Fractography of sintered steels (original) (raw)
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Fractography of sintered iron and steels
The processing technology of sintered steels cannot avoid typical microstructural discontinuities such as pores and their agglomerates and prior particle, interface and interphase boundaries, which all influence initiation, growth, and propagation of cracks when the sintered microstructure is mechanically loaded. Fracture paths and fracture resistance are shown to be related to details of the complex, frequently inhomogeneous, microstructures comprising, ferrite, austenite, upper and lower bainite, martensite, pores and weak interfaces. All these have characteristic fracture resistance properties resulting in, frequently combinations of, dimple rupture, cleavage, intergranular and interparticle failure micromechanisms. Keywords: fracture in metals, fractography of sintered iron and steels INTRODUCTION Metals fail in many different ways and for different reasons. The main purpose of microfractography is to determine how and why a component fails -it means to determine a mode of failu...
Understanding contribution of microstructure to fracture behaviour of sintered steels
Powder Metallurgy, 2014
Microstructural features of sintered steels, which comprise both phases and porosity, strongly condition the mechanical behaviour of the material under service conditions. Many research activities have dealt with this relationship since better understanding of the microstructure-property correlation is the key of improvement of current powder metallurgy (PM) steels. Up to now, fractographic investigation after testing has been successfully applied for this purpose and, more recently, the in situ analysis of crack evolution through the microstructure as well as some advanced computer assisted tools. However, there is still a lack of information about local mechanical behaviour and strain distributions at the microscale in relation to the local microstructure of these steels, i.e. which phases in heterogeneous PM microstructures contribute to localisation of plastic deformation or which phases can impede crack propagation during loading. In the present work, these questions are addressed through the combination of three techniques: (i) in situ tensile testing (performed in the SEM) to monitor crack initiation and propagation; (ii) digital image correlation technique to trace the progress of local strain distributions during loading; (iii) fractographic examination of the loaded samples. Three PM steels, all obtained from commercially available powders but presenting different microstructures, are examined: a ferriticpearlitic Fe-C steel, a bainitic prealloyed Fe-Mo-C steel and a diffusion alloyed Fe-Ni-Cu-Mo-C steel, with more heterogeneous microstructure (ferrite, pearlite, upper and lower bainite, martensite and Ni rich austenite).
STATIC AND DYNAMIC FRACTURE MICROMECHANICS OF SINTERED STEELS: A REVIEW
In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are nucleated, grow and coalesce, before attaining catastrophic size (Stage III in fatigue) for which fracture mechanics holds. The mechanisms are step-wise, thus Paris type analysis, applied e.g. to artificial cracks, does not apply. These processes were studied by combining fractography with surface replication of the most highly stressed region of specimens undergoing three-point bending, as progressively the tensile stress or the number of fatigue cycles was increased. Typically 10-20 microcracks were nucleated, some were arrested, others grew and coalesced until the final coalescence resulted in the catastrophic crack. Assuming semi-elliptical shape for the surface microcracks, the local stress intensity factor, K a was calculated for each microcrack using Irwin's formula. Initial values of K a were 1-4 MPa.m 1/2 and the eventual values correspond well to K 1C , independently determined.
Microstructure and mechanical behavior of porous sintered steels
Materials Science and Engineering: A, 2005
The microstructure and mechanical properties of sintered Fe-0.85Mo-Ni steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young's modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young's modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure-based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels are discussed.
FATIGUE CRACK GROWTH OF SINTERED STEELS WITH A HETEROGENEOUS MICROSTRUCTURE
Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm3, 7.36 g/cm3, and 7.53 g/cm3. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from -2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆Kth. In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, contributed to toughening and crack deflection. These findings are supported by quantita...
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...
Powder Metallurgy, 2019
Synchrotron X-ray laminography was used to examine the time-dependent evolution of the three-dimensional (3D) morphology of micropores in sintered iron during the tensile test. 3D snapshots showed that the networked open pores grow wider than 20 µm along the tensile direction, resulting in the internal necking of the specimen. Subsequently, these pores initiated the cracks perpendicular to the tensile direction by coalescing with the surrounding pre-existing microvoids or with the secondary-generated voids immediately before fracture. Topological analysis of the barycentric positions of these microvoids showed that they form the two-dimensional networks within the ∼20 µm of radius area. These observations strongly indicate that the microvoid coalescence could occur on shear planes formed close to the enlarged open pores or between closed pores by strain accumulation and play an important role in the crack initiation.
Microstructural Effects on Fracture Behavior of Ferritic and Martensitic Structural Steels
2014
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