Ohmi Miyagawa

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.

The effects of orientation and thickness on the notch-tensile creep strength of single crystals of a nickel-base superalloy

The effects of crystallographic orientation and thickness of specimen on the notch-tensile creep strength of single crystals of a nickel-base superalloy UDIMET∗520 has been examined at 700°, 850°, and 900 °C. It was found that the notch-tensile creep strength of thin specimens depended on the crystallographic orientations not only in the tensile direction but also in the normal direction of the specimens, and that the creep strength was superior in the thin specimens with the [011] tensile and the [011] normal orientations or the [001] tensile and the [110] normal orientations. The thick-notched specimens exhibited great creep resistance regardless of the crystallographic orientations.

The deformation and fracture of austenitic heat-resisting steel with grain-boundary reaction nodules

The tensile properties in 21-4N austenitic engine valve steel containing pearlitic nodules due to the grain-boundary reaction have been investigated at room temperature. Some theoretical considerations on the deformation and fracture behaviour of this steel are also presented. In the steel with grain-boundary reaction nodules more than about 10% in area fraction, the ductility decreases considerably with their increase owing to the brittle fracture of rod-like precipitates in the nodules. Although the grain-boundary reaction also decreases the tensile stress and 0.2% proof stress, its effect is more noticeable on ductility than on strength. A theory is developed to explain the workhardening behaviour as well as the fracture mechanism using a model for composite materials and a good agreement between theoretical and experimental results is obtained.

Effect of grain and grain boundary strengths on the life of main creep crack initiation in a notched plate specimen.

Effect of grain and grain boundary strengths on the life of main creep crack initiation in a notched plate specimen was investigated, using an austenitic heat resisting steel. In a range of lower grain strength below about HV 300, the life of main crack initiation increases with an increase in grain strength, independent of grain boundary strength. In a range above HV 300, however, it decreases drastically with increasing grain strength when a specimen has a weaker grain boundary strength. On the contrary, if a specimen has a stronger grain boundary strength, it continues to increase with increasing grain strength. Further, a relationship between the main crack initiation life, ti, and the mean notch opening displacement rate, V^^·m, that is, tiV^^·nm=C (n and C are constant), was obtained which was analogous to the Monkman-Grant relationship. Finally, the effect of grain and grain boundary strengths on ti was discussed in terms of the relationship tiV^^·nm=C, considering the effect of both strengths on V^^m and a critical notch opening displacement at the time of main crack initiation.

On the improvement of high-temperature low-cycle fatigue property by grain-boundary reaction precipitates.

The effect of grain-boundary reaction (GBR) on high-temperature low-cycle fatigue life was investigated for austenitic 21-4N heat-resisting steel at 973K. The grain-boundary was considerably serrated by GBR during aging. The high-temperature fatigue life was remarkably improved by the occurrence of GBR for fast-fast and slow-slow symmetrical triangular strain wave shapes. The fatigue life was not changed by GBR for the asymmetrical triangular wave shape. The longest fatigue life was observed on the specimen with GBR nodule was internally cracked by cyclic deformation. The improvement of fatigue life by a small amount of GBR was resulted from the retardation of brittle intergranular fracture by the serrated grain-boundary configuration.

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.