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Non-Equilibrium Diagrams and Microconstituents
Published in Joseph Datsko, Materials Selection for Design and Manufacturing, 2020
The isothermal transformation diagram for any steel can be used to predict the microstructures that will most likely result from a specified thermal cycle or conversely to determine what cooling rate is necessary to obtain a desired microstructure by modifying the IT diagram to a CT (continuous transformation) diagram. This modification is as simple to make as that required to change an equilibrium phase diagram to an equilibrium microconstituent diagram in that the change usually can be visualized without actually constructing any additional lines.
Modern Bainitic Steels
Published in H. K. D. H. Bhadeshia, Bainite in Steels, 2015
A particular form of cast irons is where the matrix of spheroidal graphite cast iron is not pearlite, but bainite (Fig. 13.67). This results in a major improvement in toughness and strength. The bainite is obtained by isothermal transformation of the austenite at temperatures below that at which pearlite forms.19
The Alloys of Iron, Their Physical Nature, and Behavior During Deformation
Published in William L. Roberts, Cold Rolling of Steel, 2017
As indicated by the isothermal transformation diagram shown in Figure 8-48, both pearlite and bainite form isothermally from austenite, pearlite being the decomposition product at temperatures between A1 and the “nose” of the diagram and bainite the product at lower temperatures.
A novel observation of Widmanstätten ferrite and pearlite by thermo-mechanical processing of interstitial-free steel
Published in Materials and Manufacturing Processes, 2022
The formation of acicular ferrite is rejected in the present case. The logic is that the nucleation of acicular ferrite prefers inclusion sites.[13] The synthesis of IF steel by a vacuum degassing technique rules out that one by minimizing the impurity-addition. The SEM images are given in Fig. 3a,b for higher magnification. It shows a lamellar type of structure corresponding to pearlite by the isothermal transformation. The observation is a surprise since the pearlite-start and -finish lines in the CCT diagram are available at a longer duration by the incubation time (Fig. 2c). Due to this unfavorable condition, the lamellar structure is underdeveloped with a finer size (compare Fig. 3a with b). The colonial growth with different orientations is hard to be observed. Usually, pearlitic from austenite appears at higher temperatures.[20] The preoccupation of diffusional ferrite over there pushes pearlite down to the bainitic transformation temperature (Fig. 2c). Thus, the lamellar growth gets hampered to land up with fragmented pearlite (Fig. 3a,b). The following section displays all the phases in a single frame with the EBSD, owing to different structure sizes unattainable by the SEM.
Predicting the cooperative effect of Mn–Si and Mn–Mo on the incomplete bainite formation in quaternary Fe–C alloys
Published in Philosophical Magazine Letters, 2018
Hussein Farahani, Wei Xu, Sybrand van der Zwaag
Recently, the Gibbs energy balance (GEB) model has been successfully introduced to predict the effect of alloying elements on the degree of the ICT in isothermal bainite transformation more accurately [11–13]. The GEB model, closest related to the diffusional theory, focusses on the solute drag effect of substitutional alloying elements at the migrating austenite/bainitic ferrite interface [14]. In this model, the velocity of migrating austenite/bainitic ferrite interface is calculated by matching the chemical driving force of the isothermal transformation as a function of the degree of transformation with the energy required to move the austenite-ferrite locally enriched by solute atoms trapped in the interface. The model assumes the carbon to partition to proceed with such a high speed that a mean field approximation can be applied. In the GEB model the bainite reaction will stop if the available energy is insufficient to drive the interface forward with a realistic velocity. For ternary, Fe-C-X alloys, it has been shown that the GEB model predictions of the degree of incomplete transformation as a function of the alloy composition and the transformation temperature are significantly better than those of the model with a constant value for the strain energy contribution [13].
A microstructure evaluation of different areas of resistance spot welding on ultra-high strength TRIP1100 steel
Published in Cogent Engineering, 2018
Iman Hajiannia, Morteza Shamanian, Masoud Atapour, Ehsan Ghassemali, Rouholah Ashiri
As shown in Figure 2a, the TRIP1100 steel microstructure consists of ferrite and bainite, RA and martensite/austenitic island (M/A). Regarding the microstructure, retained austenite can be obtained in the bainite areas formed during the isothermal transformation at 350°C. A small amount of block RA is also observed in polygonal ferrite grains (Tang, Ding, Du, & Long, 2007). Although microscopic images and microstructural similarities can partly identify the RA phase, it is necessary to make calculations and accurate analyses to prove the presence of RA. It is also essential to know the properties of the microstructure and the existing phases. For this purpose, electron diffraction analysis from EBSD return electrons was used to study phase fractions. Figure 2b and 2c illustrate the crystallographic directions along with inverse pole figure (IPF) with image quality map (IQ), and phase fraction for TRIP1100 steel derived from EBSD analysis. With respect to Figure 2a, the size of ferrite grains of polygons ranges from 0.5 to 4 µm, and phases islands of M/A are between 1 and 2 µm.