Cracking Analysis and Improvement of Stellite Overlay on Domestic Turbine-Driven Pump Balance Disc

Literature Overview

The 2020 study by Liu Xing from Fuqing Nuclear Power Company addresses a critical failure mode encountered in domestic turbine-driven pump (TDP) balance discs in nuclear power plants. The balance disc is a precision component that regulates axial thrust in centrifugal pumps, and it is typically overlaid with Stellite alloy (Co-Cr-W hardfacing) to provide erosion and corrosion resistance. The study investigates the root cause of cracking in the Stellite overlay layer and proposes engineering improvements, representing a significant contribution to nuclear-grade repair welding technology.

Core Technical Content

Failure Mode Description

The balance disc cracking manifested as circumferential cracks in the Stellite overlay layer, typically originating near the weld toe or at the overlay-substrate interface. The cracks propagated in a semi-circumferential pattern, with crack lengths ranging from 2-15 mm. The failure was detected during periodic in-service inspection, raising concerns about the structural integrity of the component.

Root Cause Analysis

The investigation employed a multi-faceted approach combining macroscopic examination, metallographic analysis, scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS):

Thermal Cracking Mechanism: The primary cracking mechanism was identified as solidification cracking (hot cracking) in the Stellite overlay. This occurred due to:

• High cooling rates during overlay welding of thin sections (balance disc thickness typically 20-40 mm)
• High dilution with the carbon steel substrate, which altered the solidification characteristics
• Sensitive temperature range for Stellite alloys (approximately 1200-1350°C) where low-melting eutectics form at dendrite boundaries

Residual Stress Contribution: The residual stress analysis revealed compressive stresses near the surface transitioning to tensile stresses at depth. The maximum tensile residual stress reached approximately 250-320 MPa, exceeding the yield strength of the as-welded Stellite deposit.

Microstructural Evidence:

• Liquid film formation at interdendritic regions in the as-welded microstructure
• Intergranular cracking along prior-dendrite boundaries
• Presence of sulfide inclusions (MnS) in the substrate acting as crack initiation sites
• Carbon segregation at grain boundaries due to dilution with carbon steel

Metallurgical Analysis Summary

Process Improvement Measures

Welding Process Optimization

The study proposed several engineering improvements based on the root cause analysis:

1. Preheat and interpass temperature control: Increase preheat to 400-500°C and maintain interpass temperature above 350°C to reduce cooling rate and minimize thermal gradients
2. Welding parameter adjustment: Reduce current density and travel speed to lower the heat input concentration, promoting more uniform solidification
3. Multi-pass technique: Use a multi-pass approach with smaller individual pass widths to distribute the thermal cycle more evenly
4. Post-weld heat treatment: Implement post-weld stress relief at 650-700°C for 2-4 hours to reduce residual stresses below the cracking threshold

Material Selection Improvements

• Use low-sulfur base metal (S < 0.015%) for the balance disc substrate to minimize MnS inclusion formation
• Consider using Stellite 6 with modified composition (reduced C to 1.0-1.2%) for improved crack resistance
• Apply a transition layer (e.g., Ni-Fe or Ni-Cr alloy) between the carbon steel substrate and the Stellite overlay to reduce dilution and improve metallurgical compatibility

Inspection Protocol Enhancement

The study recommended enhanced inspection protocols for nuclear-grade overlay welds:

• 100% magnetic particle testing (MT) of the overlay surface after each welding pass
• Ultrasonic testing (UT) of the overlay-substrate interface to detect lack of fusion
• Hardness mapping across the entire overlay area with a grid spacing of 25 mm
• Residual stress measurement using X-ray diffraction (XRD) at critical locations

Engineering Practice Integration

Nuclear-Specific Considerations

Nuclear power plant components require adherence to stringent standards including ASME Section III (Nuclear Power Plant Components), ASME Section IX (Welding and Brazing Qualifications), and RBB-NP-T00001 (Chinese nuclear welding procedure qualification standard). The balance disc repair must be performed under a qualified welding procedure specification (WPS) with documented qualification records.

The study highlights an important distinction between repair welding in conventional power plants and nuclear power plants. In nuclear applications:

• Welding procedure qualification requires demonstration of weldability over a wider range of variables
• In-service repair requires justification of the repair's effect on component lifetime
• Post-repair inspection must include both conventional NDT and potentially radiographic testing (RT) or phased array ultrasonic testing (PAUT)
• Documentation requirements are significantly more extensive, with full traceability of materials, consumables, and personnel

Practical Lessons for Industry

The case study provides several valuable lessons for engineers working with overlay welds on critical components:

1. The balance disc geometry creates inherent challenges due to its thin section thickness and high aspect ratio, which amplify thermal stresses
2. Domestic manufacturing of nuclear-grade components requires careful attention to material purity (particularly sulfur control) and process discipline
3. The transition from imported to domestic components necessitates thorough qualification and validation to ensure equivalent performance
4. In-service repair of overlay welds requires a systematic approach that addresses both the immediate defect and the underlying process causes

Key Questions and Reflections

A critical question emerging from this study is whether the cracking represents a fundamental limitation of Stellite overlay on carbon steel for this specific application, or whether it is a process control issue that can be fully resolved. The evidence suggests that both factors contribute, and that a combination of process optimization and material selection is required.

Another reflection is the broader implication for domestic nuclear equipment manufacturing. The balance disc failure occurred in a domestically manufactured component, raising questions about the maturity of domestic welding technology for nuclear-grade applications. The study's proposed improvements suggest that the gap can be closed through systematic process development, enhanced material control, and rigorous qualification protocols.

Study Insights and Implications

This case study is a valuable contribution to the field of nuclear-grade repair welding technology. It demonstrates the importance of a systematic root cause analysis approach (combining macroscopic, microscopic, and analytical techniques) in diagnosing overlay weld failures. The proposed improvements are practical and implementable, and they provide a roadmap for enhancing the reliability of domestically manufactured nuclear components. For engineers working in the nuclear industry, the key message is that overlay weld cracking is a multifactorial phenomenon that requires a holistic approach to prevention—one that addresses material selection, process parameters, thermal management, and inspection protocols in an integrated manner.