection.

Key Questions and Reflections

The study raises several important considerations for the nuclear industry. First, the long-term performance of the Inconel 625 overlay under the actual service conditions of a PWR steam generator, including the effects of thermal cycling, radiation, and coolant chemistry, requires continued monitoring and research. Second, the impact of the post-weld heat treatment on the residual stresses and dimensional stability of the thick tube sheet warrants further investigation. Third, the qualification and certification requirements for nuclear-grade overlay welding impose significant costs and schedule constraints, which must be carefully managed during project planning.

The dilution control strategy employed in this study—using multiple passes with progressive dilution reduction—represents a sound engineering approach that can be adapted for other nuclear component applications requiring nickel-based alloy overlays. The emphasis on rigorous NDT and metallurgical characterization reflects the appropriate level of quality assurance required for safety-critical nuclear components.

Study Insights and Implications

This study exemplifies the intersection of advanced materials technology and rigorous quality assurance in nuclear power equipment manufacturing. The successful development of a reliable nickel-based alloy overlay welding process for steam generator tube sheets demonstrates that modern welding technology can meet the demanding requirements of nuclear applications when properly qualified and controlled. For engineers involved in nuclear component fabrication, the key takeaway is that process development must be approached with a systematic methodology that integrates metallurgical understanding, process optimization, and comprehensive quality assurance from the outset. The lessons learned from this work are directly transferable to other nuclear applications requiring corrosion-resistant overlay layers, including reactor pressure vessel internals, feedwater heaters, and spent fuel storage components.