Papers by Keyword: Widmanstätten Ferrite

Abstract: Under loading above the Ae₃ temperature, austenite transforms displacively into Widmanstätten ferrite. Here the driving force for transformation is the net softening during the phase change while the obstacle consists of the free energy difference between austenite and ferrite as well as the work of shear accommodation and dilatation during the transformation. Once the driving force is higher than the obstacle, phase transformation occurs. This phenomenon was explored here by means of the optical and electron microscopy of a C-Mn steel deformed above their transformation temperatures. Strain-temperature-transformation (STT) curves are presented that accurately quantify the amount of dynamically formed ferrite; the kinetics of retransformation are also specified in the form of appropriate TTRT diagrams. This technique can be used to improve the models for transformation on accelerated cooling in strip and plate rolling.

Abstract: This work is devoted to formation of the Widmanstatten ferrite in the zone of complete recrystallization of the base metal during welding of low-carbon steels and cladding of hard coatings on the surface of low-carbon steels. The methods to reduce the brittleness of the ferrite in the overheated zones are proposed.

Abstract: Seven-pass strip rolling simulations were carried out on a 0.06%C and a 0.09%C-0.036%Nb steel. The rolling loads (mean flow stresses or MFS’s) did not increase as the temperature decreased during the simulation. This is ascribed to the occurrence of dynamic transformation. The simulation results are compared to the high temperature flow curves determined on eight plain C and Nb-modified steels in both compression and torsion and at a series of temperatures and strain rates. When the associated MFS’s are plotted against inverse absolute temperature in the form of Boratto diagrams, the stress drop temperatures, normally defined as the upper critical temperature applicable to rolling, A_r3*, are shown to be about 40 degrees above the paraequilibrium and about 20-30 degrees above the orthoequilibrium A_e3’s. These drops are ascribed to the dynamic transformation of austenite to ferrite, a softer phase. The characteristics of the ferrite produced dynamically are described and the transformation is shown to be displacive in nature, leading to the appearance of fine Widmanstätten plates. These plates coalesce into polygonal grains on further deformation and on holding.

Abstract: The solid phase transformation of a metastable phase into a stable phase needs the activation energy. The energy is usually supplied in the form of thermal energy. When the nucleation takes place, the strain energy may develop in the metastable matrix and the stable nucleus. The strain energy can result from differences in density of the nucleus and matrix and the lattice mismatch between the nucleus and matrix. The stable-metastable interface region has the highest strain-energy density in the maximum Youngs modulus direction of the stable phase. Accordingly, the growth rate of the stable phase is the highest in its highest Youngs modulus directions. As the transformation temperature decreases, the strain energy contribution increases and the growth rate anisotropy is likely to increase. When austenite transforms into ferrite at low temperatures, the directed growth of ferrite is observed as forms of Widmanstätten ferrite plates and acicular ferrite plates. The maximum growth direction of ferrite is along the maximum Youngs modulus direction of ferrite, <111>_α, and the broad interfaces are parallel to the maximum growth direction and formed so that they minimizes the shear strain energy in the interface layer. The directed growth results in the Kurdjumov-Sachs orientation relationship between austenite and ferrite, <111>_α//<110>_γ and {110} _α //{111}_γ.

Abstract: The integrated simulation model for microstructural design of Fe-C alloy using the phase-field method and the homogenization method is proposed. First, the phase-field simulation is performed to simulate the morphological change of the grain boundary ferrite to Widmanstätten ferrite. Then, in order to clarify the effects of the morphology of the ferrite phase on the micro- and macroscopic mechanical properties, the finite element analysis based on the homogenization method is conducted with the representative volume element obtained from the phase-field simulation. This numerical approach provides a powerful tool to investigate systematically the micro and macroscopic mechanical behavior with the morphological change of the ferrite phase in the Fe-C alloy.

Abstract: This paper addresses the physical mechanisms of the transformation of deformed austenite into allotriomorphic ferrite and Widmanstätten ferrite. The possible implementation of deformation in currently available transformation models for allotriomorphic ferrite and Widmanstätten ferrite is discussed based on a series of deformation dilatometry experiments. It is concluded that a small amount of deformation already gives significantly faster transformation kinetics and a significant decrease in fraction Widmanstätten ferrite, and that the key to understanding the effect of deformation on transformation lies in the nucleation of allotriomorphic and Widmanstätten ferrite. For Widmanstätten ferrite also the growth needs further study.