Crack Mechanism and Residual Stress Evolution in Nuclear-Grade Valve Sealing Surface Surfacing Layers

Literature Overview

This paper by Yan Xiangyu and colleagues from Dalian University of Technology and CGN Power Operation Co., Ltd. addresses a critical safety issue in nuclear power plant operation: cracking in the surfacing layers of nuclear island valve sealing surfaces. The study combines experimental investigation of crack initiation mechanisms with thermal-elastoplastic finite element analysis of residual stress evolution. Supported by the National Key R&D Program (2022YFB2503503) and the National Natural Science Foundation (52375310), this work has direct implications for nuclear safety and valve reliability.

Background and Problem Statement

Valves are critical control components in the nuclear island piping system, where they serve as the primary means of isolating sections of the plant during normal operation, maintenance, and emergency conditions. The sealing surfaces of these valves are typically surfaced with hardfacing alloys such as Norem 02 (a nickel-based alloy) to provide wear resistance and maintain sealing integrity under cyclic loading.

In engineering practice, cracking in valve surfacing layers has been observed during operation, leading to potential leaks in nuclear island piping systems. Such failures pose serious safety risks in nuclear power plants, where even small leaks can lead to radiological contamination and require plant shutdown for repair. Understanding the crack initiation mechanism and the role of residual stress is therefore of paramount importance for nuclear safety.

Experimental Investigation

The authors fabricated valve simulation specimens and subjected them to both mechanical load fatigue testing and thermal load fatigue testing to investigate the crack initiation mechanism in Norem 02 surfacing layers. The experimental approach allows for isolation of the contributions of mechanical and thermal loading to crack formation.

The key finding is that the cracks observed in valve sealing surface surfacing layers are brittle through-thickness cracks caused by the combined action of welding residual stress and thermal stress. This finding has important implications for crack prevention strategies, as it indicates that residual stress management during manufacturing and thermal stress management during operation are both critical factors.

Residual Stress Analysis

Thermal-elastoplastic finite element analysis was performed to calculate the residual stress distribution in the valve sealing surface surfacing layer. The numerical results were validated using X-ray diffraction (XRD) measurement of residual stresses, providing confidence in the accuracy of the simulation.

The analysis reveals that surfacing layer thinning (due to wear or erosion during operation) results in minimal changes to the residual stress state. However, under 1000°C thermal loading, the surfacing layer stress increases significantly. The thermal load is identified as the dominant factor leading to material failure in the surfacing layer.

Crack Mechanism Analysis

The brittle through-thickness crack morphology observed in the surfacing layer is consistent with the combined action of:

• Welding residual stress: residual tensile stresses introduced during the surfacing process, particularly in the as-deposited condition
• Thermal stress: stresses induced by thermal cycling during valve operation, including startup, shutdown, and transient conditions

The brittle nature of the cracks indicates that the surfacing layer material lacks sufficient toughness to arrest crack propagation under the combined stress state. This is consistent with the known behavior of nickel-based surfacing alloys, which, while offering excellent wear resistance, may have limited toughness at elevated temperatures.

Engineering Implications and Mitigation Strategies

Based on the findings of this study, several mitigation strategies can be identified for preventing cracking in nuclear valve surfacing layers:

1. Residual stress reduction: Post-weld heat treatment to relieve residual stresses introduced during surfacing deposition
2. Thermal stress management: Control of thermal gradients during valve operation through optimized heating and cooling rates
3. Surfacing layer design: Consideration of surfacing layer thickness, composition, and microstructure to improve toughness
4. Operational monitoring: Implementation of non-destructive testing (NDT) programs to detect crack initiation before through-thickness failure occurs

The identification of thermal load as the dominant failure factor has direct implications for nuclear plant operating procedures. Thermal cycling rates during reactor startup and shutdown, as well as transient events such as loss-of-coolant accidents (LOCAs), should be carefully managed to minimize thermal stress in valve surfacing layers.

Study Insights and Reflections

This research represents an important contribution to nuclear valve reliability engineering. The combination of experimental investigation and numerical simulation provides a comprehensive understanding of the crack initiation mechanism, which is essential for developing effective mitigation strategies.

The validation of finite element analysis results through XRD residual stress measurement is a methodological strength of this work. The agreement between calculated and measured residual stresses provides confidence in the predictive capability of the numerical model, which can be used to evaluate alternative surfacing designs and process parameters.

For nuclear power plant operators, the findings underscore the importance of considering the entire lifecycle of valve surfacing layers—from manufacturing through operation—to maintenance. Residual stress introduced during manufacturing, combined with thermal stresses during operation, can lead to crack initiation and eventual failure. A holistic approach to valve reliability, encompassing manufacturing quality control, in-service inspection, and operational procedures, is essential for ensuring nuclear safety.

The study also highlights the need for continued research into surfacing alloy development for nuclear applications. Future alloy designs should aim to balance wear resistance with toughness, and should be qualified through comprehensive testing that simulates the actual operating conditions of nuclear island valves.