非金属介在物を含むS40C材の疲労強度に及ぼす引張加工の影響

抄録

The tests were carried out of specimens of specially prepared medium carbon steels which were designated as A1, A2 and B2. A1 and A2 steels contained respectively Mn-silicate and Al-silicate inclusions. B2 steel was the top part of ingot, and had a number of large flaws. The fatigue limits due to rotary bending for these materials were obtained after the specimens were preworked by statical tensile deformations beyond the yield point.
As the supplementary test, the torsional fatigue machine was stopped at intervals after additional cycling in order to investigate the initiation and propagation of fatigue cracks for annealed specimens, and photomicrographs were taken in the same area on the surface of the specimen
The results obtained are as follows.
(1) Comparing with the results for annealed states, the fatigue limits in A1 and A2 materials decrease few percent for small strains, and increase for large strains of preworking. This tendency is similar to that obtained previously for 0.15% C steels, and the effect of the presence of inclusions can not be found.
(2) In B2 steel, however, the above-mentioned tendency is not found, and also the scatter of fatigue test results for S-N curves is obtained. It should be pointed out that the influence of the large flaws on the fatigue strength is much more than that of the structural variation of metal by tensile preworking.
(3) The specimens made of B2 steel contained a number of large flaws as longitudinal directed inclusion lines parallel to the specimens axis, but these flaws had no harmful influence on the fatigue life of specimens.
(4) The surface of specimens not broken at 10⁷ torsional stress cycles was found a large number of small cracks, but the number of small cracks in B2 specimens was much less than that in A1 and A2 specimens.