Characterization of hardening behavior of carbon steels at low temperature irradiations

Abstract

• Embrittlement factors of carbon steels irradiated at low temperature were fine dislocation loops and solute clusters of Mn-Si-Cu atoms. • Carbon steels and their weld metals containing high concentrations of Cu, Mn and Si had relatively high radiation hardening due to promotion of solute atom clustering. • Low temperature irradiation resulted in higher radiation hardening due to the higher number density of dislocation loops, despite the lower number density of solute clusters. • Low dose rate irradiation (electron irradiation) resulted in greater radiation hardening due to the higher number density of solute clusters than high dose rate irradiation (ion irradiation) did. Overpacks used for geological disposal of high-level radioactive waste will be irradiated by neutrons and γ rays with very low dose rate for a long time. A possible way to predict effects of such radiations is that microstructure changes under the expected irradiation conditions are estimated by both experiments and analytical modeling and then the changes in hardness and fracture toughness from the microstructure changes are estimated. In the present study, carbon steels, which are one of the candidate materials for overpacks, model alloys and a weld of carbon steel were irradiated with either Fe ions or electrons at 90 or 290 °C, and then the microstructure and hardness were investigated to gain an understanding about embrittlement factors of carbon steels and effects of both material composition and irradiation conditions (temperature, dose rate) on radiation-induced changes in microstructure and hardness. Formations of fine dislocation loops and solute clusters containing Mn, Si and Cu atoms were confirmed by microstructure analysis using transmission electron microscopy and atom probe tomography. Hardness increase was well correlated with the evolution of dislocation loops and solute clusters, indicating that these defects are embrittlement factors of carbon steels. Higher radiation hardening was confirmed for the materials containing a high density of Mn, Si and Cu, and the irradiation conditions of low temperature and low dose rate.