The Influence of Temperature on Microstructural Change due to Fatigue

Abstract

The study of fatigue behavior of metals at high temperature seems to be increasingly demanded by increasing requirement in various industries for use of metals at elevated temperature. According to the reports hitherto made about fatigue strength of metals subjected to heat, the fatigue strength falls with rise of temperature during the fatigue test, except for the effect of substitutional solution hardening called blue shortness. Such fatigue behavior is considered to be due to the rise of temperature facilitating the mobility of dislocation, though it is open to question whether the mobility of dislocation is the sole factor to determine the fatigue strength at elevated temperature or not.
It has been reported that formation and development of sub-structures in the metals during their fatigue is closely related to fatigue fracture. In view of the fact that the rise of temperature facilitates rearrangement of dislocation and its polygonation, it is conceivable that its mobility may have relation to formation of sub-structures.
In the present study, the influence of temperature on fatigue strength is studied from these view points by using a new fatigue testing machine for high temperature in inert gaseous environment, and by observing the micro-structure by means of an X-ray micro-beam.
From the results of the experimental study the following conclusions have been derived.
(1) The experiment at the room temperature shows that the fatigue life is longer in argon gaseous environment than in atmosphere when stress amplitude is comparatively high, but the environment almost ceases to make any difference when stress amplitude sufficiently falls.
(2) The fatigue strength at high temperature both of S45C carbon steel and of 0.04%C low carbon steel, per hour in each respective fatigue cycle, falls gradually at first with rise of temperature up to ca. 200°C, and then tends to rise under the influence of blue shortness.
(3) The rise of testing temperature has more effect on the crystalline grains on the surface of the specimens. They begin to show deformed diffraction arc of X-rays when temperature rises, but the grains, in which sub-structures are formed by the fatigue show but minor fractions of them.
(4) With the exception of the temperature range of blue shortness, to the higher temperature the specimens are subjected, and to the higher stress amplitude, the shorter their fatigue life is, and the more sub-structures develop in them.