Nature of Microcracks in Ferritic Steels Occurred during Fracture under Conditions of Ductile-Brittle Transition Temperature Region (original) (raw)
Related papers
Brittle-ductile transition and scatter in fracture toughness of ferritic steels
2005
Here we analyze a dislocation simulation model for the brittle-to-ductile transition of Ferritic Steels, a typical multiphase material. The crack tip plastic zones are simulated as arrays of discrete dislocations emitted from crack-tip sources and equilibrated against the friction stress due to lattice and obstacles. The crack-tip gets blunted and the emitted dislocation arrays modify the elastic field of the crack. The combined stress field of the crack and the emitted dislocations describe an elasto-plastic crack field. The simulated crack system involves microcracks embedded in the plastic zone of a macrocrack. The inherent scatter in fracture toughness measurements are studied by using a size distribution for microcracks, distributed on the plane of the macrocrack. The scatter in fracture toughness measurements is found to be an effect of the size distribution of microcracks rather than their spatial distribution on the matrix ahead of the crack plane.
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
Your article is protected by copyright and all rights are held exclusively by ASM International. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to selfarchive your work, please use the accepted author's version for posting to your own website or your institution's repository. You may further deposit the accepted author's version on a funder's repository at a funder's request, provided it is not made publicly available until 12 months after publication.
The effect of grain size on crack propagation in a ferritic steel
Ecf19, 2013
Polycrystalline ferritic steels show ductile fracture at higher temperatures and brittle fracture at relatively lower temperatures. The brittle fracture can be either transgranular cleavage or intergranular. The crack initiation and propagation in a polycrystalline ferritic steel, EN1, thermo-mechanically treated to produce material with a defined combination of grain size has been studied. This treatment produced a layered microstructure comprising grains of ~20 μm in one layer and ~40 μm in the other. Small three point bend specimens with a dimension of 2.7 mm×5 mm×20 mm have been tested at a constant displacement rate at a temperature of -196°C using a Zwick tensometer equipped with an environmental chamber. The influence of grain size on crack initiation and propagation has been studied with attention given to brittle cracks propagating from the coarser to finer grain structure and vice versa. The results are discussed with particular reference to changes in fracture morphology.
Zeitschrift für Metallkunde
The aim of this study is to clarify the role of the microstruc-ture in damage evolution. The influence of the transformation induced plasticity effect on the crack initiation and the impact of the different phases in development of cracks are important factors that control the fracture mechanisms. The fracture mechanisms of multiphase steels have been investigated on the basis of extensive light optical microscopy and scanning electron microscopy investigations. On the micro-scale, two failure modes appear: cleavage and ductile fracture, depending on the stress – strain conditions, the internal cleanness, the volume fraction of the retained auste-nite and the position of the austenite and the martensite grains. If a martensite or an austenite grain fails inside a bai-nitic island, the crack develops rapidly leading to cleavage fracture. If the failure starts inside the ferritic matrix due to void initiation at hard phases, the emerged void causes duc-tile damage to the surrounding f...
Micromechanics of cleavage fracture and the associated tongue formation in ferritic steel
Materials Science and Engineering: A, 2023
The present work investigates the micro-mechanisms of cleavage fracture in a low carbon ferritic steel, subjected to Charpy impact test at low temperature. EBSD crack path analysis reveals {100} crystallographic plane as the predominant cleavage plane in ferrite, with occasional cracking along the secondary {110} cleavage plane. {110} cleavage cracking is particularly feasible when the {100} planes within a given grain exhibit significant twist angles with the preceding crack plane. Moreover, the fracture surface also exhibited numerous "tongue" like features that are intermittently aligned parallel to a common <110> crack front, as evidenced from the point EBSD analysis directly on the fracture surface. To illuminate the origin of such intermittent tongues, we performed molecular dynamics fracture simulation on a pure Fe single crystal, considering a (001) edge crack with a large [110] crack front. It was found that the development of intermittent tongues mainly originates from the intermittent nucleation of symmetric {112}<111> type twins across the entire crack front. Such intermittent twin nucleation results in a sinusoidal crack front, characterized by twin boundary cracking in one region and radial expansion of cleavage cracking in the adjacent regions, which can ultimately lead to the formation of tongues on the fracture surface. Finally, based on the simulation results and experimental observations, we propose a step by step mechanism that potentially explains the formation of intermittent tongues when a sufficiently large <110> straight crack front propagates along a {001} cleavage crack plane.
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.
Dislocation simulation of brittle-ductile transition in ferritic steels
Metallurgical and Materials Transactions A, 2006
Two dimensional discrete dislocation simulations of crack-tip plasticity of a macrocrackmicrocrack system representing the fracture behavior in Ferritic Steels are presented. The crack tip plastic zones are represented as arrays of discrete dislocations emitted from cracktip sources and equilibrated against the lattice friction. The dislocation arrays modify the elastic field of the crack and the resulting field describes the elasto-plastic crack field. The simulated crack system involves a microcrack in the plastic zone of the macrocrack (elastoplastic stress field). Effects of crack-tip blunting of the macrocrack is included in the simulations; as dislocations are emitted the microcrack is kept at a constant distance from the blunted tip of the macrocrack. The brittle-ductile transition curve is obtained by simulating the fracture toughness at various temperatures. Considering the effects of blunting is found to be critical in predicting the sharp upturn of the brittle-ductile transition curve. The obtained results are compared with existing experimental data and are found to be in reasonable agreement. .
A study on fracture properties of multiphase microstructures of a CrMo steel
Materials Science and Engineering: A, 2008
An investigation into the fracture properties of tempered martensite (TM) and ferrite-bainite-martensite (FBM) microstructures was carried out for a CrMo steel, with a particular focus on the role of interphase FBM microstructure boundaries. The fractographic investigations showed the fracture mechanism of transgranular cleavage for the FBM microstructures. The obvious presence of bright facets on broken sample surfaces was indicative of the effect of inter-phase boundaries on the cleavage fracture. The appearing of bright facet surfaces in FBM microstructures was attributed to the high concentration of ferrite/bainite and ferrite/martensite inter-phase boundaries.