Rotating bending fatigue tests were carried out on two kinds of solution-treated austenitic stainless steels (Type 321 and 347) at room temperature, 400, 500, 600 and 700°C with a frequency of 7500r.p.m. and their fatigue strength at 108
cycles was obtained by the staircase method. The specimens of Type 347 steel aged for 24hrs at 750°C were also tested at 600 and 700°C. The plastic deformation behavior under a constant stress amplitude was observed at room temperature, 500 and 700°C for Type 321 steel. The results were compared with those on other austenitic stainless steels (Type 304 and 316) which had been obtained previously by the authors.
The results obtained are summarized as follows:
(1) The following phenomena were observed commonly for the four kinds of solution-treated steels investigated.
(i) Endurance limits are clearly found in the temperature range from room temperature to 600°C. At 700°C a clear endurance limit appears below 107
cycles, but the fatigue fracture starts to occur again after 107
(ii) The ratio of fatigue strength at 108
cycles to 0.2% proof stress is 1.2∼1.7 at 400∼600°C.
(iii) The coaxing effect is distinctly observed in the temperature range from 400 to 600°C. At 700°C it is scarcely observed after 108
(2) The aged 347 steel showed no stepwise S-N
curve at 700°C and its cycles to failure continued to increase with decreasing stress amplitude up to 108
(3) The plastic strain range for the stress amplitude near 0.2% proof stress decreased rapidly with increasing stress cycling at 500°C, and the material became to show substantially elastic behavior.
(4) The above phenomena (1) commonly observed for the solution-treated steels are considered to be caused by the strengthening of the material due to aging during the fatigue test at 400∼700°C, and by the softening due to overaging after 106
cycles at 700°C.