Grain Boundary Phosphorus Segregation in Thermally Aged Low Alloy Steels

Abstract

Intergranular embrittlement due to grain boundary segregation of phosphorus is recognized as one of the potential degradation factors in irradiated reactor low alloy steels at high neutron fluence. In this study, low alloy steels thermally aged at 400-500 C were investigated to evaluate the correlation between phosphorus segregation and intergranular embrittlement. Phosphorus segregation determined using Auger electron spectroscopy increased after thermal aging above 450 C and was in good agreement with the calculated value based on McLean's model. No influence of thermal aging on tensile properties or hardness was observed. The ductile brittle transition temperature determined using a one-third size Charpy impact test increased at a P/Fe peak ratio of 0.14. These results indicated that there is a threshold level of phosphorus segregation for non-hardening embrittlement. The ductile to brittle transition temperature (DBTT) increased with the proportion of intergranular fracture, so this result shows that there is a relationship between DBTT and the proportion of intergranular fracture. The fracture stress decreases due to non-hardening embrittlement on the thermally aged material with high proportion of intergranular fracture.

Current understanding of radiation-induced degradation in light water reactor structural materials

“…P segregation at GBs during thermal ageing is treated as thermal equilibrium segregation. Model estimation shows that the P concentration at GBs remains almost unchanged due to thermal aging alone for 60 years at around 300 • C in commercial low-alloy steels [49,115]. (2) Neutron irradiation significantly enhances P segregation at GBs.…”

Effects of Long-Term Service on Microstructure and Impact Toughness of the Weld Metal and Heat-Affected Zone in CrMoV Steel Joints

“…Therefore, the poor toughness of the WMs and intergranular cracking was caused by the high segregation level of P at PAG boundaries. Additionally, the obvious difference in toughness between the turbine part WM and the compressor part WM was mainly due to the different level of segregation of P. Nakata H et al [21] found that the fracture mode changed from transgranular fracture to intergranular fracture at the P/Fe peak height ratio of 0.14 for low-alloy steels. The P/Fe peak height ratio refers to (height of P peak)/(height of Fe peak) of the Auger electron spectrum, which was used to roughly evaluate the segregation level of P in low-alloy steels [21].…”

Low-Temperature Micro-Fracture Toughness Testing of Grain Boundaries in Steel

“…This type of intergranular fracture has been explained by the segregation of impurity elements, such as phosphorus (P) into the prior £-grain boundaries during tempering, inducing a decrease in intergranular strength. 1,2) In addition, the toughness value has been reported to decrease with an increase in the amount of P segregation, and the amount of decrease in grain boundary strength has been estimated from the corresponding decrease in the toughness value. 3,4) However, grain boundary properties like strength and fracture toughness have not yet been experimentally investigated.…”