Effect of surface preparation on the corrosion of austenitic stainless steel 304L in high temperature steam and simulated PWR primary water

The corrosion behavior of 304L grade stainless steel (SS) in high-temperature steam and in a simulated Pressurized Water Reactor (PWR) is studied. The goal was to characterize the nature of the oxide coating generated during 500 h exposure of samples in a 400 °C steam (200 bar) or a 340 °C simulated PWR. Accelerating the effect of the steam environment as well as the influence of surface preparation have been studied. Two initial sample surfaces were used: mechanical polishing and finishing grinding. Oxide coatings were investigated using TEM imaging coupled with EELS spectroscopy and R – SIMS (Secondary Ion Mass Spectroscopy)

Comment on "Effect of carbide distribution on the fracture toughness in the transition temperature region of an SA 508 steel"

International audienceCritical cleavage stress values and carbide sizes obtained in a recent work by Lee et al. [Acta Mater. 50 (2002) 4755] are recalculated using 2D and 3D finite element simulations and new constitutive law, instead of small scale yielding assumption and likely improper choice for the behavior of SA 508 steel. A new model based on the weakest link concept with the determined carbide size distribution (CSD) is succesfully applied to predict KJc measurements obtained on SA 508 steel and on a similar material (22NiMoCr3-7)

Micropillar compression study of Fe-irradiated 304L steel

Stainless steel used in nuclear reactors are experiencing heavy neutron irradiation that modify their microstructure, and therefore their mechanical properties. To assess the irradiation-induced hardening and the modification of deformation modes at the grain scale on 304L steels, indentations [1] and in situ microcompression tests were conducted on Fe-ions irradiated and non-irradiated FIB-made pillars [2]. 10 MeV and up to 8 dpa Fe irradiations were conducted at 450°C to surrogate neutron irradiation. Size effect was detected on unirradiated but not on irradiated pillars, revealing a strong impact of the microstructure on the mechanical behavior. Surprisingly, smoother plastic deformation took place in irradiated pillars while localized shear bands were observed in unirradiated ones. TEM investigations helped elaborating some hypothesis for this different behavior. Please click Additional Files below to see the full abstract

Local approach to fracture based prediction of the ΔT56J and ΔTKIc 100 shifts due to irradiation for an A508 pressure vessel steel

International audienceNuclear pressure vessel steels are subjected to irradiation embrittlement which is monitored using Charpy tests. Reference index temperatures, such as the temperature for which the mean Charpy rupture energy is equal to 56 J (T56J), are used as embrittlement indicators. The safety integrity evaluation is performed assuming that the shift of the nil-ductility reference temperature RTNDT due to irradiation is equal to the shift of T56J. A material model integrating a description of viscoplasticity, ductile damage and cleavage brittle fracture is used to simulate both the Charpy test and the fracture toughness test (CT geometry). The model is calibrated on the Charpy data obtained on an unirradiated A508 Cl.3 steel. It is then applied to irradiated materials assuming that irradiation affects solely hardening. Comparison with Charpy energy data for different amounts of irradiation shows that irradiation possibly also affects brittle fracture. The model is then applied to predict the fracture toughness shifts (ΔTKIc,100) for different levels of irradiation

Ductile to brittle transition of an A508 steel characterized by Charpy impact test, part I., experimental results

International audienceThis 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 coalescence 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 mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile–brittle transition range using fully coupled local approach to fracture is presented

Ductile to brittle transition of an A508 steel characterized by Charpy impact test, part II., Modeling of the Charpy transition curve

International audienceA finite element simulation of the Charpy test is developed in order to model the ductile to brittle transition curve of a pressure vessel steel. The material (an A508 steel) and the experimental results are presented in a companion paper (Part I [Engng. Fract. Mech.]). The proposed simulation includes a detailed description of the material viscoplastic behavior over a wide temperature range. Ductile behavior is modeled using modified Rousselier model. The Beremin model is used to describe brittle fracture. The Charpy test is simulated using a full 3D mesh and accounting for adiabatic heating and contact between the specimen, the striker and the anvil. The developed model is well suited to represent ductile tearing. Using brittle failure parameters identified below −150 °C, it is possible to represent the transition curve up to −80 °C assuming that the Beremin stress parameter σu is independent of temperature. Above this temperature, a temperature dependent Beremin stress parameter, σu, must be used to correctly simulate the transition curve. Quasi-static and dynamic tests can then be consistently modeled

Local approach to fracture based prediction of the ∆T56J and ∆T K1C100 shifts due to irradiation for an A508 pressure vessel steel

International audienceA material model integrating a description of viscoplasticity, ductile damage and brittle fracture is used to simulate both the impact (Charpy) test and the toughness (CT) fracture test. The model is calibrated on the Charpy data obtained on an unirradiated A508 Cl.3 steel. It is then applied to irradiated material assuming that irradiation affects solely hardening. Comparison with Charpy energy data for different amounts of irradiation shows that irradiation probably also affects brittle fracture. The model is then used to predict the DTKIc100 shifts for different levels of irradiation

Micro-mechanical approach of the intergranular stress corrosion cracking of austenitic stainless steels in PWR environment

Austenitic stainless steels are used in the nuclear industry to make the internals parts of Pressurized Water Reactors (PWR) such as baffle and former plates. Numerous Baffle-to-Former Bolts (BFB) intergranular failures have been reported as a result of Irradiation Assisted Stress Corrosion Cracking (IASCC) phenomenon. In order to predict the cracking of the grain boundary through a micro-mechanical approach, it is necessary to determine the intragranular mechanical behavior of the steel and the grain boundary strength. Please click Download on the upper right corner to see the full abstract

Cleavage fracture micromechanisms related to WPS effect in RPV steel

International audienceThe main objective of this paper is to characterise the changes caused by warm pre-stress (WPS) on the cleavage fracture micromechanisms of a 18MND5 (A533B) reactor pressure vessel (RPV) steel. In this purpose, different WPS fracture test results obtained on compact tensile (CT) and notched tensile (NT) geometries are presented and compared with isothermal test data. While confirming some well-established features of WPS, these experimental results, analysed through extensive fractographic investigations and finite element (FE) calculations, demonstrate a strong material aspect to WPS. One unveiled characteristic of the WPS effect is the deactivation of particles at high temperature through plastic straining under low stress, which enables breaking, or detaching from the matrix, the most critical particles without causing unstable cleavage propagation. In 18MND5 steel, these particles are TiN and carbide particles: according to the pre-stress level and fracture geometry, there is a progressive shift from TiN- to carbide-induced cleavage, up to the replacement of both by ductile type nucleation mechanisms