Crack Cause Analysis and Residual Stress Evolution in Nuclear Grade Valve Sealing Surface Overlay Layer

Literature Overview

This paper, published in Heat Processing Technology (2026, Vol. 55, No. 15, pp. 37-45), presents a comprehensive investigation into the cracking mechanism of Norem 02 iron-base alloy overlay layers on nuclear grade valve sealing surfaces. The research, conducted by Dalian University of Technology and CGN Nuclear Power Operation Co., Ltd., and funded by the National Key R&D Program (2022YFB2503503) and NSFC (52375310), addresses a critical safety issue in nuclear power plant operation: the failure of valve overlay layers leading to nuclear island pipeline leakage.

Background and Significance

Valves are critical control components in nuclear island piping systems, where they must maintain reliable sealing under extreme conditions—high pressure, high temperature, and radioactive environments. The sealing surfaces of these valves are typically protected by overlay layers of specialized alloys (such as Norem 02, an iron-base alloy with high chromium and nickel content) to provide corrosion and erosion resistance. Cracking of these overlay layers is a serious concern because it can lead to loss of containment, radioactive leakage, and potential nuclear safety incidents.

The paper employs a multi-faceted approach combining experimental investigation, finite element analysis, and X-ray diffraction (XRD) measurement to establish a comprehensive understanding of the cracking mechanism.

Experimental Investigation of Crack Formation

Mechanical Load Fatigue Testing

The authors prepared valve mock-up specimens with Norem 02 overlay layers and subjected them to mechanical load fatigue testing. This simulates the cyclic pressure loading and flow-induced vibration experienced by valves during normal operation and transients.

Key observations from mechanical fatigue testing:

Cracks initiated at the overlay-substrate interface and propagated through the overlay thickness
Crack morphology exhibited features of brittle fracture—flat fracture surfaces with minimal plastic deformation
The number of cycles to crack initiation was significantly lower than expected for the overlay material alone, indicating that the residual stress state from welding accelerates fatigue crack growth

Thermal Load Fatigue Testing

Thermal fatigue testing simulated the temperature cycling experienced during reactor start-up, shutdown, and power transients. The thermal cycling imposes alternating tensile and compressive stresses due to the differential thermal expansion between the overlay and substrate.

Key observations from thermal fatigue testing:

Cracks initiated at the overlay surface and propagated inward
The crack pattern was characteristic of thermal fatigue—multiple parallel cracks spaced at regular intervals
The thermal stress amplitude was the dominant factor in crack initiation, with mechanical loading acting as a secondary contributor

Crack Mechanism Synthesis

The combined experimental evidence leads to the conclusion that the valve sealing surface overlay cracks are brittle through-thickness cracks caused by the combined action of welding residual stress and thermal stress. The welding residual stress provides a baseline tensile stress state that reduces the threshold for crack initiation, while the thermal stress from operating temperature cycling provides the driving force for crack propagation.

Residual Stress Analysis

Finite Element Modeling

The authors performed thermo-elastoplastic finite element analysis of the multi-layer, multi-pass overlay welding process. The model accounted for:

Sequential deposition of multiple weld passes
Thermal cycling during each pass
Material phase transformations and associated volume changes
Plastic deformation during both deposition and cooling
Constraint effects from the valve body geometry

The ring-shaped, multi-layer, multi-pass nature of the valve sealing surface overlay introduces complex stress states that are difficult to predict analytically. The finite element model captures the interaction between successive passes and the cumulative effect of residual stresses.

XRD Verification

X-ray diffraction measurements were performed to validate the finite element predictions. The XRD method measures the lattice strain in the near-surface region (typically within 20-50 μm depth) and converts it to residual stress using the sin²ψ method.

The agreement between FEA predictions and XRD measurements confirms the accuracy of the model and provides confidence in the stress predictions for regions not directly accessible to measurement.

Residual Stress Evolution

Condition	Stress Change	Dominant Factor
As-welded state	High tensile residual stress	Welding thermal cycle
Overlay thinning (wear)	Minimal stress change	Geometric modification
1000°C thermal load	Significant stress increase	Thermal expansion mismatch
Combined thinning + thermal	Moderate stress increase	Thermal load dominant

A critical finding is that overlay thinning due to wear results in very little change in residual stress. This is counterintuitive but can be explained by the fact that the residual stress field is established during welding and is largely self-equilibrating; removing a small amount of material from the surface does not significantly alter the overall stress balance.

However, the application of 1000°C thermal load causes a significant increase in residual stress. This is attributed to the thermal expansion mismatch between the overlay and substrate at high temperatures, combined with the constraint imposed by the valve body geometry. The thermal stress can add to the existing welding residual stress, pushing the total stress state beyond the material's fracture toughness limit.

Failure Analysis and FMEA

Applying a systematic failure analysis framework to the valve overlay cracking problem:

Failure Mode	Root Cause	Contributing Factors	Detection Method	Prevention Strategy
Through-thickness brittle crack	Residual stress + thermal stress	Poor weld quality, inadequate PWHT	UT, MT, eddy current	Stress relief, optimized welding sequence
Interface delamination	Dilution mismatch, weak bonding	Inadequate surface preparation	UT, tapping test	Proper joint design, transition layers
Surface cracking	High carbon equivalent, hydrogen	Inadequate preheat, high HAZ hardness	PT, visual inspection	Preheat control, low-C fillers
Thermal fatigue cracking	Cyclic thermal loading	Low toughness, high residual stress	Eddy current, UT	PWHT, thermal barrier coatings

Engineering Practice Recommendations

Based on the findings of this study, the following recommendations are proposed for nuclear grade valve overlay welding:

Welding sequence optimization: For ring-shaped, multi-pass overlays, the welding sequence should be designed to minimize residual stress accumulation. Symmetric welding patterns and back-step welding are recommended.
Post-weld heat treatment: Stress relief annealing at 650-750°C for Norem 02 alloy can reduce welding residual stresses by 50-70%. However, the PWHT temperature must be carefully controlled to avoid sensitization or intergranular corrosion.
Interpass temperature control: Maintaining interpass temperatures below 150°C helps control grain growth and reduces the risk of thermal cracking.
Thermal stress mitigation: In high-temperature service, the thermal stress contribution to crack initiation is dominant. Design modifications such as thermal expansion joints, flexible piping connections, or thermal barrier coatings can reduce the thermal stress amplitude at the overlay.
Inspection protocols: Regular in-service inspection using UT, eddy current, and penetrant testing is essential for early detection of overlay cracking. The inspection frequency should be increased for valves experiencing frequent thermal transients.
Material selection: Consideration of alternative overlay alloys with higher toughness and lower thermal expansion coefficient may reduce the thermal stress mismatch.

Key Questions and Reflections

The study's finding that thermal stress is the dominant factor in overlay cracking has profound implications for nuclear valve design and maintenance. It suggests that simply improving the welding quality or residual stress relief may not be sufficient to prevent cracking in high-temperature service. Instead, a holistic approach that addresses the thermal stress environment—through design modifications, operational procedures, and material selection—is required.

An important question not addressed in the paper is the effect of neutron irradiation on the overlay microstructure and mechanical properties. In a nuclear environment, the overlay is exposed to radiation that can cause embrittlement, swelling, and changes in residual stress. The combined effect of irradiation and thermal cycling on overlay cracking resistance requires further investigation.

Additionally, the study focuses on Norem 02 alloy, but the cracking mechanism may differ for other overlay alloys used in nuclear valves, such as cobalt-base alloys (Stellite) or nickel-base alloys (Inconel 625, Hastelloy). A comparative study of cracking susceptibility across different overlay alloy systems would provide valuable guidance for material selection.

Study Insights and Implications

This paper makes a significant contribution to nuclear safety by establishing, through rigorous experimental and computational analysis, that thermal stress is the primary driver of overlay layer cracking in nuclear grade valves. The finding that overlay thinning has minimal effect on residual stress, while 1000°C thermal loading significantly increases stress, redirects the engineering focus from wear management to thermal stress management. For nuclear power plant operators, this means that valve maintenance strategies must incorporate thermal stress considerations—such as optimized start-up and shutdown procedures, thermal barrier coatings, and enhanced inspection protocols. For welding engineers, it reinforces the importance of residual stress control through optimized welding sequences and post-weld heat treatment. The combination of experimental validation and finite element modeling presented here provides a robust analytical framework that can be applied to other nuclear component overlay systems, contributing to the overall safety and reliability of nuclear power plants.