Analysis of Creep Behavior of Alloy 617 for Use of VHTR System

Abstract

Alloy 617 is the one of the leading candidate materials for intermediate heat exchangers (IHX) application of a Very High Temperature Reactor (VHTR) system for economic production of electricity and hydrogen. Creep rupture data for Alloy 617 were obtained from a series of creep tests with different applied stresses at 850oC, 900oC and 950oC. On the basis of a systematic analysis of the experimental data, creep behaviour for Alloy 617 was analyzed using various creep relationships and laws, such as Norton's power law, Monkman-Grant Relationships (MGR), Modified Monkman-Grant Relationships (MMGR), creep damage tolerance factor λ, and Zener-Hollomon Parameter (Z), and the creep constants used in each equation were reasonably determined. The creep curve of Alloy 617 was somewhat different from those of typical heat-resistance steels, and it did not exhibit a textbook creep curve. The secondary creep stage is unclear, and it appeared to exhibit a predominantly accelerated creep rate from the start of a tertiary creep stage in short time ranges. The creep deformation was dominantly developed from the formation and growth of cavity in creep damage process. The MMGR appeared to be more narrowed in data scattering than the MGR, and it followed well a straight line of m ≅ 1.0 as m=0.97. In the plot of the Z parameter vs. stress, it obeyed a straight line with a slope of n’=5.87 regardless of the three different temperatures. It would be inferred that the same creep mechanism was operative within the present stress and temperature ranges, and the creep damage tolerance factor of Alloy 617 was found to be 2.40.