浸炭焼入材の疲労特性に及ぼす不完全焼入層の影響とそれに対する平均応力と残留応力の寄与

抄録

Non-martensitic structure is formed in the surface zone of carburized steel as a result of the local reduction of hardenability caused by internal oxidation of Cr and Mn during the gas carburizing process. Plane bending fatigue tests were carried out to investigate the effect of non-martensitic structure on the fatigue behavior in relation with the mean stress and residual stress. The results obtained are as follows;
(1) Non-martensitic structures formed during the gas-carburizing process lower the hardness and compressive residual stress at the surface of carburized steel.
(2) Both the residual stress and mean stress give the same effect on the fatigue strength of carburized steel, whether non-martensitic structures exist or not in the surface zone.
(3) The increase of fatigue strength resulting from the removal of non-martensitic structures by electro-polish is mainly caused by the increase of compressive residual stress at the surface.
(4) The S-N curves for the specimens tested under high tensile mean stress and low compressive residual stress show one discontinuity but those for the specimens tested under low tensile mean stress and high compressive residual stress show no discontinuity.
(5) A semicircular crack propagation region is observed on the fracture surface by means of scanning electron microscope.