Papers by Author: Kazuhiro Morino

Abstract: In order to investigate the effect of radical nitriding on fatigue strength of spheroidal graphite cast iron FCD400, rotating bending fatigue tests were carried out at room temperature. Fatigue strength was increased by nitriding. Fracture started from microshrinkages or abnormal graphites in the same manner as that of as cast iron. However, fracture mode was different according to the stress level. That is, fracture occurred from specimen surface at high stress levels and it occurred from defects in subsurface at low stress levels. The cause of these results was the hardened layer and the compressive residual stress, which suppressed the initiation of the surface crack.

Abstract: Ultrasonic and rotating bending fatigue tests were carried out for aged and nitrided Ni-base super alloys to investigate the effects of loading frequency and nitriding on fatigue strength. Loading frequencies were 19.5 kHz under ultrasonic and 50 Hz under rotating bending, respectively. Fatigue strength under ultrasonic was higher than that under rotating bending in both alloys. Moreover, in both tests, fatigue strength was improved by nitriding. The increase in fatigue strength by nitriding was large in ultrasonic fatigue. These results were discussed through the successive observation of fatigue process at specimen surface and fracture surface observation.

Abstract: In order to investigate the effect of nitriding on the crack initiation and propagation behavior of Ni-base super alloy, Alloy 718, rotating bending fatigue tests were carried out until 108 cycles at room temperature. By nitriding at 500°C for 12h, compound layer of about 5μm in thickness was formed and the initiation of a fatigue crack was strongly suppressed causing the increase in fatigue strength. A crack initiated in brittle manner at the compound layer in all of fractures. However the crack propagated in ductile manner controlled by the property of the base alloy. That is, there is no or little influence of nitriding on the crack growth rate of the alloy.

Abstract: Rotating bending fatigue tests were carried out to investigate the effects of temperature on the fatigue strength and the fracture mechanism of an 18 % Ni maraging steel at room and elevated temperatures of 473K and 673K. Fatigue strength was higher at elevated temperatures than at room temperature, though static strength was decreased by softening at elevated temperature. There was no effect of temperature on crack morphology and fracture mechanism. On the other hand, during fatigue process at elevated temperature, the specimen was age-hardened and the specimen surface was oxide. That is, the increase in fatigue strength at elevated temperature was mainly caused by the increase in hardness due to age-hardening and suppression of a crack initiation due to surface oxidation.

Abstract: Rotating bending fatigue tests were carried out to investigate the influence of grain size on the resistance to crack growth of Ni-base super alloy, Inconel 718, using the materials with grain sizes of about 20, 50,100 and 200 たm, at room temperature. The larger grain size, the smaller crack growth rate, though the static strength was decreased with increase in grain size. The growth rate of a small crack was uniquely determined by the term ja na (Small crack growth law), where ja and a are the stress amplitude and the crack length, respectively, and n is a constant. The resistance to crack growth among materials was evaluated based on the law and showed a good correlation with tensile strength jB, meaning that fatigue life can be predicted by using only j B BB.

Abstract: Fatigue properties of Inconel 718 at 500°C were investigated under rotating bending. Fatigue strength was higher at 500°C than at room temperature. Fracture occurred by the initiation and propagation of a surface crack in short life region. The early propagation of a surface crack smaller than 20-30 μm was suppressed by oxidation at 500°C. This is a main reason for the higher fatigue strength at 500°C. However, the crack growth after the retardation of a small crack growth at 500°C was accelerated reflecting the lower static strength. On the other hand, in long life region, though a surface crack initiated at the early stage of fatigue life similar to the case in short life region, the crack did not propagate over 20-30 μm, and an internal fracture occurred. Consequently S-N curve at 500°C showed a duplex property and the fatigue limit was not recognized. The subsurface damage was initiated at the early stage of fatigue life.