A Dislocation-Based Cleavage Initiation Model for Pressure Vessel Steels

Abstract

Efforts are under way to develop a theoretical, multi-scale model for the prediction of fracture toughness of ferritic steels in the ductile-to-brittle transition temperature (DBTT) region that accounts for temperature, irradiation, strain rate, and material condition (chemistry and heat treatment) effects. This new model is intended to address difficulties associated with existing empirically-derived models of the DBTT region that cannot be extrapolated to conditions for which data are unavailable. Dislocation distribution equations, derived from the theories of Yokobori et al., are incorporated to account for the local stress state prior to and following initiation of a microcrack from a second-phase particle. The new model is the basis for the DISlocation-based FRACture (DISFRAC) computer code being developed at the Oak Ridge National Laboratory (ORNL). The purpose of this code is to permit fracture safety assessments of ferritic structures with only tensile properties required as input. The primary motivation for the code is to assist in the prediction of radiation effects on nuclear reactor pressure vessels, in parallel with the EURATOM PERFORM 60 project. This paper begins with a brief overview of the strategy for implementing the new model into the DISFRAC computer code. The balance of the paper focuses on efforts to model the nucleation of a carbide particle crack near an existing macrocrack under applied load. The carbide microcrack initiation model applies dislocation mechanics to assess the stress intensity exerted on a stiff, elastic carbide particle embedded in an elastic-plastic ferrite matrix near a macrocrack tip. The paper derives and discusses the governing equations for the model; including (1) computation of a slip band dislocation pileup distribution by enforcing equilibrium with the macrocrack-induced elastic-plastic stress field, (2) calculation of the mode I stress intensity on the particle crack plane due to the dislocation pileup and (3) determination of the particle fracture toughness. Together, these calculations provide the basis for determining the applied load required to initiate particle fracture. This paper demonstrates how the prediction of particle fracture depends on various microstructure parameters.