Flaw evaluation of cast stainless steel pipe for revision of JSME rules on fitness for service

Abstract

The current JSME rules on Fitness-for-Service only applies the Elastic Plastic Fracture Mechanics (EPFM) method, including the two-parameters method for flaw evaluation of cast stainless steel (CASS) pipes, while plastic collapse is expected to occur in the case of low ferrite content or low thermal aging condition. The EPFM method of the JSME rules introduces Z-factor for simplification similar to ASME Sec. XI. The Z-factor equation for CASS pipe was derived with the assumption of a penetrated circumferential flaw, 20% ferrite content, J-R curve of fully aged material, and stress-strain (S-S) curve of unaged material, which are very conservative conditions. In order to revise the current JSME rules for a more practical and accurate flaw evaluation of CASS pipe, this paper introduced to the rules, a limit load method for low ferrite content or low thermal aging condition, and prediction models of S-S and J-R curves for thermal aging. These were verified by fracture tests using flat plate specimens with a surface flaw and detailed FE analyses. Also applicability of a Z-factor equation for an axial flaw was proposed by other authors. This paper is a technical basis for the code revision of the JSME rules.

Keywords

• cast stainless steel
• net section criterion
• limit load
• ductile fracture
• j-integral
• two-parameters method
• j-t criterion
• allowable flaw
• rules on fitness-for-service
• j-r curve
• thermal aging
• epfm
• z-factor