Jaegu Choi

Effects of main and trace elements on the formation of oxide in the oxidation behavior of Inconel 600 and 625 were investigated in this study using various calculated phase diagrams. After the isothermal oxidation tests at 1000 and 1100 °C, Nickel 201 for comparison showed significant growth of an oxide layer corresponding to NiO, and the NiO layer consisted of oxide crystals with both equiaxed and columnar shapes. As the temperature increased, the growth of the oxide layer proceeded mainly through columnar crystals. On the other hand, Inconel 600 and 625 showed relatively thin oxide layers, which were confirmed to be mainly composed of Cr₂O₃. The Cr₂O₃ layer was composed of equiaxed fine crystals. In the case of Inconel 625, an intermediate layer was formed at the interface between Cr₂O₃ and the base metal. It was found that the surface segregation of Cr as the main alloying element influenced the formation of the oxide layer of two Inconel alloys, and that some elements disappeared from the base metal near the surface due to continuous surface segregation and vaporization. On the other hand, even some elements with trace amounts showed strong surface segregation and contributed to the formation of oxide layer.

The leak-before-break design concept is based on J-R curves, which are obtained by J-R tests on various types of specimens and are known to be dependent on the cyclic load history. The J-R curves of standard specimens suggested by the American Society for Testing Materials are determined based on quasi-static tensile loading. However, seismic loading induces a reverse cyclic loading that alternately applies a tensile and a compressive load to nuclear plant piping. Therefore, it is very important to obtain the fracture toughness characteristics under reverse cyclic loading for the integrity estimation of nuclear plant piping. The objective of this paper is to study the effects of reverse cyclic loading on the fracture toughness characteristics of SA312 TP316L stainless steel, which is a nuclear plant piping material. J-R tests on a flat, equivalent stress gradient specimen with varying incremental displacement were carried out. The test results were reviewed by comparing the J-R test results under quasi-static loading. In addition, the safety margin of the nuclear plant piping was evaluated using a crack driving force diagram method. For the SA312 TP316L stainless steel, the results showed that the J-R curves were decreased with a decrease in the incremental displacement. When the incremental displacement was set to 0.25 mm, the unstable crack growth point value was about 73.0% of those for the quasi-static loading conditions.