Further application of the cleavage fracture stress model for estimating the T0 of highly embrittled ferritic steels (original) (raw)
Related papers
Journal of Nuclear Materials, 2007
Irradiation embrittlement studies rely very often on Charpy impact data, in particular the ductile-to-brittle transition temperature (DBTT). However, while the DBTT-shift is equivalent to the increase of the fracture toughness transition temperature of ferritic steels, it is not the case for ferritic/martensitic steels. The aim of this study is to critically assess experimental data obtained on a 9%Cr-ferritic/martensitic steel, Eurofer-97, to better understand the underlying mechanisms involved during the fracture process. More specifically, a dedicated analysis using the load diagram approach allows to unambiguously reveal the actual effects of irradiation on physically rather than empirically based parameters. A comparison is made between a ferritic and ferritic/martensitic steel to better identify the possible similarities and differences. Tensile, Charpy impact and fracture toughness tests data are examined in a global approach to assess the actual rather than apparent irradiation effects. The adequacy or inadequacy of the Charpy impact test to monitor irradiation effects is extensively discussed.
Fracture Toughness of Ferritic Steels in the Ductile-to-Brittle Transition Region
Fracture Mechanics - Properties, Patterns and Behaviours, 2016
Ferritic steels, as other materials, have different failure modes depending on the temperature. At elevated temperatures, they behave as ductile materials, while at low temperatures they are brittle. There is an intermediate temperature region where these alloys have a failure mode resulting from the competition between cleavage and ductile mechanisms. This region is known as the ductile-to-brittle transition zone. The characterization of fracture resistance of ferritic steels in the ductile-to-brittle transition region is problematic due to scatter in results, as well as size and temperature dependences. American Society for Testing and Materials (ASTM) has standardized the determination of a temperature reference (T 0) for the fracture toughness characterization of ferritic steels in this region. This chapter presents the evolution of the statistical treatment of fracture toughness data until the present, including some comments on T 0 determination, and some aspects that require a deeper analysis.
Effect of thermal ageing on the impact fracture behaviour of Eurofer’97 steel
Journal of Nuclear Materials, 2009
Ferritic-martensitic reduced activation steel Eurofer'97 is candidate structural material for in-vessel components of proposed fusion reactors. The use of the steel is limited up to a temperature about 550°C. On the other hand the efficiency enhancement of the fusion reactors to the level suitable for energy production is predetermined by an increase of temperature in reactor. The long-term exposition of the steel at high temperatures leads to microstructural changes. The aim of the work was to investigate the influence of thermal ageing on fracture properties of Eurofer'97 steel. Thermal ageing of the steel was simulated by step cooling treatment. Charpy impact tests were performed before and after thermal ageing. No evident changes in impact properties have been registered when comparing the properties of the steel in as-received state and in state after step cooling.
Fracture Behavior of EUROFER´97 Steel After Thermal Ageing
Ecf17 Brno 2008, 2013
Ferritic-martensitic reduced activation steel Eurofer´97 is candidate structural material for in-vessel components of proposed fusion reactors. The use of the steel is limited up to a temperature about 550 °C. On the other hand the efficiency enhancement of the fusion reactors to the level suitable for energy production is predetermined by an increase of temperature in reactor. The long term exposition of the steel at high temperatures leads to microstructural changes. The aim of the work was to investigate the influence of short and long term thermal ageing on fracture properties of Eurofer´97 steel. Short term thermal ageing of the steel was simulated by step-cooling treatment and long term thermal ageing was simulated by isothermal annealing treatment at 550 °C/5000 h. Charpy impact tests were applied to compare the fracture resistance of the sheet before and after thermal ageing. No evident changes in impact properties have been registered comparing the properties of the steel in as-received state and in state after step-cooling. Outstanding embrittlement of the thermally aged steel was observed after long term isothermal ageing.
International Journal of Fracture, 2017
Effect of loading rate on cleavage failure probability for ferritic/martensitic steels using Weibull stress analysis is studied. Calibration of Weibull slope for two grades of fusion reactor blanket steels namely, Indian Reduced Activation Ferritic/Martensitic Steel referred as In-RAFMS, F 82H and a non fusion grade modified 9Cr-1Mo steel (P91) are performed for the first time. The calibrated values of Weibull slope is used to predict the fracture behaviour of In-RAFMS at three different loading rates. The effect of loading rate is also examined on reference transition temperature using Wallin's correlation based on Zener-Hollomon strain rate parameter.
Procedia Engineering, 2014
For physical reasons fracture toughness K Jc of ferritic steels in the brittle-to-ductile transition regime is affected by a pronounced scatter, which requires statistical methods to be applied for evaluation of test results as well as for application in safety analysis of structures. For this purposes the probabilistic Master-Curve-approach according to ASTM E1921 is often used. However, for engineering purposes like a screening safety analysis of a defect-containing component it is usually preferable to use a deterministic lower bound. However, within the framework of ASTM-standards there is no possibility to determine K Ic experimentally. So K Ic needs to be determined indirectly from the reference temperature T 0. In the present paper it is shown how lower bounds of fracture toughness-either plane strain K Ic for large components or thickness-dependent K Jc for smaller onescan be derived from T 0 , and how the latter can be determine with sufficient accuracy from one or a few K Jc values.
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2023
A major drawback in ferritic steels production and usage is their mechanical brittleness at temperatures close to ambient temperature. Precipitation and grain size appear as two major parameters in such cleavage brittle behavior. This is why six model microstructures have been elaborated from the same base of chemical composition, but with different elements additions and thermal treatments. The base composition is 18% chromium and 2% molybdenum to ensure an entirely ferritic matrix at any temperature even with 0.015% of both carbon and nitrogen. The addition of titanium or niobium changed the nature, size and location of the carbides and nitrides, while carefully chosen heat treatments varied the size of the grains. Microstructure characterizations down to very fine scales (TEM, SANS) combined with thermodynamics and diffusion modeling allowed analyzing precipitates formation as well as remaining interstitial elements (carbon and nitrogen) in solution in the ferritic matrix. This multi-scale analysis of the microstructures is important to understand the mechanical behavior of the alloys, which will be presented in a companion paper.
Competing fracture mechanisms in the brittle-to-ductile transition region of ferritic steels
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2004
Theoretical models are used to investigate the propagation of fracture in the transition region of ferritic steels. Four mechanisms of fracture are allowed: transgranular brittle and ductile and grain boundary brittle and ductile. As fracture propagates decisions are made at each stage about which of these mechanisms will be operative in the next grain or grain boundary. These decisions are based on the relative energies of the different mechanisms, which are functions of temperature, and the orientation of the stress axis. The simulations, which are two-dimensional, enable the proportions of the four mechanisms to be deduced and hence the overall energy of the fracture surface to be determined. The most striking feature of the results is that there is a much greater scatter of mechanisms and of energies than is found in corresponding low temperature and high temperature simulations. This is consistent with experimental results obtained on ferritic steels.
Microstructural Effects on Fracture Behavior of Ferritic and Martensitic Structural Steels
2014
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