Technical Letter Report of Slow-Strain-Rate Tensile Tests on Irradiated Stainless Steels in a PWR Environment

Abstract

Slow-strain-rate tensile (SSRT) tests have been performed on irradiated specimens in a simulated pressurized water reactor (PWR) environment. The samples are miniature tensile specimens of various austenitic stainless steels (SSs) with different thermal-mechanical treatments commonly used for reactor core internal components. Neutron irradiation was carried out in the BOR-60 reactor, a sodium-cooled, fast breeder reactor in Russia, at ~320°C. The damage doses of the specimens are 5, 10, and 48 dpa (displacements per atom). All irradiated materials show significant irradiation hardening and loss of ductility in the SSRT tests. The yield strengths of cold-worked are higher than that of solution-annealed samples at all doses up to 48 dpa. While the irradiation hardening seems to saturate between ~5 and ~10 dpa, the loss of ductility continues to increase above 10 dpa. Strain softening is also observed for all irradiated materials above 5 dpa. Fractographic examinations show that ductile dimple fracture is the dominant morphology for all SSRT tests in the PWR environment. Small areas of transgranular, mixedmode and cleavage fractures are seen on most fracture surfaces in PWR water tests. Intergranular cracking is also observed in 48-dpa Types 316 and 347 SSs. Cracking susceptibility of the tested materials was evaluated with fracture morphology and time to failure. In general, high-doses cold-worked SSs are more susceptible to transgranular cleavage cracking in the PWR environment. Solution-annealed Type 347 SS is susceptible to intergranular cracking at 45 dpa in the PWR environment.

Technical Letter Report on the Cracking of Irradiated Stainless Steels in Low-Corrosion-Potential Environments

“…Note that a blunting line of four times the flow stress (4 f ) is recommended by Mills 23 for materials with high strain hardening coefficients. Although the stain hardening coefficient is greatly reduced for irradiated SSs, 24,25 the (4 f ) blunting line is still assumed in the present study to be more conservative in determining the J 0.2 values.…”