In the nuclear energy sector, every fastening element must undergo extremely stringent quality checks.
This case study describes a test campaign conducted by an accredited laboratory, Sigma Tre Lab (UNI CEI EN ISO/IEC 17025:2018), commissioned by EFS Srl, on fasteners made from Inconel 718 for use in nuclear plants.
Material selection and operating context
The Inconel 718 superalloy was chosen for its ability to perform under critical conditions—offering high resistance to fatigue, creep, temperatures up to 700 °C, and corrosion. These properties make it ideal for mechanical components in nuclear power plants, where every part is exposed to significant thermal and mechanical stress.
The testing campaign had two main objectives:
• To verify compliance with applicable regulations for components intended for the nuclear sector
• To guarantee mechanical and chemical reliability for each production batch, enhancing operational safety
Here are the various tests typically applied to these fasteners:
• Compositional analysis, to confirm the correct presence of Ni, Cr, Fe, Nb
• Microstructural evaluation through metallographic analysis, to exclude defects such as cracks, segregations and inclusions
• Impurity inspection, ensuring the absence of inclusions that could compromise mechanical strength
Case study: Non-destructive testing and mechanical checks
This case focuses on two special parts in Inconel 718 intended for SMRs (Small Modular Reactors), the next-generation nuclear reactors that Italy is also planning to adopt.
The fasteners were custom-made based on engineering drawings and included:
• One fully threaded hex head bolt
• Two studs with different eccentricities
For each production batch, the following tests were conducted in order:
I – Dimensional control and traceability
Each piece underwent precise metrological inspection using CMM and laser gauges, ensuring correct tolerances for thread profiles, diameters, and lengths.
II – Non-destructive testing with liquid penetrants
This method detects micro-cracks and surface discontinuities, even down to a few microns. It is cost-effective compared to X-ray or ultrasound methods and easy to apply. However, it does not detect subsurface or internal flaws.
The testing process included:
• Surface cleaning before application
• Application of red penetrant across the surface
• Removal of excess penetrant to avoid false positives
• Application of developer to bring out penetrant indications from surface discontinuities
• Final visual inspection to check for micro-cracks or critical surface defects
III – Hardness and impact tests
These tests assess performance under shock and vibrations.
Tensile testing, which evaluates breaking strength and yield load, was not feasible due to the small dimensions of the fasteners. However, all obtained values were compared against regulatory limits, ensuring full compliance of each fastener batch.
EFS Srl maintains its focus on stringent testing protocols and safety assurance, upholding a culture of continuous improvement and quality through adherence to standards, including ISO 9001 certification.
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