It is studied in this report how the toughness of tempered Ni-Cr steel changes after this steel had been loaded at elevated temperature for long time. The main results are as follows: (1) The behavior of transition temperature depends upon the loading method, loading temperature and loading time. The rise of transition temperature is more remarkable in the case of higher loading temperature and of longer loading time. And the dynamic loading such as rotating bending makes the transition

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... ure to rise higher than the static tensile loading. (2) The rise of transition temperature corresponds with the increase of the carbon and chromium contents in deposit and the extent of boundary etching. It is concluded from these results that the tempered Ni-Cr steel should be used carefully, since this steel is remarkably embrittled under the condition of dynamic loading at elevated temperatures for long time.