Fracture Toughness Evaluation of Carbon Steels in Piping and Valve for Reactor Primary System

Abstract

UK very high integrity (VHI) component classification includes design, manufacturing, and inspection requirements that go beyond those established in ASME BPVC Sec. III Subsection NB [1]. One of these requirements is to ensure the component is tolerant of manufacturing defects. This can be demonstrated using a Defect Tolerance Assessment (DTA) based on two parameters fracture mechanics method. The brittle fracture parameter of this assessment requires the analysis of stress occurring in the component against the plane strain fracture toughness, KIC of the material. This work focuses on the practical determination of KIC for materials chosen for a Boiling Water Reactor (BWR) Main Steam Piping (MSP) and Main Steam Isolation Valve (MSIV), which carbon steel seamless pipe SA-106 Grade C and carbon steel casting SA-216 Grade WCB, are respectively. These materials are usually tested by Charpy impact testing specified in [1], but there are not many studies reporting their KIC, and there is not enough information concerning actual piping and valve materials. Thus the authors implemented fracture toughness testing using J-resistance curve according to ASTM E 1820 [2] for test pipe and test casting block simulating actual MS Piping and MSIV, and evaluated KIC(J) to be used in DTA. KIC(J) is evaluated from elastic-plastic fracture toughness, JIC, gained from the J-resistance curve, and equivalent to KIC [3]. KIC(J) corresponds to KJIc in ASTM E 1820. There were some cases, however, in which valid JIC values could not obtained, because of the materials high toughness, test specimen size limitations, and uneven final crack sizes. When valid JIC can’t be obtained, retesting or remanufacturing would significantly affect plant construction schedule. Hence, alternative evaluation methods by which JIC can certainly be obtained are desired. In this study, the authors focused on two types of alternative JIC evaluation methods. The first one is the Stretch Zone Width (SZW) method, in which JIC is calculated from SZW measurements of crack tip plastic blunting on fracture toughness test specimens. The SZW method was well studied in the 1970s, and experimental data showed a clear correlation between JIC values obtained from J-resistance curves and JIC values obtained from SZW measurements [4]. The second method is by correlation of JIC with the energy absorbed during Charpy testing. As represented by Rolf’s study [5], it has been reported that there are correlations between Charpy absorbed energy and KIC for high tensile strength steels. In this study, the validity of the SZW method was first evaluated by comparing its results with JIC obtained from J-resistance curves. Then, the applicability of the JIC values to DTA of actual products was discussed. Finally, by comparing Charpy absorbed energy and KIC(J), the validity and applicability of KIC determination method with Charpy absorbed energy was discussed.