An assessment of tensile, irradiation creep, creep rupture, and fatigue behavior in austenitic stainless steels with emphasis on spectral effects

Abstract

A review of mechanical properties of austenitic stainless steels is made to assess their behavior in fusion reactors. Since the first walls of fusion devices are expected to range in temperature from 100 to over 500°C, behavior over a wide range of temperatures is reviewed. For tensile properties, the neutron spectrum has little effect on strength, but appears to influence plastic deformation and ductility. The effect is apparent at temperatures above 400°C where ductility begins to increase. Ductility appears to be higher for alloys irradiated in fast reactors where little helium is produced, but even for fast reactor irradiation, ductility begins to drop above 700°C. This behavior, which is enhanced by higher helium concentrations, is believed to be a result of helium embrittlement. Irradiation creep is very weakly dependent on temperature, with nearly constant creep rates from room temperature to half the melting point. There are not yet sufficient data to determine the effect, if any, of helium on irradiation creep. However, creep rupture is exacerbated by helium since bubble formation at grain boundaries is the primary mechanism of in-reactor failure, especially at high temperatures. Fatigue is also dependent upon high temperature helium embrittlement. High temperatures, high helium concentrations, and low strain rates enhance reduction in fatigue life. At temperatures as high as 550°C, there is no apparent effect of helium even at concentrations of 500 appm. at strain rates of 10 −3 s −1 but either a decrease in strain rate by a factor of 100 or an increase in helium concentration by a factor of 6 produces a degradation in fatigue life. In general, helium affects mechanical properties of austenitic stainless steels; however, it is mostly a high temperature phenomenon. Caution must be exercised at temperatures below 250°C, where uniform tensile elongation is extremely low in irradiated alloys. Low-temperature embrittlement must be investigated further to determine if incremental hardening by helium contributes.