Cleavage fracture of RPV steel following warm pre‐stressing: micromechanical analysis and interpretation through a new model

Abstract

A B S T R A C T In this paper, the warm pre-stress (WPS) effect on the cleavage fracture of an 18MND5 (A533B) RPV steel is investigated. This effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following a prior loading at high temperature, has received great interest in light of its significance in the integrity assessment of structures, such as nuclear pressure vessels, subjected to thermal transients. Several loading cycles between room temperature (RT) and −150 • C are considered: Load-Unload-Cool-Fracture (LUCF), Load-Cool-Fracture (LCF) and Load-Cool with Increasing K-Fracture (LCIKF). All experiments complied with the conservative principle, which states that no fracture will occur if the applied stress intensity factor (SIF) decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. The experimental results indicate that an effective WPS effect is present even at small pre-load (K wps = 40 MPa √ m), and that a minimum critical slope (− K / T) in the LCIKF cycle has to be exceeded to induce cleavage fracture between RT and −150 • C. Numerical modelling was performed using mixed isotropic and kinematic hardening laws identified on notched tensile (NT) specimens, tested in tension to large strains (up to 40%), followed by large compressive strains. Detailed microstructural investigations on compact tensile (CT) and NT fracture test specimens were performed so as to determine the nature of the cleavage initiation sites, as well as the local mechanical conditions at fracture. Based on this local information, a new cleavage model was calibrated and applied to predict the probability of cleavage fracture after WPS: it is shown that the predictions are in good agreement with the experimental results.a 0 = crack length in CT toughness specimen (mm) d = distance (mm) K = SIF increment between RT and −150 • C (MPa √ m) (− K/ T) = transient slope of the LICKF cycle between RT and −150 • C (MPa √ m· • C −1 ) K, K el = stress intensity factor (MPa √ m) K frac = stress intensity factor at fracture (MPa √ m) K Ic , K Jc = mode I fracture toughness for the virgin material (MPa √ m)K wps = stress intensity factor after warm pre-stressing (MPa √ m)

Experimental and analytical analyses of the cutting process in the deep hole drilling with BTA (Boring Trepanning Association) system

“…Several studies are conducted on similar steels in term of chemical composition, like 16MND5. Studies concerning the 16MND5 are mainly oriented to the integrity of reactors vessels and trait aspects of the materials tenacity [48][49][50][51][52]. According to Vereecke [51], originally the manganese composing the 18MND5 has been chosen to improve the hardenability and weldability of the steel, but also to trap sulphurs to form manganese sulphides, preferable to iron sulphides which have a negative effect on the mechanical resistance.…”

Cleavage Fracture Micromechanisms Related to WPS Effect in RPV Steel

“…All these results suggest that the higher Kwps, the higher the enhancement in toughness at low temperature (note however that an excessive pre-load may cause ductile-type damage that opposes this trend [2]). They also indicate that a minimum critical slope (-∆K/∆T) has to be exceeded to induce cleavage fracture during cooling between room temperature and -150°C, which further comforts the conservative principle stated in the introduction.…”

“…Based on the location of the initiation site in the deformed NT and CT geometries, the local normal (opening) stress and plastic strain at fracture were evaluated from finite element calculations, whose details can be found in [2]. For simplification, it was assumed that, for both NT and CT geometries, the initiation points lay on the mid-plane normal to the loading direction [2].…”

“…Full details of the mechanical tests are given in [2]. The fracture specimens, all tested under quasi-static conditions, consist of:…”

“…For simplification, it was assumed that, for both NT and CT geometries, the initiation points lay on the mid-plane normal to the loading direction [2]. An axisymmetric FE model was used for the NT geometry, while 2D plane strain modelling was used for the CT geometry.…”