Observations of secondary carbide precipitation and its relation to high-temperature flow and fracture in HT-9 stainless
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Table I. Nominal Composition of HT-9 (Weight Percent) Fe
C
Si
Mn
Cr
Ni
Mo
W
V
Observations of Secondary Carbide Precipitation and Its Relation to High-Temperature Flow and Fracture in HT-9 Stainless Steel
Bal
0.20
0.4
0.55
11.5
0.5
1.0
0.5
0.3
4-
R.J. DiMELFI and YUHENG LI The martensitic stainless steel HT-9 has been of interest to the advanced nuclear energy community for many years, primarily because of its low swelling properties in the fast neutron environment. We at Argonne have studied this material in connection with its role as fuel cladding in the integral fast reactor (IFR). tl,21 Very similar steels constitute the metal matrix in dispersion-strengthened alloys proposed for use in Japanese advanced reactor designs, and HT-9 itself is still considered as an option for use in fusion reactor first wall structures. All of these applications require an understanding of the high-temperature mechanical behavior of HT-9 over a wide range of loading conditions, including the effects of temperature, stress, and rate. In this context, we have studied and modeled the mechanical properties of this material with an eye toward predicting IFR fuel-cladding life and the conditions under which cladding can fail. The goal of this work has been to provide the knowledge base needed to avoid such failures and their consequences and to provide for a safe and long reactor life. Our findings, however, are relevant to many other advanced energy technologies. Key to our understanding of the deformation and fracture behavior of this alloy is an understanding of the evolution of its microstructure during high-temperature flow. In this article, we present for the first time microstructural evidence that is connected with several important mechanical behavior observations and which supports our models u,21 of that behavior. Many advanced energy technologies require broadly applicable models for high-temperature flow and fracture capable of following complex loading histories and thermal/mechanical transients. We have met with some success in this context for systems involving solid-solutionstrengthened austenitic steels, t31 In the case of HT-9, however, this task is made more complicated by the inherent instability of the alloy's microstructure at high temperature. The composition of HT-9 is shown in Table I. For the IFR application, the alloy begins its life in the as-heattreated state with a rather uniform fine tempered martensite rnicrostructure. One would expect this microstmcture to coarsen during long-time exposure at high temperatures, resuiting in a reduction in strength. However, there is evidencevl that there are microstmctural changes occurring during longtime deformation at high temperatures that lead to increased high-temperature strength and delayed rupture. It is important
R.J. DiMELFI, Staff Materials Scientist, is the Director of the Engineered Materials Center, Argonne National Laboratory, Argonne, IL 60439. YUHENG LI, formerly Postdoctoral Fellow, Experimental Facilities Division, Argonne Natio
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