Dynamic Strain Aging of Nickel-Base Alloys 800H and 690
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y engineering alloys used in the nuclear power industry experience dynamic strain aging (DSA). This effect has been extensively studied in 316 stainless steel, but there have been only a few DSA studies published on alloys 690 and 800H. Alloy 690 is the primary replacement for alloy 600 to mitigate the problem of stress-corrosion cracking. Alloy 800H is one of the proposed alloys for the heat exchanger/steam generator in a very high-temperature reactor (VHTR). Both of these alloys experience DSA when strained, which may impact the cracking behavior in service. The chemical compositions of the commercial heats of alloys 690 and 800H used in this study are shown in Table I. Alloy 800H samples were prepared from heat 35175 in the as-received condition, which consisted of a hot rolling and anneal at 1450 K (1177 C) for 2 hours followed by a water quench. The alloy 690 sample was prepared from heat NX3151 HK in the as-received condition, which consisted of hot rolling and annealing at 1313 K (1040 C). Alloy 800H had a grain size of 150 lm and TiN particles distributed in the matrix. Alloy 690 had a grain size of 22 lm and had discontinuous carbides along the grain boundaries with TiN particles distributed in the matrix. The microstructure of each alloy after being etched in Marble’s reagent is shown in Figure 1. A series of constant extension rate tensile (CERT) tests were performed with initial strain
TYLER E. MOSS, Graduate Student Research Assistant, and GARY S. WAS, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. Contact e-mail: [email protected] Manuscript submitted April 24, 2012. Article published online August 11, 2012 3428—VOLUME 43A, OCTOBER 2012
rates of 107, 106, 105, and 104 s1over a temperature range from 295 K to 673 K (22 C to 400 C). The samples were strained in an autoclave with an argon environment in the High Temperature Corrosion Laboratory at the University of Michigan. This system can strain up to four samples at once using a stepper motor (4k Interactive Instruments, Inc., Scotia, NY). The temperature was controlled by heating bands surrounding the autoclave, providing an accuracy of ±0.25 K (±0.25 C). Sample extension was measured using a linear variable differential transformer with an accuracy of ±0.5 lm. To map the load serration occurrence based on strain rate and temperature, two samples of alloy 800H and one sample of alloy 690 were repeatedly strained over the previously stated temperature and strain rate range. The straining history for each sample is detailed in Table II. The engineering stress–strain curves for sample 800H-2 at a constant strain rate of 107 s1 over the temperature range 295 K to 673 K (22 C to 400 C) are shown in Figure 2 as an example of the observed load serration behavior. The yield stress of each strain increment is noted above the stress–strain curve. Changes in the yield strength between strain increments are due to strain hardening and the order in which experiments were conducted.
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