Mechanisms of Fracture in Neutron-Irradiated 15Ch2MFA Steel

The influence of irradiation on fracture properties of 15Ch2MFA pressure vessel steel is studied. The distribution of inclusions and carbides is characterized. The quantitative fractography of broken Charpy specimens is carried out. The correlation of crack initiation energy and the area of ductile fracture adjacent to the notch is identified. It is shown that most of the absorbed energy belongs to the stage preceding the cleavage crack initiation. From the fracture surface of Charpy specimens the distribution of ductile dimples is investigated. The relationship between the distributions of ductile dimples and second phase particles is discussed.Изучено влияние облучения на разрушение корпусной стали 15Х2МФА. Показано рас­пределение включений и карбидов. Выпол­нен анализ образцов, разрушенных по мето­ду Шарпи, с помощью количественной фрактографии. Выявлена корреляция между энергией зарождения трещины и зоной вяз­кого разрушения вблизи надреза. Установ­лено, что большая часть энергии поглоща­ется на стадии, предшествующей возникно­вению трещины скола. Исследовано распре­деление ямок вязкого разрушения на поверхности образцов Шарпи. Оценена зави­симость между распределением ямок и частицами вторичных фаз

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses ofhigh-strength steels (HSS) sheets used in the car industry are inapplicablefor standardized testingprocedures. The aim ofthis study is topropose an appropriate methodologyfor testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests andfracture toughness tests were used in order to compare thefracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J —Δα) curves were measured. From thefracture toughness linked to the specimen thicknesses the value of fracture toughness KIc was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to —100° C).Толщины листовой высокопрочной стали, применяемой в автомобильной промышлен­ности, не соответствуют требованиям стан­дартных методов испытаний. Цель настоя­щего исследования - предложить приемле­мую методологию испытаний и сравненить между собой тонколистовые высокопрочные сталей. Микроструктуру изучали с помо­щью оптической и сканирующей электронной микроскопии. Для сравнения особенностей разрушения трех различных сталей (Docol 1200, M. Multiphase 1200 и BTR165) использовали модифицированный метод ударных испытаний по Шарпи и метод оценки вязкости разрушения. Были получены кривые перехода из вязкого состояния в хрупкое и кривые сопро­тивления разрыву (J —Δα). По вязкости раз­рушения, зависящей от толщины образцов, оценивали величину KIc . Фрактографический анализ разрушенных образцов пока­ зал, что вследствие тонкой микроструктуры смешанного феррита-мартенсита сохраняет­ся вязкий механизм разрушения даже при низких температурах (до —100° C)

Mechanical properties and fracture behavior of high-strength steels

Typical thicknesses ofhigh-strength steels (HSS) sheets used in the car industry are inapplicablefor standardized testingprocedures. The aim ofthis study is topropose an appropriate methodologyfor testing and comparing of thin HSS sheets. Microstructures were observed by means of light and scanning electron microscopy. The modified Charpy impact tests andfracture toughness tests were used in order to compare thefracture properties of three different HSS sheets (Docol 1200 M, Multiphase 1200 and BTR 165). Ductile-to-brittle transition curves and tearing resistance (J —Δα) curves were measured. From thefracture toughness linked to the specimen thicknesses the value of fracture toughness KIc was estimated. Fractographic analysis of broken specimens has revealed that due to the fine microstructure of mixed ferrite-martensite fracture mechanism remains ductile even at low temperatures (down to —100° C).Толщины листовой высокопрочной стали, применяемой в автомобильной промышлен­ности, не соответствуют требованиям стан­дартных методов испытаний. Цель настоя­щего исследования - предложить приемле­мую методологию испытаний и сравненить между собой тонколистовые высокопрочные сталей. Микроструктуру изучали с помо­щью оптической и сканирующей электронной микроскопии. Для сравнения особенностей разрушения трех различных сталей (Docol 1200, M. Multiphase 1200 и BTR165) использовали модифицированный метод ударных испытаний по Шарпи и метод оценки вязкости разрушения. Были получены кривые перехода из вязкого состояния в хрупкое и кривые сопро­тивления разрыву (J —Δα). По вязкости раз­рушения, зависящей от толщины образцов, оценивали величину KIc . Фрактографический анализ разрушенных образцов пока­ зал, что вследствие тонкой микроструктуры смешанного феррита-мартенсита сохраняет­ся вязкий механизм разрушения даже при низких температурах (до —100° C)

Influence des inclusions sur la rupture d'un acier faiblement allié

L'étude de la rupture d'un acier faiblement allié dans le domaine de la transition ductile fragile a permis de mettre en évidence la présence croissante, avec la température de sollicitation, d'amas d'inclusions de seconde phase sur les surfaces de rupture. On montre, à l'aide de modélisations par éléments finis, que ces amas jouent néanmoins un rôle mineur dans le déclenchement du clivage. En revanche, leur influence sur la propagation de la rupture ductile est importante. On peut alors expliquer l'anisotropie de la résilience en prenant en compte la géométrie des inclusions et leur répartition spatiale

Stress Corrosion Cracking of Austenitic Stainless Steels in Simulated Nuclear Reactor Conditions

Stress corrosion cracking (SCC) susceptibility of selected austenitic stainless steels was investigated within the SAFELIFE and HPLWR-2 projects. The aim was to identify and describe the specific failure mechanisms in simulated reactor conditions, i.e., a combination of slow strain rate and high temperature in different environments. Slow J-R and standard SCC tests of AISI 304L stainless steel with different degree of cold working, were carried out under BWR condition. Slow strain-rate tensile (SSRT) tests of 316L stainless steel were carried out in ultra-pure demineralized supercritical water (SCW) solution. Tested specimens were subjected to fractographic analysis in order to characterize the failure mechanisms. The specimens failed due to a combination of transgranular SCC and transgranular ductile fracture. The ratio of SCC and ductile fracture was affected by the parameters of tests, in particular by temperature, pressure, oxygen content and strain rateJRC.DDG.F.4-Safety of future nuclear reactor

Development of Crack Growth Rate SCC Devices for Testing in Super Critical Water (SCW) Environment

A detailed investigation of detrimental processes such as corrosion and stress corrosion cracking (SCC), ageing embrittlement, creep and fatigue etc., which may significantly affect the materials performance in specific water chemistry environments, is of fundamental importance for the development of the Supercritical Water (SCW) reactor concept. In particular, SCC crack growth rate testing is difficult to perform due to complex requirements for the experimental facilities which include water chemistry loops equipped with relevant pH, conductivity, dissolved O2 and H2 sensors, autoclaves with mechanical loading devices, incorporating crack growth rate measurement by direct current potential drop (DCPD) and displacement measurement by linear variable differential transducers (LVDT). All these requirements significantly advance the total materials qualification costs. JRC in cooperation with VTT has been working on a new type of loading devices which are expected to decrease these costs and at the same time guarantee enough reliability and flexibility for both SCC and future irradiation assisted stress corrosion cracking (IASCC) testing to be performed in SCW environments. This paper summarizes the first results of SCC crack growth rate measurements in SCW by using a pneumatic bellows based loading device developed in the frame of JRC-VTT cooperation. Furthermore, the results of crack growth rates in SCW (550oC, 8000 ppb of dosed oxygen) are compared to those measured by using the same loading device in 288oC subcritical water. Finally, the development of another miniature size autoclave concept together with preliminary results obtained from it is described in detail.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

Microstructure and mechanical properties of hot rolled Fe-40 at-%Al intermetallic alloys with Zr and B addition

The microstructure and mechanical properties of hot rolled Fe-40 at-%Al based intermetallic alloys, with 0.1 at-%Zr and different additions of B (varying from 0.01 to 0.1 at-%), are characterised. The additions of Zr and B improve tensile properties at room and elevated temperatures. Increasing B content is also associated with a number of other effects. First, the fracture mode changes from intergranular decohesion to cleavage, which correlates with significant increases in the fracture toughness. Second, there is a certain stabilisation of dislocations arranged in parallel systems of slip bands, as shown by transmission electron microscopy. Numerous complex stacking faults on {100} planes are also observed in the alloy with the highest B content. Third, B is found to modify the formation of second phase particles; such particles (coarse and fine) are analysed by energy dispersive X-ray spectroscopy and electron energy loss spectrometry to obtain compositional information

Pre-qualification of cladding materials for SCWR fuel qualification testing facility

In frame of the project “HPLWR – Phase 2” (Sept. 2006 to Feb. 2010), prospective cladding materials have been tested up to 650°C and stainless steels, such as 1.4970 which had already been used successfully in sodium cooled fast reactors, turned out to be among the most promising candidates. The main target of the follow-up FP7 project “Fuel Qualification test for SCWR” is to make significant progress towards the design, analysis and licensing of a fuel assembly cooled with supercritical water in a research reactor. The program of dedicated WP4-pre-qualification was focused on evaluation of general corrosion resistance of three austenitic stainless steels 08Cr18Ni10Ti (equivalent of 321), 347H and 316L, which should be pre-qualified for application as a cladding material for fuel qualification tests in SCW conditions. Therefore, the experiments in support of WP4 concentrated on 2000 h corrosion exposures in 25 MPa SCW at two different temperatures 550 and 500oC dosed with 2000 ppb of dissolved oxygen. Coupons manufactured according to ASTM G1-03 with milled surface finish were exposed for 600, 1400 and 2000 h in JRC IET autoclave connected to recirculation loop allowing continual water chemistry control during the test. Following examination of the exposed specimens consisted of weight change calculations and detailed macro and microscopic investigation of oxide layers using SEM, EDX. Furthermore, linear extrapolation to one year of operation in SCW was performed as the most conservative approach. With respect to general corrosion results, all tested steels showed sufficient corrosion resistance in SCW conditions taking into account the conditions foreseen for future fuel qualification test in the research reactor in Rez. When comparing the results of weight change calculations obtained for all three materials, it was found out, that the corrosion resistance increased in the following order: 316L<347 H<08Cr18Ni10Ti.JRC.F.4-Nuclear Reactor Integrity Assessment and Knowledge Managemen

EBSD Study of Cleavage Crack Initiation.

Available from STL Prague, CZ / NTK - National Technical LibrarySIGLECZCzech Republi