About: Ferrite (iron) is a(n) research topic. Over the lifetime, 20700 publication(s) have been published within this topic receiving 234474 citation(s).
01 Jan 2001-
Abstract: The mechanism of the bainite transformation in steels is reviewed, beginning with a summary of the early research and finishing with an assessment of the transformation in the context of the other reactions which occur as austenite is cooled to temperatures where it is no longer the stable phase. The review includes a detailed account of the microstructure, chemistry, and crystallography of bainitic ferrite and of the variety of carbide precipitation reactions associated with the bainite transformation. This is followed by an assessment of the thermodynamic and kinetic characteristics of the reaction and by a consideration of the reverse transformation from bainite to austenite. It is argued that there are useful mechanistic distinctions to be made between the coherent growth of ferrite initially supersaturated with carbon (bainite), coherent growth of Widmanstatten ferrite under paraequilibrium conditions, and incoherent growth of ferrite under local equilibrium or paraequilibrium conditions. The nature of the so-called acicular ferrite is also discussed.
Abstract: Steel with an ultimate tensile strength of 2500 MPa, a hardness at 600–670 HV and toughness in excess of 30–40 MPa m 1/2 is the result of exciting new developments with bainite. The simple process route involved avoids rapid cooling so that residual stresses can in principle be avoided even in large pieces. The microstructure is generated at temperatures which are so low that the diffusion of iron is inconceivable during the course of the transformation to bainite. As a result, slender plates of ferrite, just 20–40 nm thick are generated, giving rise to the extraordinary properties.
01 Jan 2011-Acta Materialia
Abstract: Three ferrite/martensite dual-phase steels varying in the ferrite grain size (12.4, 2.4 and 1.2 μm) but with the same martensite content (∼30 vol.%) were produced by large-strain warm deformation at different deformation temperatures, followed by intercritical annealing. Their mechanical properties were compared, and the response of the ultrafine-grained steel (1.2 μm) to aging at 170 °C was investigated. The deformation and fracture mechanisms were studied based on microstructure observations using scanning electron microscopy and electron backscatter diffraction. Grain refinement leads to an increase in both yield strength and tensile strength, whereas uniform elongation and total elongation are less affected. This can be partly explained by the increase in the initial strain-hardening rate. Moreover, the stress/strain partitioning characteristics between ferrite and martensite change due to grain refinement, leading to enhanced martensite plasticity and better interface cohesion. Grain refinement further promotes ductile fracture mechanisms, which is a result of the improved fracture toughness of martensite. The aging treatment leads to a strong increase in yield strength and improves the uniform and total elongation. These effects are attributed to dislocation locking due to the formation of Cottrell atmospheres and relaxation of internal stresses, as well as to the reduction in the interstitial carbon content in ferrite and tempering effects in martensite.
Abstract: An experimental silicon steel has been used in a detailed kinetic and structural study of the bainite transformation in an attempt to resolve some of the controversies concerning the reaction mechanism. Distinct reaction ‘C’ curves and transformation mechanisms were observed for the upper and lower bainite reactions. The observed set of three minima in transformation kinetics were found to be incompatible with the solute drag explanation of the kinetic Bs temperature. Transmission electron microscopy indicated the growth of both upper and lower bainite by the propagation of displacive subunits, with adjacent nucleation in the latter case. Definite evidence for carbon supersaturation was obtained for the lower bainitic ferrite. The results are best explained in terms of a shear mechanism for the ferritic component of bainite rather than a ledge mechanism (as is observed in Widmanstatten ferrite growth). Carbide precipitation events were also characterized and the evidence suggested that precipitation resulted from the aging of a supersaturated matrix in lower bainite. The evidence also suggests that carbide precipitation events are of secondary importance to the essence of bainite formation. It was further proven that the concept of a metastable equilibrium1 controlling the transition from upper to lower bainite was not applicable to the present steel and indeed, if any metastable equilibrium does exist in any other steel, it does not constitute a general phenomenon and hence is not essential to the bainite transformation mechanism.
Abstract: A model is reviewed, that describes the endpoint of carbon partitioning between supersaturated ferrite and retained austenite. A new process, quenching and partitioning (Q&P), has been developed recently to intentionally employ such partitioning in creating useful ferrous microstructures containing retained austenite. The process involves quenching austenite below the martensite-start temperature, followed by a partitioning treatment to enrich the remaining austenite with carbon, thereby stabilizing it to room temperature. Recent experimental studies have confirmed that Q&P provides a viable means to create microstructures containing carbon-enriched retained austenite, and attractive property combinations have been achieved in a variety of materials, while opportunities remain for further optimization. Furthermore, some implications of the partitioning model with respect to fundamentals of the bainite transformation are discussed, including the possibility of displacive growth under carbon diffusion control, with an austenite composition at the α/γ interface represented by the (adjusted) T0 composition. It is suggested that individual movements of iron atoms are likely during growth of Widmanstatten ferrite, and that there may be a need for further consideration of thermally activated iron-related processes in general.