1977 Volume 63 Issue 6 Pages 962-970
Fatigue strength of two-uctile-phase steels is studied in order to clarify the effects of volume fraction of the harder phase (f), C*-value (=0.2% proof stress of the harder phase /0.2% proof stress of the softer phase), and shape of the harder phase grains on the fatigue strength. The alloys used are classified into three groups with various values of f: Fe-Cr-Ni alloys composed of austenite and ferrite (A), Fe-C steels composed of ferrite and martensite (C), and Fe-Cr-Ni alloys composed of austenite and martensite (D). The C*-value is changed by aging (A) or tempering (C). The mean grain size (d) of each alloy is adjusted to be nearly the same by heat treatment. The main results obtained are as follows:
(1) When the shapes of grains (both of the harder phase and of the softer phase) are nearly spherical, the effects of f, C*-value, and d are as follows.
(a) Effects of f when d is constant; Three regions are observed in the relation of the endurance limit (σW) vs f. Namely, according to an increase in f, σW hardly changes at first (lst region), then increases rapidly (2nd region) and again hardly changes (3rd region).
(b) Effect of C*-value; When alloys belong to the 1st region, σW is independent of C*-value in the case of an alloy of the 2nd or 3rd region.
(c) Effect of d; σW is estimated to increase with decreasing d, which plays an important role to obtain a two-phase alloy with excellent fatigue strength.
(2) In case of thin plate-like grains of the harder phase, σW increases with an increase in f even if f is small. That is, the range of each region mentianed above changes by the shape of the harder phase.
(3) The above conclusions (1) and (2) are explained qualitatively by the observed preferential path of fatigue crack growth.