Fundamental studies on electron beam welding on heat-resistant superalloys for nuclear plants, (3)

In this report, microcracks in the electron beam welds of the superalloys for nuclear plants were metallurgically investigated. Obtained conclusions can be summarized as follows; 1) The precipitates adjacent to the fusion line of Hastelloy X, Inconel 617 and Incoloy 807 which accompany microcracks change and melt clearly. On the other hand, those of Inconel 625 and SUS 316 in which microcracks never occur remain unchanged, and the precipitates remain nearly in the original condition in the case of Incoloy 800 which accompanies microcracks. 2) Main precipitates identified for every superalloy are M23C6 and M6C for Hastelloy X, M6C for Inconel 617, NbC and Ti(C,N) for Incoloy 807, NbC for Inconel 625, Ti(C,N) for Incoloy 800 and M23C6 for SUS 316. 3) The liquation of precipitates is considered to take place only when either melting temperature of precipitates or eutectic temperature between precipitates and matrix γ is lower than the temperature of matrix to start melting. 4) It is easily presumed that a microcrack will initiate at the melted precipitate at the grain boundary of the heat affected zone adjacent to the fusion line where the distribution of temperature, stress and strain is of peculiar characteristics. This distribution will be described in the following report in details. (author)