A New Strategy to Prevent The Undesirable High Temperature Fatigue Reduction due to Cellular Formation in Microstructurally-Controlled Ni-Base Superalloys : -For Refurbishment and Recoating Technology-

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Recently industrial gas turbine power plants have been the major baseload source of power for many countries. These applications require a highly reliable, long-term load mechine which can operate using cheap, available fuels. Nobody doubts that repair, recoating and refurbishment of the hot section components in service under extremely sever condition will be inevitable technology in very near future. Nevertheless, there has been no established method for these technologies yet. Main objective in this work is to get basic understandings on the refurbishment and rcoating technology of microstructurally-controlled Ni-base superalloys, from a viewpoint of material relaibility.At first the microstructural changes in a single crystal Ni-base superalloy, CMSX-4, and a directicnally-solidified alloy, CM247LC, are shown in this work. After a plastic straining at room temperature into these alloys to simulate several kinds of in-service damages, the re-heat treatments as a usual course of dama … More ge recovery process was performed at elevated temoeratures ranged from 1080℃ to 1300℃. A siginificant change in microstructure, or cellular formation, was found to occur, depending on the re-heat treatment temperature. It was also found that the change was quite anisotropic, being influenced by the strain field and the microsegregation in the material.Next the effect of the carried out at 950℃ in air at a frequency of 10 Hz under fully-reserved loading ratio, by means of an electro-hydraulic fatigue testing machine. The experiments indicated that once the cellular formation was reached, the fatigue strength was remarkably reduced : an indication of a serious role of the cellular formation was reached, the fatigue strength was remarkably reduced : an indication of a serious role of the cellular formation for recoating and refurbishment technology.Finally the method to reduce and prevent the above undesirable effect is shown. It was found by the fatigue tests that the application of the surface modification technique, or protective coating might be a promising method. It was suggested that the control of alloy composition of the coating could also be a useful technique to reduce the harmful effects. Less