Effect of temperature and deformation rate on fracture characteristics of nuclear reactor pressure vessel steel in dynamic strain aging region

Abstract

The influence of dynamic strain-ageing(DSA) on deformation and fracture characteristics was investigated on ASME SA508 class3 nuclear pressure vessel steel. Serrated flow in stress-strain curve that is a typical phenomenon of DSA was observed in the temperature range 140-340$^\circ\!$C with four different strain rates varying from $\sim$ 10$^{-5}$ to $\sim$10$^{-3}$/sec, and the region depended on microstructure condition. The onset temperature of serration for furnace-cooled condition was lower than that for as-received, tempered martensite condition. It can be considered that since the long tempering process of as-received condition that transfers more interstitial to the carbide and nitride in matrix than the case of furnace-cooling, the serration map for as-received condition was constituted in a higher temperature region. It was also noted that DSA caused a sharp rise in ultimate strength and a marked decrease in ductility and a negative strain-rate sensitivity of the flow stress. The temperature and strain-rate dependence of the onset of serrations yields an activation energy of 21.57kcal/moe for as-received condition and 16.53kcal/mole for furnace-cooled condition which suggests that the process is controlled by interstitial diffusion, probably of carbon and/or nitrogen in ferrite. On the other hand, the Ji value obtained from the direct current potential drop (DCPD) method, for crack initiation, was lowered by DSA. The drop in Ji at upper shelf region may be because of the interaction of the interstitial impurities with dislocations at the crack front. Comparing JIC from ASTM E813, the Ji by DCPD was a good parameter to detect the fracture toughness drop in the upper shelf region. After crack initiation, the crack propagation was also affected in terms of tearing modulus. The tearing modulus of the serrated region was about 30-40\% smaller than that at room temperature. As like the tensile test, negative strain rate sensitivity was applicable to the var...