Abstract
Recently, surface hardening treatment is being given on many machine parts in order to improve their fatigue strength. Although these parts are often used under such environments that a corrosive action may cause a considerable change in fatigue strength, the fracture mechanism of corrosion fatigue on such a hardened steel has been hardly made clear. So, in this study, a fractographical investigation was carried out to clarify the relation between fatigue strength and corrosion fatigued fracture patterns for the smoothed specimens of tufftrided steel.
The experimental results are summarized as follows.
(1) In air, the 1st and 2nd stages of crack propagation were confirmed on the fracture surface of tufftrided steel when the fracture originated from surface. Its complicated fracture zone was wider than that of non-tufftrided steel and its succeeding zone where striations could be observed distinctly was limited to the region where HV hardness before fatigue was less than 180.
(2) In pure water, tufftrided steel was ruptured by only one crack which nucleated at a corrosion pit, while non-tufftrided steel was ruptured by joining of several cracks. A zone of cleavage or quasi-cleavage fracture followed a mixed zone of intergranular and cleavage fractures as the crack propagation proceeded. The decrease in the applied stress level, i. e., the increase in fatigue life, brought the transition from the cleavage or quasi-cleavage fracture to the ductile fracture showing striations under the nitrogen diffusion layer.
(3) On the final fracture, cleavage fracture dominated in pure water, while dimple fracture dominated in air. The size of the final fracture zone in pure water was larger than that in air.
