Improvement of fatigue and stress corrosion properties by surface treatment and microstructure control

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1.On improvement of fatigue and stress corrosion properties by microstructure controlRotating bending fatigue tests were carried out to investigate the effects of humidity and reverted austenite on a crack initiation and its propagation of a maraging steel. Many cracks were initiated and intergranular cracks were observed on the fracture surface, when humidity was high. Both of the crack initiation and the early stage of the crack propagation of were accelerated by humidity, especially the effect on the crack initiation was large. However, the effect of humidity was suppressed by the formation of reverted austenite.2.On improvement of fatigue properties by surface treatmentBy nitriding and shot peening, fatigue strength was markedly improved. This was mainly caused by the surface hardening and formation of compressive residual stress in both surface treatments. Under low stress levels, fracture occurred from subsurface, while under high stress levels, fracture occurred from surface. Co … More nsequently, S-N curves had double stage. The effect of nitriding on the fatigue strength depended by nitriding condition. That is, when nitriding was carried out under under-aged condition, the increase in fatigue strength was large, while the increase was small under the over-aged condition.3.On improvement of fatigue properties by combined treatment of microstructure control and shot peeningRotating bending fatigue tests were carried out for a shot peened maraging steel in order to investigate the influence of reverted austenite on the internal fracture. Fracture initiated from an internal inclusion regardless of the formation of reverted austenite in long life region, while fracture occurred from a rough surface of specimen in short life region. Although fatigue strength was decreased when surface fracture occurred, that for an internal fracture was increased conversely by the formation of reverted austenite. Many intergranular cracks generated after the formation of flat facets around an inclusion and then fractured by the growth of a surface crack in case of no reverted austenite, while by the formation of reverted austenite intergranular cracking was suppressed and the growth process of an internal crack presented after intergranular cracking. Consequently, an orbit trace was formed clearly at the boundary between the growth of an internal crack and that of a surface crack. Less