Zhuojin Pipe Fitting Co., LtdDelamination Mechanism of Stainless Steel Strip Electrode Submerged Arc Overlay on Nuclear Island Main Equipment
Literature Overview
The paper by Yang Chengdong, Ru Xiangkun, and Tang Weibao from Shanghai Electric Nuclear Power Group Co., Ltd. (2023, Pressure Vessel, Vol. 40, Issue 6, pp. 22–26) presents a detailed failure analysis of stainless steel strip electrode submerged arc overlay (SES-SSAO) layers on low-alloy steel base materials used in nuclear island main equipment. This work is of critical importance given the demanding quality requirements for nuclear-grade components, where overlay delamination represents a catastrophic failure mode that compromises structural integrity and containment safety.
Failure Mechanism Analysis
The investigation identified a multi-factorial failure mechanism involving both metallurgical and mechanical factors. The transition zone between the stainless steel overlay and the low-alloy steel base metal exhibits a hardened layer with an average thickness of approximately 25 μm. This hardened layer consists of lath martensite with high hardness and low plasticity, formed due to the rapid cooling rates and significant dilution between the stainless steel deposit and the low-alloy steel base during the submerged arc welding process.
Identified Failure Factors
| Factor | Classification | Description | Consequence |
|---|---|---|---|
| Hardened lath martensite layer | Internal cause | ~25 μm thick, high hardness, low ductility | Crack initiation site |
| Type II boundary | Internal cause | Interface between hardened layer and base metal | Crack propagation path |
| Porosity in hardened layer | Internal cause | Gas pores within the hardened zone | Stress concentration sites |
| High residual stress | External cause | Reaches 380 MPa at 6 overlay passes | Drives crack extension |
The Type II boundary, as classified in the boundary typology of weld interfaces, represents a metallurgical discontinuity where the microstructure transitions sharply between the hardened zone and the base metal. This boundary acts as a preferential path for crack propagation because it combines a zone of low ductility with a plane of metallurgical weakness. The porosity within the hardened layer provides additional stress concentration sites that facilitate crack nucleation.
Residual Stress Evolution
A critical finding of this study is the quantification of residual stress evolution with increasing overlay pass number. The residual stress at the overlay interface increases progressively with each additional welding pass, reaching a critical value of 380 MPa when the overlay layer count reaches 6 passes. This progressive stress accumulation occurs because each subsequent welding pass introduces new thermal cycles that superimpose additional residual stresses onto the previously deposited layers, without adequate stress relief between passes.
The combination of the internal metallurgical defects (hardened layer, Type II boundary, porosity) and the external mechanical driving force (high residual stress) creates a synergistic failure mechanism. The Type II boundary serves as the crack initiation and propagation path, while the residual stress provides the thermodynamic driving force for crack extension. At lower pass counts, the residual stress may be insufficient to overcome the fracture resistance of the boundary, but beyond a critical threshold (approximately 6 passes in this case), delamination becomes inevitable.
Engineering Countermeasures
Based on the identified failure mechanism, several engineering countermeasures can be implemented:
- Interpass stress relief: Applying localized stress relief treatments between welding passes to prevent cumulative residual stress buildup.
- Process parameter optimization: Adjusting welding current, voltage, and travel speed to reduce dilution and minimize the thickness and hardness of the transition zone.
- Limiting overlay pass count: Restricting the number of overlay passes to below the critical threshold where residual stress exceeds the fracture resistance of the interface.
- Post-weld heat treatment: Implementing stress relief annealing after the complete overlay build-up to reduce residual stress below the critical level.
- Consumable selection: Using filler metals with compositions that promote a more gradual microstructural transition and reduce the formation of brittle martensitic phases in the transition zone.
Key Reflections and Study Insights
This failure analysis exemplifies the importance of understanding the interplay between metallurgical structure and mechanical state in overlay welding applications. The identification of a critical pass number (6 passes) at which residual stress reaches a dangerous level provides a practical design limit that can be incorporated into welding procedure specifications for nuclear-grade overlay applications. The methodology of combining microstructural analysis with residual stress measurement is a best practice approach that should be adopted for all critical overlay welding applications. The findings also underscore the need for rigorous post-weld stress relief protocols in nuclear island equipment manufacturing, where the consequences of overlay delamination are unacceptable.