Ductile to brittle transition of an A508 steel characterized by Charpy impact test

Abstract

This study is devoted to the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain rates. Effects of impact velocity on ductile-brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids colaescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that Manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentionned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [1], the numerical simulation of the Charpy test in the ductile-brittle transition range using fully coupled local approach to fracture is presented.

Development of the Local Approach to Fracture over the Past 25 years: Theory and Applications

“…As stated previously the Beremin model is essentially based on the description of the propagation of an existing critical defect belonging to a single population. This is a simplification which might explain why in the application of this model over a wide range of temperatures, a number of investigators have reported that it was necessary to assume the normalizing stress, σ u , to be an increasing function of temperature (see, e.g., Tanguy et al, 2005a, b). This might simply reflect the existence of critical different steps depending on temperature, as indicated earlier.…”

Modeling ductile to brittle fracture transition in steels—micromechanical and physical challenges

“…15a show that the predictions are in good agreement with the experiments for Variation of the Charpy energy (CVN) with crack extension from the notch. Experimental results at −60 • C. Calculated probabilities to initiate cleavage fracture (P R = 0.10, 0.50, 0.90) for two temperatures (−60 and −40 • C) (Tanguy et al 2005a, b) temperatures lower than −80 • C. Above this temperature the model largely underestimates the Charpy energy. Similar results have been reported in the literature (Bernauer et al 1999;Rossol et al 2002a, b).…”

“…However when properly instrumented and numerically modeled this test can also be used as a tool to measure the intrinsic fracture properties of a given steel. This is now briefly illustrated using the results of a study performed on RPV A 508 steel (Tanguy 2001;Tanguy et al 2005a, b).…”

“…Similar results have been reported in the literature (Bernauer et al 1999;Rossol et al 2002a, b). The transition curve was adjusted with a value of σ u increasing with temperature (Tanguy et al 2005a, Tanguy et al b). As observed in Fig.…”

“…15 A 508 RPV steel. Prediction of the Charpy V transition curve assuming (a) a constant value of the normalizing σ u of the Beremin model, (b) an increasing value of σ u with temperature (Tanguy et al 2005a;Pineau 2006) Local strain rates as large as 10 3 s −1 take place during the deformation of the notch and during crack propagation. Modeling the mechanical response of the specimen requires to properly capture the strain rate effect in the constitutive equations.…”