In the fatigue test at elevated temperatures, the test pieces must be pre-heated to the prescribed testing temperature. In LTQ low carbon steel, therefore, the age-hardening takes place during such pre-heating. In the present study, the rotary bending fatigue test was performed at elevated temperatures ranging from room temperature to 500°C and under the running speed of 3500rpm, after pre-heating the specimens for 1hr. The main results were as follows.
(1) In the pre-heating at temperatures from room temperature to 150°C, the surface hardness increased with heating time. The hardness reached maximum at 150°C after 1hr. At 300°C and 400°C, the over-age hardening phenomenon was seen after 1hr. This is considered due to carbide dispersions.
(2) The hardness values at 1hr heating were Hv
125 and Hv
120 at room temperature, 100°C, 150°C, 300°C and 400°C, respectively. In the subsequent fatigue test at 150°C, the hardness decreased with increasing the number of stress cycles. On the other hand, in the tests at 100°C and 300°C, the hardness increased with increasing the number of stress cycles.
(3) The fatigue strength at 107
at room temperature, 100°C, 150°C, 300°C and 400°C, respectively. These fatigue strengths seem to be unrelated with the hardness values at the start of fatigue tests after 1hr pre-heating.
(4) All of the values of σw
for LTQ steel were higher than those for annealed steel. It seems that this fact in LTQ steel is due to the combined contribution of age-hardening in pre-heating and cyclic stressing. On the other hand, in annealed steel, the effect of age-hardening is less than that in LTQ steel.
(5) In the LTQ steel as well as in the annealed steel, in the temperature range up to 500°C the fatigue strength became maximum at 100°C and 300°C, and the value at 300°C was higher than that at 100°C.