Dynamic Strain-Aging Effect on Fracture Toughness of Vessel Steels

Abstract

The effect of dynamic strain aging (DSA) on fracture is investigated on the quenched and tempered specimens of American Society of Mechanical Engineers (ASME) standard SA508 class 3 nuclear pressure vessel steel. Serrated flow by DSA is observed between 180 and 340-degrees-C at a tensile strain rate of 2.08 x 10(-4)/s and 1.25 x 10(-3)/s. The DSA causes a sharp rise in the ultimate tensile strength and a marked decrease in ductility. The DSA range shifts to higher temperatures with increased strain rates. The temperature and strain rate dependence of the onset of serrations yields an activation energy of 16.2 kcal/mol, which suggests that the process is controlled by interstitial diffusion of carbon and nitrogen in ferrite. The J(i) value obtained from the direct current potential drop (DCPD) method, for true crack initiation, is lowered by DSA. The drop in J(i) at elevated temperatures may be because of the interaction of the intersitial impurities with dislocations at the crack front. Compared with J(IC) from American Society for Testing and Materials (ASTM) standard E813, the J(i) by DCPD is a better parameter to use to detect the DSA effect on fracture toughness. Crack propagation is also affected in terms of the tearing modulus. The tearing modulus in dynamic strain aging is approximately 30% smaller than that at room temperature.