Summary of Studies of Aging and Environmental Effects on Inconel 617 and Haynes 230

A number of very high temperature helium-cooled reactors have been built and operated for extended periods. The helium coolant in the primary circuit has been found to contain low levels of impurities after steady-state operation that can lead to an environmental degradation of the high temperature alloys used for internals and heat exchangers. Depending on the impurity concentration and the temperature, high temperature alloys can undergo oxidation, carburization, or decarburization. The concentration of H2O and CO is of particular interest because they essentially control the oxygen partial pressure and carbon activity, respectively. The optimum coolant chemistry for long-term stability of high temperature alloys is slightly oxidizing and results in formation of a tenacious and protective Cr2O3 scale. The most critical metallic component of the Next Generation Nuclear Plant (NGNP) is the heat exchanger. Inconel 617 is the primary candidate alloy for this application because of its superior creep resistance. The mechanisms of environmental interaction between this alloy and prototype Very High Temperature Reactor (VHTR) helium chemistries have been extensively studied. A modified type of Ellingham diagram that maps the ranges of carbon activity and oxygen partial pressure that result in each of the degradation mechanisms has been developed. The NGNP materials program has designed and built three test loops to extend previous studies on environmental effects of prototype impure helium on Inconel 617 by increasing temperatures and using test coupons that incorporate fusion welds in controlled impurity experiments. In addition, parallel studies have been initiated with a less well-characterized alloy, Haynes 230. The goal of this work is to determine the range of gas chemistries that give rise to stable oxide formation for these alloys at temperatures up to 1000 C. Stability of the microstructure and properties of Inconel 617 and Haynes 230 after extended exposure to the elevated temperatures expected in the NGNP heat exchanger are also a potential issue. An aging program has been initiated for these alloys to characterize changes resulting from prolonged high temperature exposure. Furnace aging Inconel 617 in air results in formation of an adherent oxide scale and a carbide depleted zone beneath the oxide. There is some grain growth with increasing time at elevated temperature and the distribution of grain sizes becomes broader. There are modest changes in the mechanical properties for the aging conditions that have been examined. Decreased yield strength with increasing time at elevated temperature is consistent with the increasing grain size. Aging under load results in notable redistribution of carbides to some of the grain boundaries that experience tensile loading for some temperatures and applied stress values. Details of this phenomenon are being examined in detail.