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The degradation of fracture resistance of ferritic steels subjected to neutron irradiation can be qualitatively evaluated by examining the shift in the ductile-to-brittle transition temperature and the drop in the upper-shelf energy as determined in the Charpy impact test. That is an empirical test, however, and its usefulness is derived primarily from either correlation to more quantitative measures of fracture toughness or comparison with other Charpy test results for different materials or conditions of interest. Any departures from the standard testing procedures, including specimen dimensions, will yield results which will vary in some degree from those of a standard test.
In examining the effects of neutron irradiation on the fracture resistance of the high-chromium ferritic steels which are candidates for the first wall of magnetic fusion reactors, it is necessary to severely limit the size of the test specimens to be fit within high-flux test reactors. Impact specimens with cross sections as small as ⅓ by ⅓ cm are employed. To evaluate the effectiveness of small specimens in measuring the relative shift in transition temperature and drop in upper-shelf energy, compared with a standard Charpy specimen, a study of the impact properties of 9Cr-1Mo-V-Nb steel in both the normalized and tempered and the quenched-only condition was conducted. Full-size and various subsize Charpy V-notch specimens of two orientations were examined in both material conditions.
Comparing the test results of the tempered with the embrittled 9-Cr-1Mo-V-Nb steel showed that larger transition temperature shifts and smaller fractional drops of upper shelf were measured with the small specimens relative to the standard-size specimens. Attempts were made to improve the correspondence of data by fracture area and volume normalization. No improvement in the shift or shelf drop was obtained. However, a fair agreement for the higher energy tests of absolute energy was obtained with fracture volume normalization.
impact testing, subsize specimens, size dependence, ductile-to-brittle transition, upper-shelf energy, embrittlement, 9Cr-1Mo-V-Nb steel, Charpy specimens, fracture
Development Engineer, Oak Ridge National Laboratory, Oak Ridge, TN
Virginia Polytechnic Institute and State University, Blacksburg, VA