A hydrostatic stress ratio of greater than 0.9 was proposed as one of the most significant criterion for void consolidation after closing during open die forging, on the basis of some physical modeling tests on 3.5NiCrMoV-rotor steel billet with a hole at the centerline and numerical simulation using the three-dimensional rigid-plastic finite-element method. To satisfy the criterion for void consolidation in a forging process of large rotor shafts, it was found that the use of a precooling ingot forging method for a rectangular cross-section-ingot is highly effective since the hydrostatic stress ratio at the center of the ingot remarkably increases with the increase of aspect ratio in the cross section from 1.0 to 1.8. The enlargement of width in the rectangular cross section enabled the development of the optimum maufacturing process of an ultralarge low-pressure turbine rotor shaft with 2800 mm and over diameter.

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Although the closure of defects at the center of billets on rolling such billets is an important subject, a method of quantity evaluation for such closure has not yet been clarified. Therefore, to establish a method for such quantity evaluation, we study such defects using experiments and finite element analysis including asymmetric rolling, especially with respect to the integration of the hydrostatic stress Gm. The results showed that stress was widely distributed, as determined in the experiment and in the FEM analysis of rolling at the edge around an artificial central hole in which defects were simulated. Therefore, it was suggested that, to evaluate the behavior of defect closure, the size of defects and the shape of the grooved roll used should be taken into account.

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Porous concrete is expected as an environmental preservation-type concrete but causes concern about its low freeze-thaw resistance compared with that of normal concrete. The purpose of this study is to clarify the effect of silica fume and fine fibers, a new material, on the freeze-thaw resistance of porous concrete. This study also included a vegetation experiment on porous concretes of the same kinds to observe the growth of plants. As a result, the freeze-thaw resistance of porous concrete was significantly improved when10-30% silica fume and 1Vol% fibers were used, with large differences in the state of deterioration and number of cracks being observed between normal porous concrete and fiber reinforced porous concrete containing silica fume. The growth of plants on fiber reinforced porous concrete was found to be comparable to that on normal porous concrete.

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Open-die forging experiments using different die geometries under hot isothermal conditions and three-dimensional simulations using rigid-plastic finite-element method were performed to formulate a void-closing behavior using only two factors : the integral of hydrostatic stress and the equivalent strain. First, upsetting, side-upsetting and V-shape die cogging of several cylinders with a spherical void at the center are carried out and the information on the void volume reduction is obtained. Seconds, the same forgings, but without voids is treated numerically and the development of stress and strain at the location of voids is investigated. Then, by combining these results, and using regression analysis, it is found that the void volume reduction is expressed as a polynomial function of the two factors. When the polynomial function is used, various forging methods can be evaluated quantitatively in terms of void-closing behavior. Therefore it is beneficial to optimize the forging process for a large rotor shaft.

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Austenite grains in the core of large scale turbine rotor forgings are refined by repeating annealing treatments afterforging in order to improve the ability of detection of flaw by ultra sonic wave. The treatments, however, require high cost and long time. The grain growth behavior of austenite during holding at high temperature following hot deformation was investigated using hot deformation simulator to achieve refinement in austenite grain by controlling hot forging.
Austenite grain markedly grew to about 4mm during holding for 5000s at 1250°C after a small amount of deformation of 5% at 1250°C. In the specimens in which extremely large austenite grains were formed, deformed austenite recrystallized statically by bulging mechanism and there were many grain boundaries with unstable triple point in recrystallized austenite before grain growth. On the other hand, only a few grain boundaries with unstable triple point were observed in samples deformed at more than 10%. Since a small strain is applied to forgings at the last stage of forging, it appears extreme grain growth occurs. There is a possibility that extreme grain growth can be prevented by accumulating strain to more than 8%, which can be achieved by forging with a period within 30s between deformations at a certain position.

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In order to develop a forging process that enables effective closure and consolidation of voids in the centerline of a heavy ingot for ultralarge rotor shafts, optimization of the combination of die configuration and billet section geometry was performed using a 3-dimensional finite-element method and high-temperature modeling experiments on NiCrMoV-rotor steel. The results indicate that a pair of indenter flat dies with the initial die width-to-billet height ratio of 0.5 for a round billet is more efficient than the conventional broad dies and V-shaped dies because it is shown under the non excessive load requirement that good closure of voids at the center of the billet is achieved, and the amount of longitudinal elongation of the billet is smaller.

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