Nuclear-Grade Valve Sealing Surface Overlay Crack Analysis and Residual Stress Evolution

Literature Overview

Yan Xiangyu and colleagues from Dalian University of Technology, in collaboration with China General Nuclear Power Operation Co., Ltd., present a comprehensive study on crack formation mechanisms in Norem 02 iron-based alloy overlay layers on nuclear-grade valve sealing surfaces. The research was supported by the National Key R&D Program (2022YFB2503503) and the National Natural Science Foundation (52375310). Published in Heat Processing Technology (2026, Vol. 55, No. 15, pp. 37-45), this work addresses a critical safety issue in nuclear power plant operations where overlay cracking has led to serious leakage incidents.

Core Technical Findings

Through mechanical load fatigue experiments and thermal load fatigue experiments on valve simulation specimens, the researchers identified that cracks in the valve sealing surface overlay are brittle through-thickness cracks caused by the combined action of welding residual stress and thermal stress. Thermoplastic finite element calculations were performed to model the residual stress distribution in the multi-layer multi-pass overlay, and X-ray diffraction (XRD) measurements were used to verify the numerical simulation results.

The study revealed two key findings regarding residual stress evolution. First, during normal valve operation, the thinning of the overlay layer due to wear results in minimal changes in overlay stress, indicating that wear-induced geometry changes are not a primary crack initiation mechanism. Second, under thermal loading at 1000°C, the overlay stress increases significantly, demonstrating that thermal cycling is the dominant factor leading to overlay material failure.

Crack Mechanism Analysis

The brittle through-thickness cracking mechanism identified in this study is consistent with the high residual stress levels typically found in multi-layer multi-pass overlay welds. During the welding process, each successive pass imposes tensile residual stress on the previously deposited layers. In a thick overlay, these stresses can accumulate to levels approaching or exceeding the yield strength of the overlay material, creating a highly stressed state that is susceptible to crack initiation.

When the valve is subjected to thermal loading during operation, the temperature gradient between the hot sealing surface and the cooler valve body creates additional thermal stresses. The combination of pre-existing welding residual stresses and operational thermal stresses can exceed the fracture toughness of the overlay material, leading to brittle crack propagation through the full thickness of the overlay.

The finding that overlay thinning due to wear has minimal effect on stress levels is particularly significant for maintenance planning. It suggests that periodic thickness monitoring alone is insufficient to predict crack initiation risk. Instead, engineers must consider the thermal cycling history and residual stress state when evaluating overlay integrity.

Residual Stress Characterization

The thermoplastic finite element analysis employed in this study is appropriate for modeling the complex stress evolution during multi-pass welding. The model accounts for the nonlinear material behavior of the overlay material as it undergoes repeated heating and cooling cycles. XRD measurements provide a non-destructive means of validating the simulation results, offering confidence in the predicted stress distributions.

Typical residual stress levels in multi-layer overlay welds can reach 200-400 MPa in the transverse direction, with compressive stresses near the surface and tensile stresses in the interior. The specific stress distribution depends on welding sequence, interpass temperature, and post-weld treatment. Stress relief heat treatment is commonly employed to reduce residual stresses, but for nuclear-grade applications, the heat treatment must be carefully controlled to avoid adversely affecting the overlay microstructure and properties.

Engineering Practice and Safety Implications

For nuclear power plant operations, this study provides critical insights for overlay integrity management. The identification of thermal loading as the dominant crack initiation mechanism suggests that thermal cycling control is essential for preventing overlay failure. Engineers should implement thermal gradient management strategies, including controlled warm-up and cool-down rates, to minimize thermal stresses during valve operation.

The multi-layer multi-pass overlay design must be carefully considered to minimize residual stress accumulation. Welding sequence optimization, such as using a back-step welding pattern or symmetric deposition, can help distribute stresses more uniformly. Post-weld stress relief treatment should be evaluated for its effectiveness in reducing residual stresses without compromising overlay properties.

Inspection protocols for nuclear-grade valve overlays should incorporate both surface and subsurface examination methods. While visual inspection and magnetic particle testing can detect surface cracks, ultrasonic testing or eddy current testing may be necessary to detect subsurface crack initiation. The residual stress state should be periodically assessed using XRD or similar non-destructive techniques.

Study Insights and Reflections

This research represents a valuable integration of experimental mechanics and computational modeling for addressing a real-world nuclear safety issue. The identification of thermal loading as the dominant crack initiation mechanism provides clear guidance for preventive maintenance and design improvement strategies.

One notable aspect of this study is the use of simulation specimens rather than actual in-service valves. While this approach allows for controlled experimentation and detailed characterization, the actual stress state in operating valves may be more complex due to additional factors such as vibration, pressure cycling, and chemical exposure. Future work should validate the findings on actual valve components extracted from service.

The thermoplastic finite element approach offers a powerful tool for predicting overlay performance under various operating conditions. Engineers can use this methodology to evaluate design modifications, such as overlay thickness optimization or welding sequence changes, before implementation in production. This predictive capability is particularly valuable for nuclear applications where safety margins must be rigorously maintained.

The findings from this study underscore the importance of considering residual stress evolution throughout the service life of nuclear-grade valve overlays. A comprehensive integrity management program should incorporate residual stress assessment, thermal cycling history tracking, and periodic non-destructive examination to ensure long-term reliability and safety.