Hastelloy® N alloy was developed in the 1950’s as ‘INOR 8’ by the Oak Ridge National Laboratory to resist molten salts used as a fuel and coolant in the early development of molten salt nuclear reactors for propulsion and power generation. China has recently expressed interest in Hastelloy N for use in prototype and demonstration components for a high-temperature, uranium-fueled, molten-salt cooled reactor for the production of electricity. An ASME Section III NH Code Case will be necessary to move Alloy N forward commercially. This paper discusses the guidelines for design data requirements necessary to satisfy the Boiler Code for elevated temperature nuclear applications where creep effects are significant. The historic tensile and creep properties data for Alloy N (N10003) were collected and re-analyzed in accordance with current ASME procedures. The collected data will be uploaded into the ASME Materials Properties Database to support the NH Code Case development. Paper published with permission.

HAYNES 230 and 617 alloys are competing for use on Generation IV, high temperature gas cooled reactor components because of their good high temperature creep strength in the temperature range of 760°C and 982°C and resistance to attack in the gas cooled reactor environment. A review of the metallurgy affecting the properties in each alloy is provided. It is shown that the grain size and carbide precipitation developed during manufacture affect short term and long term ductility, fatigue life, and creep strength. For example, 230 alloy has a finer grained structure which promotes fatigue strength with a slight sacrifice in creep strength. The 617 alloy has a coarser grain structure which provides slightly higher creep resistance while sacrificing some fatigue strength. Thermal aging also introduces gamma prime precipitation to 617 alloy in addition to grain boundary carbides. This, along with grain boundary oxidation, reduces the low cycle fatigue strength of 617 alloy compared to 230 alloy. Independent studies have shown that 230 alloy possesses higher resistance to thermal fatigue than 617 alloy. However, welds of both base metals with similar weld composition have about the same thermal fatigue life. Cooling rates from solution annealing temperatures during processing affect the ductility and creep strength of these alloys with the highest cooling rates preferred for retention of ductility and creep strength. Slow cooling rates promote carbide precipitation in the grain boundaries which reduces ductility and creep strength.

In the past, the prediction of the fatigue life of a part was often compared with Miner's rule (Σn/N = 1) wherein damage was computed by a linear accumulation of growth under individual loads. The difference between the predicted and observed damage for variable amplitude tests showed up as either retarded or accelerated fatigue life depending on test conditions. In this program, fatigue crack growth tests were performed with the compact tension specimen under aircraft spectrum loading to develop techniques for predicting fatigue life. Two models were investigated for their application to retardation behavior—linear cumulative growth and Wheeler. Constant amplitude crack growth rate data represented by the Forman equation was used as source data for the computer models. These models appear suitable for comparison with actual test results for diffusion-bonded Ti-6A1-4V and HP-9Ni-4Co steel at 0.3 and 0.2 carbon contents for the aircraft spectrums investigated. The 2219-T851 aluminum alloy, on the other hand, exhibited accelelerated behavior in relation to the linear cumulative growth scheme. An explanation is proposed by relating recent crack closure and spike overload studies in aluminum with the format of the applied spectrum. Also, the Wheeler model was found dependent on material and spectrum variables, and the Forman equation did not accurately represent crack growth rate below a rates of 10−5 in/cycle.