Papers by Author: Jilt Sietsma

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Authors: Maria Giuseppina Mecozzi, C. Bos, Jilt Sietsma
Abstract: A three-dimensional cellular automata (CA) model is developed for the kinetic and microstructural modelling of the relevant metallurgical mechanisms occurring in the annealing stage of low–alloy steels: recrystallisation, pearlite–to–austenite transformation and ferrite–to–austenite transformation on heating and austenite–to–ferrite transformation on cooling. In this model the austenite–to–ferrite transformation is described by a mixed–mode approach, which implies that the transformation kinetics is controlled by both the interface mobility and the diffusivity of the partitioning elements. This approach also allows incorporation of the ferrite nucleation occurring on structural defects. The developed CA algorithm, in which the transformation rules for the grain boundary and interface cells are controlled by the growth kinetics of the forming phase, allows three-dimensional systems to be treated within relatively short simulation times. The simulated microstructure reproduces quite well the microstructure observed in experimental samples. A good agreement is obtained between the experimental and simulated ferrite recrystallisation and ferrite and austenite transformation kinetics. The present approach also models the development of the carbon concentration profile in the austenite, which is, for instance, essential for subsequent martensite formation.
Authors: Richard G. Thiessen, Jilt Sietsma, I.M. Richardson
Abstract: This work presents a unique approach for the modelling of the austenitisation of martensite in dual-phase steels within the phase-field method. Driving forces for nucleation and growth are derived from thermodynamic databases. Routines for nucleation are based on a discretisation of the classical nucleation theory. Validation is given via dilatometric experiments.
Authors: Y. van Leeuwen, A.W. Luinenburg, M. Onink, Jilt Sietsma, Sybrand van der Zwaag
Authors: Dominic Phelan, Nicole Stanford, B. Thijsse, Jilt Sietsma
Abstract: The deformation behaviour of magnesium single crystals under plane strain conditions has been examined using molecular dynamics modelling. The simulations were based on an existing atomic potential for magnesium taken from the literature. A strain of 10% was applied at rates of 3x109s-1 and 3x107s-1. The simulations predicted the formation of mechanical twins that accommodated extension in the c-axis direction of the hexagonal unit cell. However, the predicted twin is not of the same kind found in magnesium, but is that commonly observed in titanium. It is believed that further analysis of the physical properties predicted by this interatomic potential will shed more light on the atomic processes controlling twinning in Magnesium alloys. It also highlights the need for improvements to the interatomic potential such that more accurate deformation behaviour can be attained.
Authors: V.I. Savran, Y. van Leeuwen, Dave N. Hanlon, Jilt Sietsma
Abstract: The first step in the heat-treatment processes for a vast majority of commercial steels is austenitization. There is much less research put in this field comparing to the cooling transformation, but the interest is continuously increasing especially in view of the development of TRIP and Dual-phase steels. The microstructural evolution during continuous heating experiments has been studied for a series of C-Mn steels with carbon contents in the range 0.35-0.45 wt. % using optical and scanning electron (SEM) microscopy. It is shown that the formation of the austenitic phase is possible in pearlitic as well as in ferritic regions, although in the former it proceeds at a much faster rate due to the shorter diffusion distances. Thus a considerable overlap in time of the ferriteto- austenite and the pearlite-to-austenite transformations is likely to occur. Another observation that was made during the experiments is that depending on the heating rate, the pearlite-to-austenite transformation can proceed in either one or two steps. At low heating rates (0.05 °C/s) ferrite and cementite plates transform simultaneously. At higher heating rates (20 °C/s) it is a two-step process: first ferrite transforms to austenite within pearlite grains and then the dissolution of the cementite lamellae takes place.
Authors: Haiwen Luo, Jilt Sietsma, Sybrand van der Zwaag
Abstract: The austenite recrystallization kinetics in the intercritical region of a C-Mn steel is investigated by means of stress relaxation tests. It is found that the Avrami exponent, n, decreases significantly with decreasing temperature, i.e. with increasing ferrite fraction. This behaviour deviates from that of austenite recrystallization in the purely austenitic state, in which case the Avrami exponent is constant and independent of temperature and deformation. To interpret this, the influence of spatial variation of the plastic strain in the intercritical austenite grains on recrystallization kinetics is modelled quantitatively. The modelling results seem to indicate that the strain heterogeneity is responsible for the decreasing Avrami exponent with decreasing intercritical temperature.
Authors: P.A. Duine, Jilt Sietsma, A. van den Beukel
Authors: Dominic Phelan, T. Zuidwijk, L. Strezov, Jilt Sietsma, Rian Dippenaar
Abstract: The strip casting of steel, whereby liquid steel is solidified between twin water cooled copper rolls directly into its final shape, is a radical, energy efficient, cost effective route for the production of steel products that also provides exciting opportunities for the development of new products. An experimental program is currently underway to study phenomena associated with rapid solidification of steel using levitating droplet techniques and Gleeble®3500 thermo-mechanical processing. For example, studies have been conducted to investigate the heat transfer, nucleation behaviour and microstructure development during solidification of a low carbon steel and a peritectic steel on copper substrates hard coated electrolytically or using Filtered Arc Deposition (FAD). It was found that peak and average heat fluxes were significantly higher for steels solidified on the first substrate than for the FAD coated substrates. Maximum heat flux on the respective substrates was 36.5 to 39.0 MW/m2 and 8.3 to 9.4 MW/m2. The average heat flux on the respective coated substrates ranged between 9.6 to 12.5 and 5.5 to 6.6 MW/m2.
Authors: S. Eric Offerman, Henrik Strandlund, Niels H. van Dijk, Jilt Sietsma, Erik M. Lauridsen, L. Margulies, Henning Friis Poulsen, John Ågren, Sybrand van der Zwaag
Abstract: Ferrite formation during austenite decomposition in carbon-manganese steel is studied during slow continuous cooling by three-dimensional x-ray diffraction microscopy at a synchrotron source. The ferrite fraction and nucleation rate are measured simultaneously and independently in real time in the bulk of the specimen. Thermodynamic calculations involving both ortho- and paraequilibrium have been performed to determine the driving force for nucleation. From the experiments and thermodynamic calculations the activation energies are estimated for nucleation and the transfer of iron atoms across the interface of the cluster during ferrite nucleation in steel.
Authors: V.I. Savran, S. Eric Offerman, Niels H. van Dijk, Erik M. Lauridsen, L. Margulies, Jilt Sietsma
Abstract: Studying austenitisation in steel, so far, was either limited to observations at the surface of the material or to the determination of the average grain growth behavior in the bulk. The development of the three-dimensional X-ray diffraction (3DXRD) microscope at beam line ID11 of the European Synchrotron Radiation Facility in Grenoble, France, made it possible to study the transformation kinetics in-situ and at the level of individual grains in the bulk of the material. Unique in-situ observations of austenite growth kinetics during continuous heating experiments were made for two commercial low-alloy steels (C22 and C35). The observed growth behavior of individual austenite grains gives a valuable contribution to understanding the phase transformations on heating, i.e. austenite formation from ferrite and pearlite.
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