Characterization of elastic-plastic fracture behavior of type 347 stainless steel piping materials

Type 347 austenitic stainless steel is a stabilized grade selected for improved resistance to sensitization and higher allowable stress levels than other standard grades of austenitic stainless steel in the operating temperature range up to 800 F. Columbium additions to Type 347 stainless steel result in the formation of columbium carbides at high temperature during the processing of the material. The presence of the columbium carbides prevents the formation of chromium carbides at grain boundaries. Formation of chromium carbides at grain boundaries causes sensitization of stainless steels with a corresponding loss of corrosion resistance of the stainless steel in certain aqueous environments. Elastic-plastic fracture toughness testing has been performed on several heats of stabilized Type 347 austenitic stainless steel piping material. J-R curve tests were performed at 315 C (600 F). The testing was performed with the specimen cracks oriented in the circumferential (LC) and longitudinal (CL) directions. The J-R curves for the Type 347 stainless steels were compared with similar data on other austenitic stainless steels. Metallographic and fractographic evaluations were performed on the material and the fracture surfaces of the broken test specimens. The columbium carbide distribution affects the fracture behavior of the material by creating sites for the nucleation of voids resulting in the ductile dimpled fracture appearance. This observation is evidenced by the difference in J-R curve behavior and by differences in the fracture appearance between the LC and CL oriented specimens. Microstructural features associated with the carbides and EDS X-ray analysis of the material and fracture surfaces are utilized to rationalize the observed behavior