Cyclic deformation mechanisms and microcracks behavior in high-strength bainitic steel (original) (raw)
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Short fatigue crack growth behaviour in ferrite-bainite dual-phase steels
2008
The influence of bainite content on the short crack growth behaviour has been examined in a series of ferrite-bainite dual phase steels containing 50.3-90.3 % bainite. Fatigue tests have been carried out using an earlier reported specimen configuration with the help of a rotating bending machine. These tests have been supplemented by characterization of the generated microstructures, determination of their hardness and tensile properties, and examinations of the nature of association of fatigue crack paths with microstructural constituents. The characteristic critical crack lengths above which linear elastic fracture mechanics may be applicable, the fatigue threshold values at the transition of short to long cracks, and the influence of microstructure on the crack path have been discussed. The maximum value of short crack fatigue threshold indicates a marginally increasing trend with increased amount of bainite. The fatigue crack path is found to be predominantly intra-granular for steels containing Ͻ70 % bainite while it is predominantly inter-granular for steels containing Ͼ70 % bainite.
Microstructural effects on the sub-critical fatigue crack growth in nano-bainite
Materials Science and Engineering: A, 2018
Bainitic transformation at low temperatures in high carbon steels leads to a dual phase nanostructure of bainitic ferrite and retained austenite. Strengths above 1 GPa can be easily achieved in these steels while also preserving significant amount of ductility. However, in order to be used in most structural applications, microstructure tailoring for high damage tolerance under fatigue loading is of utmost importance. In the current study, we have developed three distinct bainitic microstructures by austempering at 250, 300 and 350°C respectively after similar austenitization treatment. Sub-critical fatigue crack growth investigation for all the specimens has been carried out under ambient environmental conditions. It has been found that the specimen with the coarsest morphology of bainitic ferrite and retained austenite and the highest retained austenite content shows the largest crack growth threshold. Threshold has been found to be reached for the specimens when the crack tip opening displacement attains the respective bainitic lath thickness for different isothermal conditions. In Stage II, the specimens with the largest lath thickness and highest austenite content showed marginally slower crack growth rates than the other specimens. This can be attributed to transformation induced plasticity that is expected to be more severe in the specimens with the highest austenite content. The specimens transformed at the highest isothermal temperature also showed the highest plane strain fracture toughness deduced from the stress intensity value at which the crack propagates catastrophically.
Acta Metallurgica Sinica (English Letters), 2014
The aim of this work is to investigate the fatigue behavior of a new class of nanostructured, low-temperature bainitic steels, where rotating bending fatigue tests were conducted on samples isothermally transformed at 300, 250, and 200°C, after which, fracture surfaces were examined using scanning electron microscopy. Results showed that, fatigue limits of about 820, 945, and 1,005 MPa were achieved for the samples transformed at each transformation temperature, respectively. Moreover, according to the SEM micrographs, secondary crack initiation was observed in the high carbon retained austenite blocks for samples transformed at 300°C and at the interface of blocky austenite-bainitic ferrite for samples transformed at 250 and 200°C.
Influence of the crystalline orientations on microcrack initiation in low-cycle fatigue
Present study aims at analyzing the crack initiation in an austenitic stainless steel in low-cycle fatigue. A fatigue test was carried out using a polished specimen. The surface of the specimen was observed in situ during the fatigue test, in order to establish the time of slip activity or crack initiation. After a number of cycles sufficient to initiate small cracks, the test was stopped and the surface observed by scanning electron microscopy. The electron backscattered diffraction technique (EBSD) was used to identify the orientations of surface grains in the central zone of the fatigue specimen. Crack-initiation sites and slip systems associated to the initiated microcracks were identified. The criterion of the maximum Schmid factor explains two-thirds of the cracks initiated in slip systems; however if the favorably oriented slip band with respect to this criterion makes an angle of around 451 to the loading direction, a crack may initiate in another slip system.
MICROSTRUCTURAL STUDY OF THE MULTIPHASE BAINITIC STEEL THROUGH HEAVY COMPRESSION
International Journal of Modern Physics: Conference Series, 2012
High Si bainitic steel has received much interest because of combined ultra high strength, good ductility along with high wear resistance. In this study, the microstructural evolution of dual phase bainitic ferrite-austenite steel after heavy compression was investigated. Compression tests were conducted at temperature of 298K on the rectangular billets at the strain rate of 0.001s -1 . The samples were deformed to 40% and 70% of their original thickness. The EBSD results show formation of nano grains with high angle grain boundaries through 70% compression, which confirms grain refinement. Additionally, 40% deformation resulted in enhancement of the dislocation density and formation of subgrains at ferrite unites. Also, it was found during 70% compression of the steel, the austenite transforms to the martensite, which is in agreement with thermodynamic calculations.
Effects of microstructure and crystallography on crack path and intrinsic resis.PDF
The paper focuses on the effective resistance and the near-threshold growth mechanisms in the ferritic-pearlitic and the pure pearlitic steel. The influence of microstructure on the shear-mode fatigue crack growth is divided here into two factors: the crystal lattice type and the presence of different phases. Experiments were done on ferritic-pearlitic steel and pearlitic steel using three different specimens, for which the effective mode II and mode III threshold values were measured and fracture surfaces were reconstructed in three dimensions using stereophotogrammetry in scanning electron microscope. The ferritic-pearlitic and pearlitic steels showed a much different behaviour of modes II and III cracks than that of the ARMCO iron. Both the deflection angle and the mode II threshold were much higher and comparable to the austenitic steel. Mechanism of shear-mode crack behaviour in the ARMCO iron, titanium and nickel were described by the model of emission of dislocations from the crack tip under a dominant mode II loading. In other tested materials the cracks propagated under a dominance of the local mode I. In the ferritic-pearlitic and pearlitic steels, the reason for such behaviour was the presence of the secondary-phase particles (cementite lamellas), unlike in the previously austenitic steel, where the fcc structure and the low stacking fault energy were the main factors. A criterion for mode I deflection from the mode II crack-tip loading, which uses values of the effective mode I and mode II thresholds, was in agreement with fractographical observations.
Metallurgical Transactions A, 1984
Characteristics of fatigue crack propagation in dual-phase steels have been investigated in a high purity Fe-2Si-0.1C steel with the objective of developing ferritic-martensitic microstructures with maximum resistance to fatigue crack extension while maintaining high strength levels. A wide range of crack growth rates has been examined, from ~ 10 -8 to 10 -3 mm per cycle, in a series of duplex microstructures of comparable yield strength and prior austenite grain size where intercritical heat treatments were used to vary the proportion, morphology, and distribution of the ferrite and martensite phases. Results of fatigue crack propagation tests, conducted on "long cracks" in room temperature moist air environments, revealed a very large influence of microstructure over the entire spectrum of growth rates at low load ratios. Similar trends were observed at high load ratio, although the extent of the microstructural effects on crack growth behavior was significantly less marked. Specifically, microstructures containing fine globular or coarse martensite in a coarse-grained ferritic matrix demonstrated exceptionally high resistance to crack growth without loss in strength properties. To our knowledge, these microstructures yielded the highest ambient temperature fatigue threshold stress intensity range AK0 values reported to date, and certainly the highest combination of strength and AK0 for steels (i.e., AKo values above 19 MPaV~m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a tortuous morphology of crack path which results in a reduction in the crack driving force from crack deflection and roughness-induced crack closure mechanisms. Quantitative metallography and experimental crack closure measurements, applied to currently available analytical models for the deflection and closure processes, are presented to substantiate such interpretations.
Experimental characterization of short fatigue crack kinetics in an austeno-ferritic duplex steel
Procedia Engineering, 2011
The present work reports the damage evolution during low (LCF) and high (HCF) cycle fatigue behavior in an embrittled duplex stainless steel type DIN 1.4462. For LCF, the results have shown microcracks nucleating mainly along the most favorable oriented slip planes regarding the Schmid factor in the ferrite and propagating along similar planes. Occasionally, they nucleate at -grain boundaries. For these cracks, phase boundaries seem to be an effective barrier against the propagation in contrast to grain boundaries. During HCF, cracks initiate at -grain or at -phase boundaries and propagate in an intercrystalline mode and propagate in the neighbor grain. LCF and HCF characteristics have been related to the associated dislocation structure and with the local crystallography obtained by EBSD analysis in order to understand the propagation behavior.