Microstructural Features Controlling Mechanical Properties in Nb-Mo Microalloyed Steels. Part II: Impact Toughness
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current demand for ultrahigh strength steels with high toughness at low temperatures covers different structural applications, such as the oil and gas industry and heavy equipment for naval, construction, and automotive applications.[1,2] Traditional thermomechanical-controlled processing (TMCP) of microalloyed steels is employed to refine the microstructure and produce multi-phase microstructures, which provides good combinations of high strength and low temperature impact toughness. Suitable processing parameters in conjunction with microalloying help improve strength through various mechanisms: microstructural refinement, solid solution hardening, precipitation strengthening, and dislocation hardening due to the modification of the resulting microstructure.[3] However, although strength levels can be increased by different combinations of the strengthening mechanisms, toughness properties may be impaired. Grain size refinement is the only mechanism that increases yield strength and reduces the impact transition temperature. Therefore, a careful design of chemical combination and process parameters is needed in order to meet strength and toughness properties simultaneously.[2,4] NEREA ISASTI and DENIS JORGE-BADIOLA, Researchers, and PELLO URANGA, Researcher and Associate Professor, are with the CEIT and TECNUN (University of Navarra), Paseo de Manuel Lardizabal 15, 20018, Donostia-San Sebastia´n, Basque Country, Spain. Contact e-mail: [email protected] MITRA L. TAHERI, Hoeganaes Assistant Professor, is with the Department of Materials Science, Drexel University, Lebow 344, 3141 Chestnut Street, Philadelphia, PA 19104. Manuscript submitted February 12, 2014. Article published online July 16, 2014 4972—VOLUME 45A, OCTOBER 2014
The correlation between toughness properties and the different microstructural aspects is an issue that has not yet been solved. Significant efforts have been dedicated to investigating fracture toughness in different microstructures.[5] Furthermore, when the final microstructure contains non-polygonal bainitic phases, the analysis of fracture toughness becomes more complex, due to the limitations related to the optical microscopy capacity.[6] The electron-backscattered diffraction (EBSD) technique allows the mentioned limitations to be overcome and opens a broad range of possibilities in terms of microstructural characterization, which helps provide a better understanding of the influence of the microstructural parameters in the obtained mechanical properties.[7,8] For example, grain boundary misorientation is essential as boundary nature controls different mechanical properties. Medium angle boundaries are considered to control strength properties by blocking dislocation movement, whereas high angle boundaries act as effective barriers to cleavage fracture.[9] Therefore, the relations that link the microstructural parameters and transition temperatures can be expanded from ferrite– pearlite microstructures[10] to more complex microstructures[11] using EBSD. Most of the semi-empirical equations reported in t
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