The effect of dynamic strain aging on cyclic stress response and fatigue life of ASME SA508 Cl.3 forging steel for nuclear reactor pressure vessels was studied at temperatures ranging from room temperature to 500℃. Total strain ranges and strain rates were varied from 0.7 to 2.0% and from $1×10^{-2}$ to $4×10^{-4}s^{-1}$, respectively. Test environments were the atmospheres of air and argon.
Except for the temperature region of dynamic strain aging, 300℃, the initial cyclic hardening was immediately followed by cyclic softening at all strain rates in air atmosphere. The cyclic softening continued over 95% of the fatigue life. On the other hand, at the dynamic strain aging temperature, the operating temperature of nuclear pressure vessel, the variation of cyclic stress amplitude showed the primary and secondary hardening dependent on the strain rate and the total strain range. Dynamic strain aging was manifested as secondary hardening and a negative strain rate sensitivity by the maximum stress amplitude and micro-Vickers hardness. During the low cycle fatigue test at the dynamic strain aging regime in inert atmosphere, the variation of cyclic stress amplitude was nearly the same in air except for the occurrence of secondary hardening at the faster strain rate and the higher maximum strength. The earlier appearance of dynamic strain aging in inert atmosphere than in air may be caused by the reduced oxidation effect. Therefore, dynamic strain aging may be suppressed at the initial stage of deformation in air atmosphere. Dynamic strain aging in the oxidizing atmosphere enhanced the number of crack initiation sites by partitioning the local deformation but retarded the crack propagation rate by crack branching and by the suppressed plastic zone size. As the strain rate increased, the fatigue resistance increased at all temperatures. And the effect of dynamic strain aging on fatigue life in a smooth specimen was larger than oxidation effect, while the hardening by dynamic strain aging was overcome by the oxidation and easy crack initiation in a rough specimen. However, the fatigue life in inert atmosphere was independent of test temperature. Therefore, it is considered that the dynamic strain aging contended against oxidation. A modified cell shuttling model was described for the onset of the secondary hardening due to the dynamic strain aging which was based on dislocation cell shuttling motion and it was in good agreement with the experimental results. In comparison with the design fatigue curve in the ASME Boiler and Pressure Vessel Code, it was concluded that the SA508 forging steel for nuclear pressure vessel has resistance to fatigue and possesses safety margins for the integrity.
