Abstract
The influence of aging treatment on the fatigue strength of 18 Ni-maraging steels has been investigated in relation to the behavior of age hardening and the formation of reversion austenite in an 18Ni-8Co-5Mo steel. In the present study, fatigue test was performed on two series of specimens in order to make the effect of age hardening phases on fatigue property clear; in the first series, the strength of test pieces was controlled to have the same proof strength by varying aging temperature and time, and in the second series, it was changed by varying aging time at 750K, which is usually applied as the aging temperature in the commercial 18Ni-maraging steel. Fractography was also examined to study the fracture mode. The main results obtained are as follows.
(1) The fatigue limit is almost independent of the aging condition of specimens as far as they have the same proof strength. The computed fatigue strength, however, decreases when specimens have been hardened through the precipitation of low temperature phases.
(2) In the case of 750K aging, specimens strengthened to around the maximum hardness (Hv 450) have the almost same value in the fatigue limit independent of small difference in the aging time, although the fatigue limit of under-aged specimens was much lower.
(3) A small amount of reversion austenite, which is formed in an over-aging condition, is very effective in increasing fatigue resistance. An amount of 3 vol% austenite was enough for improving the fatigue limit of a maraging steel used.
