Artificial Flaw Detection with Ultrasound in Austenitic Stainless Steel
Koskinen, Tuomas Sakari
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Keinotekoisten vikojen havaitseminen ultraäänellä austeniittisessa ruostumattomassa teräksessä
Ultrasonic testing is the main tool to inspect and verify the structural integrity of the primary circuit components in nuclear power plants during in-service inspection. However, the in-service inspection is usually done in a short time during the outages in limited accessibility, which sets the need for efficient inspection procedure. Primary circuit components are made from austenitic stainless steel and designed mechanical integrity and corrosion resistance in mind. While material properties of austenitic stainless steel promote these aspects they reduce the capability of ultrasonic testing, due to scattering, diffraction and attenuation of ultrasound. In order to validate an inspection method and to train inspectors, reference points are needed. However, there are not enough real flaws available to be used for these purposes. For this reason artificial flaws must be used. Artificial flaws must reflect same or similar reflection indication so it can be used as a reference point. Otherwise this could lead to a false indications or missing of real flaws during in-service inspection. Unfortunately ultrasonic testing is highly dependent on the geometry of the component, the type of the flaw and the material in which ultrasound propagates; therefore numerous artificial flaws are needed to cover all probable flaw types and locations. The aim of this thesis is to study artificial flaws and acquire wider knowledge on ultra-sonic indications of these flaws. Another objective is to conduct an experiment with an austenitic steel weld with electric-discharge machined (EDM) notches as artificial flaws. The results of the experiment are then compared to the results from CIVA simulation. The focus is on mechanical and thermal fatigue type of flaws and also EDM notch for artificial flaws. Mechanical and thermal fatigue flaws were selected due to their occurrence during the service life of a primary circuit. These flaws can also be produced in well controlled manner as well as the EDM notches.