Papers by Author: Eric J. Palmiere

Abstract: The cementite precipitation behavior in the martensite and banite of the H21 tool steel under high temperature axisymmetric compression test and double temper was investigated. The main purpose on this work is to develop a better understanding regarding the transformation mechanism of bainite and martensite in a H21 tool steel. The selected deformation temperatures were 1100 ^oC and 1000 ^oC and the double temper process was carried out at 650 ^oC for 1 hour respectively. The results showed that the cementite was sensitive to the stress. The applied stress has affected the Fe₃C precipitation behaviour by decreasing the number of variants carbides in tempered martensite and decreasing the number of a single variant carbides in tempered lower bainite. The results were in agreement with a displacive mechanism of martensite and bainite transformation. It was also found that hot deformation temperatures selected in this work have the same contribution in decreasing number of variant carbides in tempered martensite and decreasing number of single variant carbides occurred in tempered lower bainite.

Abstract: The effects of hot axisymmetric compression to break down the primary carbide network of the H23 tool steels were studied. This current study only focused on one strain rate of 0.01 s^-1. The samples were deformed at 3 different temperatures (1000, 1050 and 1100 °C) with solutioning temperatures 1100 and 1250 °C, respectively. Afterwards, the samples were cooled by water quenching. The techniques used in this current study for investigation were the optical and electron microscopes and Vickers hardness test. The results show that hot axisymmetric compression had broken down the primary carbide network in the direction perpendicular to the compression axis and the carbides became finer. Although the highest hardness (274 HV) was achieved after solutioning at 1250 °C, followed by deformation at 1000 °C, however the microstructure analysis indicated that the optimum hot axisymmetric compression condition was solutioning at 1250 °C and deformation at 1000 °C.

Abstract: Solidification behaviour of a H21 tool steel has been studied. The aim of this work is to give better understanding of the as cast microstructure of the steel. The investigated H21 tool steel was made using induction vacuum furnace with air cooling. The microstructure observation was carried out using optical and electron microscopy. X ray diffraction and nanohardness were also used for phase and carbide identification. The results show that the as cast microstructure consisted of ferrite and coarse primary M6C carbide that is rich in tungsten. The nanohardness of ferrite was 5.1 ± 0.3 GPa and there was inhomogeneity in as cast microstructure, which is indicated by the nanohardness values from bulk grain to grain boundary precipitates varied between 4.8 to 8.9 GPa. Comparison between prediction and experimental results shows that the calculated phase diagram was not in agreement with the solidification microstructure of the H21 tool steel.

Abstract: In the present work cyclic torsion test was used to simulate hot plate rolling process in order to study the effect of strain reversal on non-recrystallisation temperature using unalloyed and microalloyed austenite model alloys. It was found that the amount of strain reversal directly influences both static recrystallisation and strain-induced precipitation process significantly delaying their kinetics. The proper assessment of the interactions between strain reversal and microstructure evolution plays a crucial role during hot rolling process - as continuous changes in the deformation mode (strain reversal) affect the level of redundant strain (in the areas near the surface of the stock) and lead to strain inhomogeneity across the plate thickness. This complex strain path introduces microstructural inhomogeneity and makes its predictions very difficult. Proper understanding of the effects of strain reversal on microstructure evolution in the austenite will help to optimise the hot rolling process.

Abstract: The Strain-Induced Dynamic Transformation (SIDT) is an efficient way to overcome the limitation of grain refinement during the conventional thermomechanical controlled processing (TMCP) of steels. The present study deals with the effects of austenite morphology on the SIDT in microalloyed and IF steels. The discussion of the processing route in terms of chemical composition, deformation schedules, heating and cooling conditions is carried out by the means of torsion tests with deformation in metastable conditions. As it was expected, the microstructure of microalloyed steel was clearly controlled by the microalloying elements what, in turn, directly affectedthe SIDT products. Examination of water-quenched microstructures, just after deformation, revealed the morphology of "strain-induced ferrite". The kinetics of SIDT were observed and analyzed using the strain-stress curves. It is shown that presence of strain-induced precipitations in microalloyed steels accelerates kinetic of SIDT - by reducing the amount of Nb in solution. The changes in ferrite refinement of the experimental steels were explained from the view of the austenite morphology and processing parameters.

Abstract: This paper presents a modelling strategy that combines neuro-fuzzy methods to dene the material model with cellular automata representations of the microstructure, all embedded within a nite element solver that can deal with the large deformations of metal processing technology. We use the acronym nf-CAFE as a label for the method. The need for such an approach arises from the twin demands of computational speed for quick solutions for ecient material characterisation by incorporating metallurgical knowledge for material design models and subsequent process control. In this strategy, the cellular automata hold the microstructural features in terms of sub-grain size and dislocation density which are modelled by a neuro-fuzzy system that predicts the ow stress. The proposed methodology is validated on a two dimensional (2D) plane strain compression nite element simulation with Al1%Mg alloy. Results from the simulations show the potential of the model for incorporating the eects of the underlying microstructure on the evolving ow stress elds. In doing this, the paper highlights the importance of understanding the local transition rules that aect the global behaviour during deformation.

Abstract: Plate steel S460 is intercritically rolled during the final stages of industrial processing. A series of experiments to represent the preliminary stages of an intercritical simulation were completed and the isothermal austenite to ferrite transformation kinetics investigated. The growth of the ferrite grains was interpreted using the classic JMAK model and the effect of processing history also examined. A double austenite deformation at 1323 K, to a true strain of 0.2, led to the most acceptable starting microstructure for the extended simulation. The role of niobium in this preliminary simulation is also considered.

Abstract: In the present study, monotonic and cyclical torsional deformations of an X-70 microalloyed steel were conducted at austenite temperatures below the recrystallisation-stop temperature (T_5%). The austenite deformation is followed by accelerated continuous cooling to allow the investigation of the strain reversal effect on the subsequent phase transformation mechanisms. The transformation behaviours were studied by a dilatometry method, and the microstructures of the transformed products have been analysed using electron back scatter diffraction (EBSD). The results of this study shows that although subjected to the same total cumulative strain and the same cooling rate, strain path reversal by cyclical torsion produces lower temperature transformation products involving mainly a displacive mechanism, comparing to simple strain path deformation which leads to higher temperature transformation by a reconstructive mechanism.

Abstract: Recent observations show that the strain reversal affects significantly and in a complex way both the static recrystallisation (SRX) and strain-induced precipitation (PPT) kinetics in Nb-microalloyed steel. It is already known that the recrystallisation stagnation is a consequence of the competition between the driving pressure for recrystallisation and the pinning pressure caused by the strain-induced precipitation of Nb (C,N) precipitates. Both of these parameters depend in turn on the local dislocation density. Thus, it is expected that a variation of the local dislocation density due to reversal of the strain will affect at the same time the local driving and the pinning pressures, which will cause the difference in the hardening levels. In the present paper, the influence of strain path change on microstructure evolution and mechanical behaviour in Nb-microalloyed steel (API X-70 grade) was studied. The deformation schedules were designed in order to investigate an effect of strain reversal on both static recrystallisation and strain-induced precipitation process kinetics. Flow curves recorded during deformation of X-70 steel showed clear influence of applied strain path on both static recrystallisation kinetics and strain-induced precipitation process.

Abstract: Thermomechanical Controlled Processing (TMCP) including accelerated cooling after the final hot rolling pass is a well-established technology, widely applied in HSLA steel plate production. However, there are still certain limitations, especially for thicker plate. The rolling schedule includes a long holding period (HP) after the roughing stage to allow the temperature to fall sufficiently for optimised TMCP during finishing. Intermediate Forced Cooling (IFC) applied during the HP can increase productivity by decreasing the required hold time, can restrict austenite grain growth, and can also improve the subsequent strain penetration in thick plate with further metallurgical benefits. Multi-pass plane strain compression (PSC) tests have been performed on the thermomechanical compression (TMC) machine at Sheffield University including different severities of IFC. Clearly it is impossible to simulate all aspects of the temperature and strain gradients present in thick plates in laboratory specimens, and most of the tests were conducted at temperatures and strains calculated by Finite Element modelling as relevant to specific positions through the plate thickness. However, some aspects of the gradients were addressed with tests using cold platens. The results have indeed shown that IFC can shorten the HP and reduce austenite grain growth and its variation across thick plate.