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Low Cycle Fatigue of Alloy 617 at 850 °C and 950The low cycle fatigue behavior of Alloy 617 has been evaluated at 850°C and 950°C, the temperature range of particular interest for the intermediate heat exchanger on a proposed high-temperature gas-cooled nuclear reactor. Cycles to failure were measured as a function of total strain range and varying strain rate. Results of the current experiments compare well with previous work reported in the literature for a similar range of temperatures and strain rate. The combined data demonstrate a Coffin-Manson relationship, although the slope of the Coffin-Manson fit is close to -1 rather than the typically reported value of -0.5. At 850 °C and a strain rate of 10~^ Is Alloy 617 deforms by a plastic flow mechanism in low cycle Jbtigue and exhibits some cyclic hardening. At 950 °C for strain rates of 1O~^-IO~ Is, Alloy 617 deforms by a solute drag creep mechanism during low cycle fatigue and does not show significant cyclic hardening or softening. At this temperature the strain rate has little influence on the cycles to failure for the strain ranges tested.

-The Next Generation Nuclear Plant project is considering

Three types of high-temperature joints were created from alloy 617 base metal: fusion welds, braze joints, and diffusion bonds. The microstructures of all joint types and tensile properties of fusion welds and braze joints were characterized. Sound fusion welds were created by the Gas-Tungsten Arc Weld (GTAW) process with alloy 617 filler wire. Cross-weld tensile strengths were equal to the parent metal at temperatures of 25, 800, and 1000°C; ductilities of the joints were only slightly lower than that of the parent metal. Failure occurred in the weld fusion zone at room temperature and in the parent metal at elevated temperatures. Incomplete wetting occurred in joints produced by vacuum brazing using AWS BNi-1 braze alloy, believed to be due to tenacious Al and Ti oxide formation. Incompletely bonded butt joints showed relatively poor tensile properties. A second set of braze joints has been created with faying surfaces electroplated with pure Ni prior to brazing; characterization of these joints is in progress. Conditions resulting in good diffusion bonds characterized by grain growth across the bondline and no porosity were determined: vacuum bonding at 1150°C for 3 hours with an initial uniaxial stress of 20 MPa (constant ram displacement). A 15 µm thick pure Ni interlayer was needed to achieve grain growth across the bondline. Tensile testing of diffusion bonds is in progress.iii

The effect of normalization temperature from 850°C to 1050°C on the structure and creep-rupture properties of modified 9Cr-1Mo steel was studied. Normalization at temperatures below 925°C resulted in structures containing significant polygonized, recovered ferrite. The ferrite structures had poor creep-rupture strength: roughly two orders of magnitude increase in minimum creep rate or decrease in rupture life for 850°C compared to 1050°C normalization at test conditions of 600°C and 145 MPa. Room-temperature strength and hardness were also reduced. The microstructure after normalization at the standard 1050°C temperature consisted of tempered martensite with fine M 23 C 6 carbide along prior austenite and lath boundaries and fine MX carbonitride precipitates within the laths. Normalization at temperatures between 925°C and 1000°C also resulted in reduced creep strength in comparison with 1050°C normalization, even though tempered martensite microstructures were formed and little change in room-temperature strength was observed; the reduction was attributed to subtle differences in the MX precipitates. The effect of reduced normalization temperature was more pronounced for higher-temperature, lower-stress creep-rupture conditions.