Yield and Toughness Transition Predictions for Irradiated Steels Based on Dislocation Mechanics

Abstract

An abundance of empirical data supports the use of the Master Curve, as proposed by Wallin, Saario and Törrönen, to describe the fracture toughness transition behavior of ferritic steels, particularly the notion of a curve shape that is invariant with steel microstructure (other than lattice structure). However, nuclear surveillance programs do not always contain samples of the steel that most limits reactor operations, making direct measurement of fracture toughness impossible. This suggests that a purely empirical argument cannot define the limits of applicability of the Master Curve or validate its use for all conditions of interest. In previous papers a microstructural basis for the existence of a single “Master” fracture toughness transition curve for all ferritic steels was established and limits of applicability have begun to be explored from a theoretical viewpoint. These previous papers established that all steels with the same lattice structure and cleavage fracture mechanism should be expected to adhere to transition behavior that can be defined by a single curve shape with variations in microstructure accounting only for a shift in the transition temperature. In this paper we explore the basis for “Master Curve” validity for irradiated steels by exploring how irradiation affects the microstructure and fracture mode and using the Zerilli-Armstrong constitutive model as the basis for predictions of irradiated steel behavior.

Local approach to fracture based prediction of the ΔT56J and shifts due to irradiation for an A508 pressure vessel steel

“…On the physical point of view, irradiation produces fine scale microstructures [17]. Their effects on the macroscopic behavior can be separated into two groups [17,18]: (i) change of the plastic hardening, (ii) embrittlement, one of the most wellknown and well-described being phosphorous segregation at grain boundaries [17]. Hardening mechanisms include matrix and age hardening.…”

“…Hardening mechanisms include matrix and age hardening. Matrix hardening is due to radiation-produced point defect clusters and dislocation loops, referred to as the matrix damage contribution [18]. Age hardening is an irradiation-enhanced formation of copper-rich precipitates.…”