Root Cause Analysis of Nonrelevant Indication of Magnetic Particle Test on 18MND5 Steel Welded Joints in the Steam Generator of a Nuclear Power Project

The 18MND5 steel is rolled and welded by the submerged arc welding process to fabricate the upper cylinder shell of the steam generator of a nuclear power project. After the shell is welded, Heat treatment is performed. Then, magnetic particle test (MT), ultrasonic testing (UT), and radiographic testing (RT) were performed on the longitudinal Joint, and no defects were found. However, there are MT nonrelevant indications characterized continuous linear at the position of the weld fusion line on both sides of the longitudinal Joint. Through the peeling observation of the Product Weld Test Coupon with the same linear indication, no defects were found. It can be determined that continuous linear indications are nonrelevant. Optical microstructure observation (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and electron backscattered diffraction (EBSD) analysis was carried out on the 18MND5 welded joints to determine the cause of MT nonrelevant indication. The result showed that there is a noticeable growth texture in the coarse-grained lath martensite (M) region of the weld with MT nonrelevant indication, which is closely related to the ferrite (F) and parent metal of the weld metal on both sides. The ferrite (growth direction and morphology of the material structure are different. This growth texture leads to a significant difference in magnetic properties at the fusion line position of the welded joint, resulting in a mutation in the magnetic properties of the coarse-grained region, resulting in an uncorrelated MT indication.