The material microstructure of cast austenitic stainless steel (CASS) materials challenges the reliability of ultrasonic examination techniques, particularly detection of axial cracks and depth sizing of circumferential cracks. As such, the qualification requirements for ultrasonic examination of CASS piping in accordance with the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (BPVC), Section XI, Mandatory Appendix VIII, have not been developed.
In addition to the limitations of ultrasonic testing (UT) examination, CASS material is subject to thermal aging, which causes degradation of its fracture toughness over time. The extent of toughness degradation depends on the ferrite phase (δ-Fe) content of the particular component.
A probabilistic fracture mechanics (PFM) approach was applied to support development of examination requirements in light of the current nondestructive examination (NDE) limitations for CASS components. The PFM evaluations were performed using a software code developed specifically for this purpose called PIPER-CASS. This paper describes the approach and results obtained for postulated axial cracking (e.g., from a preexisting fabrication flaw) in CASS piping. This work is intended to support development of ASME BPVC, Section XI, Mandatory Appendix VIII, Supplement 9, for CASS and an alternative to the current Section XI, Subarticle IWB-2500 (Examination Categories B-F and B-J) examination requirements specific to CASS piping.