Takuro Saga

Studies on the Fatigue Strength of Aluminum Coated Steel : 1st Report, The Effect of Aluminum Coating Method on the Fatigue Strength

Rotating bending fatigue tests were performed on three kinds of aluminum coated steels. The effect of three aluminum coating methods, namely electroplating, hot dip, and spraying on the fatigue strength of the steels were studied. The results obtained are as follows. (1) The effect of electroplating on the fatigue strength of the steel is not recognized at all. But a little effect of the spraying on the fatigue strength is observed. (2) In hot dip aluminum coating of the steels, a decrease of the fatigue strength is observed. This is considered to be due to the formation of cracks in a brittle alloy layer. Such cracks act as shape notches on the surface of steel base. Moreover, the formation of cracks decreases the effective area of specimen whic supports the alternating stress. (3) The addition of 1% beryllium to molten aluminum bath markedly improves the fatigue strength of hot dip materials.

On the Improvement of the Mechanical Properties of 21-4N Valve Steel by Double Aging

During the aging of the solution treated 21-4N valve steel, the interstitial elements form carbides and nitrides by precipitation reactions in the same manner as in most Cr-Ni austenitic heat-resisting steels. The morphology and distribution of these phases will have a marked effect on the mechanical properties of this steel at both room and elevated temperatures. Particularly in mechanical properties, the toughness and the fatigue strength seem to decrease remarkably by high temperature aging of 900∼1000°C at which the lamellar nodules are formed at grain boundaries. In this study, the effect of low temperature pre-aging at the temperatures of 700∼800°C on the precipitation reactions during the subsequent high temperature aging has been investigated for the commercial exhaust valves because of the“double aging process”was found to be effective for the test specimens. The effect of low temperature pre-aging on the mechanical properties after high temperature aging has also been investigated at both room and elevated temperatures.The results obtained were as follows:(1) If this steel was pre-aged at 700∼800°C for about 3 hours after solution treatment at 1200°C, lamellar grain boundary precipitates were scarcely formed and general precipitation was predominant during the subsequent high temperature aging.(2) The employment of this low temperature aging seemed to be also effective to retard the formation of lamellar nodules at grain boundaries in the commercial exhaust valves made of this steel.(3) If the solution treated steel was pre-aged at low temperature before high temperature aging, the fatigue strength of this steel was considerably increased at both room and elevated temperature, and its toughness was also improved at elevated temperatures, compared with steel not pre-aged, because lamellar grain boundary precipitates were detrimental to these properties.(4) On the other hand, the tensile strength at the temperatures up to 800°C as well as the creep rupture strength at 600°C seemed to be scarcely changed by the employment of this“double aging process”because the effect of the two types of precipitation reactions, general and lamellar precipitation, on these properties was only secondary.

Effect of Heat Treatment on the Mechanical Properties of 21-4N Valve Steel

The room and elevated temperature mechanical properties as well as the aging characteristics after various heat-treatment have been investigated on commercial 21-4N valve steel. The results obtained are as follows:(1) Solution temperature of 1200°C is suitable for this steel.(2) In aging the sufficiently solution treated steel at 900∼1000°C, both general and lamellar precipitation occur, but in the steel aged at temperatures below 800°C only general precipitates appear. In the insufficiently solution treated steel at the temperatures below 1150°C, however, only general precipitation takes place regardless of the aging temperature.(3) This steel shows high strength and toughness even in solution treated state. The aging causes only slight improvement of strength, but the toughness at room temperature decreases remarkably by aging, particularly by high temperature aging in which the lamellar precipitation occurs.