Further Validation of the Technical Basis of Code Case N-838 for Flaw Tolerance Evaluation of CASS Piping

ASME Code, Section XI Code Case N-838 provides analyses methods that may be used when performing flaw tolerance evaluation of Class 1 and 2 cast austenitic stainless steel (CASS) piping components with ferrite content exceeding 20%. Tolerable flaw sizes are provided in the Code Case that cannot be exceeded during the evaluation period from an initial postulated flaw. The tolerable flaw sizes were determined based on probabilistic fracture mechanics (PFM) techniques employing toughness and flow strength correlations developed at Argonne National Laboratory (ANL) that estimated the crack growth resistance (J-R) curve using material composition with the room temperature Charpy energy as an intermediate variable. Since the publication of the technical basis for this Code Case, additional toughness data have been identified, some of which are significantly lower than the toughness for CASS CF-8M materials considered in the ANL model.This paper reports the results of evaluations and sensitivity studies performed to examine the additional toughness and tensile data and determine the effects on the Code Case tolerable flaw size tables. The current study directly employs an expanded set of data of actual measured fully saturated high-temperature crack growth resistance curve, thereby bypassing the material composition-to-Charpy-to-toughness correlations in the ANL model. In this study, both deterministic fracture mechanics and PFM analyses were performed using the assembled data of toughness and tensile properties. The results obtained in the current PFM and deterministic analysis which use the actual toughness and tensile data are found to always provide larger tolerable flaw sizes than those obtained from the previous analyses which used ANL material correlations, thereby demonstrating the conservative nature of the technical basis for the Code Case.Copyright © 2015 by ASME