Abstract
In order to investigate the notch effect on the fatigue strength of 18-8 stainless steel at an elevated temperature, rotary bending fatigue tests were carried out at 450°C and room temperature, using specimens 12mm in diameter with V-type grooves of 60°, whose stress concentration factors αk were between 2.5 and 4.0. A Wöhler type fatigue testing machine was operated at 1900rpm. The notch effect on coaxing was also investigated at 450°C.
After the tests, the microstructures and Vickers hardness of specimens not fractured after 107 cycles of stresses at the respective endurance limit in each αk were examined.
The test results were qualitatively discussed in connection with several phenomena, such as (a) the decrease of actual stresses due to yielding at the notch bottom region, (b) work hardening and (c) shortening of the distances between the slip lines accelerated by their developments and the preferential precipitations along these lines.
The main results obtained are as follows:
(1) At room temperature, the fatigue notch factor β increases as αk increases, though β has comparatively low value close to 1. This tendency seems to be due to the decrease of actual stresses and work hardening.
(2) At 450°C, β decreases as αk increases. This phenomenon seems to be due to the differences of the distances between the slip lines, besides the effects of actual stress decrement and work hardening.
(3) At 450°C, the pronounced increment of fatigue strength is brought about by coaxing. The upper limit seems to exist, however, concerning this increment of the endurance limit. These phenomena will be explained in the same way as in (2).
