X-ray Study of Fatigue Damage of Metallic Materials (Change of Phase Transformation Micro-stress in Carbon Steel Caused by Heat Treatment Due to Aging and Fatigue)

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  • X線による熱処理炭素鋼の疲労の研究
  • Xセン ニ ヨル ネツ ショリ タンソ コウ ノ ヒロウ ノ ケンキュウ

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Abstract

The behavior of residual micro-stress caused in carbon steel by phase transformation during heat treatment was studied in the processes of aging and fatigue by observing the change of X-ray back reflection patterns. The half-value breadth was taken as the measure of micro-stress. Steel wire of 3 mm diameter and 0.78% carbon content was used for the material of specimens. Thes pecimens were prepared by quenching from 850°C with subsequent tempering at the temperatures of 100°, 300°, 500° and 800°C for half an hour each. The results obtained summerized as follows: (1) The phase transformation micro-stress diminishes markedly by heating, even at comparatively low temperature such as 300°C. (2) The micro-stress diminishes by aging, in accelerated rate during the first ten days and then gradually. (3) The material tempered at 500°C still contains a noticeable amount of micro-stress. When the material is subjected to stress alternation, the micro-stress tends to fade, in accelerated rate during the first 104 stress cycles and then gradually. It is likely that the micro-stress reaches a definite value just before fracture of the material, irrespective of the magnitude of alternating stress applied. (4) The relation between b⁄B and logn⁄N, where b and B are the current and the initial values of the breadth and n and N are the current and the ultimate numbers of stress alternation, is represented by a straight line, irrespective of the magnitude of alternating stress applied. This relation offers possibility of the non-destructive detection of fatigue damage. (5) In the case of the material tempered at the temperature lower than 100°C the micro-stress tends to increase at the early period of stress alternation and then decreases in a similar manner to the case of 3). It may be due to the transformation of residual austenite to martensite.

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