THERMEC 2018

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Authors: Navjeet Singh, Andrii G. Kostryzhev, Chris R. Killmore, Elena V. Pereloma
Abstract: Three novel low carbon microalloyed steels with various additions of Mo, Nb and V were investigated after thermomechanical processing simulations designed to obtain ferrite-bainite microstructure. With the increase in microalloying element additions from the High V- to NbV- to MoNbV-microalloyed steel, the high temperature flow stresses increased. The MoNbV and NbV steels have shown a slightly higher non-recrystallization temperature (1000 °C) than the High V steel (975 °C) due to the solute drag from Nb and Mo atoms and austenite precipitation of Nb-rich particles. The ambient temperature microstructures of all steels consisted predominantly of polygonal ferrite with a small amount of granular bainite. Precipitation of Nb-and Mo-containing carbonitrides (>20 nm size) was observed in the MoNbV and NbV steels, whereas only coarser (~40 nm) iron carbides were present in the High V steel. Finer grain size and larger granular bainite fraction resulted in a higher hardness of MoNbV steel (293 HV) compared to the NbV (265 HV) and High V (285 HV) steels.
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Authors: Li Na Yu, Kazuyoshi Saida, Kazutoshi Nishimoto
Abstract: Temper bead welding is one of effective repair welding methods in case that post weld heat treatment is not easily applied. In order to evaluate the effectiveness of temper bead welding, hardness in HAZ becomes important factor. The neural network-based hardness prediction system of HAZ in temper bead welding for A533B low-alloy steel has been constructed by the authors in the previous study. However, for HAZ hardness prediction of other steels, it is necessary to obtain hardness database for each steel which is time-cost consuming, if the same method is used. The present study has been conducted to develop the generalized hardness prediction method applicable for other steels by utilizing the hardness data-base of A533B steel assuming that the hardness in HAZ of steels after tempering have a linear relationship with LMP (Larson-Miller parameter). On using the newly proposed extended method, only a few hardness data-base for the other steels is needed to obtain. Hardness distribution in HAZ of temper bead welding for other steels was calculated by using the extended hardness prediction system. The thermal cycles used for calculation were numerically obtained by a finite element method. The experimental results have shown that the predicted hardness is in good accordance with the measured one for steels without secondary hardening. It follows that the currently proposed extended method is effective for estimating the tempering effect during temper bead welding for the steels without secondary hardening.
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Authors: Abdelhalim Loucif, Davood Shahriari, Kanwal Chadha, Chunping Zhang, Mohammad Jahazi, Rami Tremblay, Louis Philippe Lapierre-Boire
Abstract: The present study focuses on characterizing cracks and fracture that appeared during solidification in the segregated zones of the as-cast structure of a large size ingot made of high strength low alloy steel. Solidification experiment was conducted, using Gleeble® 3800 thermo-mechanical simulator, on samples taken from the ingot/hot top interface of a 40 MT (Metric Ton) ingot. The thermal cycle consisted in heating from ambient temperature to 1385 °C with a constant heating rate of 2 °C/s followed by a free cooling. Optical and scanning electronic microscopies were used to analyze and quantify the cracked regions. Microstructural observations revealed that shrinkage during rapid solidification of melted grain boundaries ultimately led to the initiation and propagation of cracks.
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Authors: Gloria Basanta, Ana L. Rivas, Ervis Díaz, Carlos Parra
Abstract: The present work has been undertaken to assess the evolution of dissolution process of large dendritic precipitates in a V-Nb-Ti microalloyed steel. The study was performed by reheating the samples at 1250°C, simulating the industrial reheating practices at laboratory scale and in situ, following industrial profile; afterwards the samples were quenched in 10%NaCl aqueous solution. The characterization of the material was carried out by scanning electron microscopy accompanied with dispersive energy spectrometry, and chemical analysis by inductively coupled plasma optical emission spectrometry. The results showed a partial dissolution of dendritic precipitates. This process ocurred by a progressive dissolving the Nb-rich shells formed over cuboidal particles and primary arm of well-faceted dendritic precipitates, and by fragmentation and spheroidization of secondary branches. These processes gave rise to spherical Nb rich precipitates and cuboidal particles at the reheating conditions used in this study. Both type of particles contain vanadium.
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Authors: Elena Astafurova, Valentina Moskvina, Galina G. Maier, Eugene Melnikov, Nina Galchenko, Sergey Astafurov, Antonina Gordienko, Alexander Burlachenko, Alexandr Smirnov, Vladimir Bataev
Abstract: A temperature dependence of the tensile mechanical properties, microstructure and fracture mechanism of high-nitrogen Fe-(19-23)Cr-(17-21)Mn-(0-3)V-(0.1-0.3)C-(0.5-0.9)N vanadium-free and vanadium-containing steels was investigated. For all steels, the 0.2% offset yield strength and strain-hardening drastically increase with a decrease in test temperature. This is associated with high interstitial solid solution strengthening of the steels and more pronounced twinning and stacking-fault formation during straining below room temperature. For the vanadium-free steel, a ductile-to-brittle transition was evaluated: at 77K specimens destroy by cleavage mechanism while at room temperature steels show ductile fracture. Vanadium-alloying provides a particle strengthening of the steels and, at the same time, reduce solid-solution strengthening. Increase of vanadium concentration fully or partially suppress brittle fracture of the steels at 77K. Particle strengthening changes interstitial solid-solution effect, dislocation arrangement and slip/twinning relation in vanadium-containing high-nitrogen steels compared to vanadium-free one.
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Authors: Yu Lin Ju, Aimee Goodall, Claire Davis, Martin Strangwood
Abstract: Low alloy quench and tempered (Q&T) steels plates up to 100mm thick are used in applications such as cranes and earth movers due to their combination of high strength and toughness. In order to ensure that appropriate tempering conditions are used to give optimum properties through thickness in Q&T steels it is desirable to be able to predict the effect of composition and tempering conditions (time and temperatures) on the microstructure and hence the hardness evolution. In this paper, the types and coarsening rates of carbides formed in a low alloyed Q&T steel have been investigated on tempering at 600 °C. It has been found that in the as-quenched condition auto-tempered martensite is present with needle-shaped epsilon and cementite particles, whilst after different tempering times (1 - 16 hours) cementite becomes the stable phase with an elliptical shape, which coarsens with time. Besides, the coarsening of inter-lath cementite with a faster rate is independent from that of intra-lath ones.
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Authors: Janusz Majta, Remigiusz Bloniarz, Marcin Kwiecień, Krzysztof Muszka
Abstract: This paper presents a summary of a preliminary research aimed at producing ultrafine-grained (UFG) and heterogeneous microstructure in microalloyed steel and testing these materials under dynamic loading conditions (strain rates 800 s-1 and 1800s-1). The UFG and bimodal-structures, due to grain size, structural composition or morphology of structural components, were produced by an advanced thermomechanical processing, namely rolling in: hot, two-phase and cold-hot combined conditions. The advantage of bimodal microstructures is their maximization of mechanical behavior under extreme loading conditions due to promoted accumulation and interactions of geometrically necessary dislocations. The dynamic work-hardening behavior has been studied as a function of solute atoms and fine-scale, second-phase particles in the UFG and bimodal-structures. The substantial complexity of the phenomena, which occur through the evolution of microstructure and texture in response to dynamic loading, presents formidable challenges to theoretical model development of plastic deformation of UFG and bimodal-structures. Such an extraordinary work hardening provides an attractive strategy to develop optimal combination of mechanical properties i.e. strength/ductility ratio. A multi-scale analysis capable of including material behavior in different scales should be applied to discuss mechanical response of mentioned above microstructures and to help to analyze their influence on mechanical behavior under dynamic loading. The investigation was performed for a material of common application: high strength microalloyed steel X70. The experimental results show that strain rate sensitivity of the heterogeneous microstructures obtained by various thermomechanical rolling routes are significant, but not by a similar magnitude with the microstructure compositions and increasing strain rate.
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Authors: Dannis Rorisang Nkarapa Maubane, Charles Witness Siyasiya, Kevin Mark Banks, Waldo Edmund Stumpf
Abstract: The influence of roughing strain on the extent of austenite recrystallisation in plain carbon steel and Nb-Ti-V microalloyed steel was investigated. Reheating and roughing simulations were conducted on a deformation dilatometer using industrial heating, soaking times, strain and strain rates. The roughing schedules comprised of varying the pass strain magnitude within a typical roughing temperature range. The double stroke method was used to determine the austenite softening fraction. The austenite grain size, prior to and after rolling, was measured on quenched specimens. Grain refinement was achieved in both steels after all rolling schedules. An applied pass strain of 0.15 was sufficient to completely soften the austenite after the first pass and produced the finest recrystallised grain size. This was attributed to sufficient nucleation sites and driving force for recrystallisation compared to lower strains. Partial recrystallisation occurred after the first pass due to the relatively coarse initial grain size. The steel chemistry played an insignificant role in controlling the recrystallisation kinetics at high roughing temperatures. The through-thickness strain distribution calculated from FEM simulations showed that, for a given applied strain, a similar magnitude of deformation is achieved at the centre of a hot-rolled plate.
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Authors: Bernd Pfleger, Marina Gruber, Gerald Ressel, Peter Gruber, Matthew Galler, Stefan Marsoner, Reinhold Ebner
Abstract: The concept of quenching and partitioning (Q&P) is a tool to generate steels with high strength and high ductility resulting from a relatively high amount of martensite and austenite. As the parameters of the Q&P treatment influence the strength and ductility properties, the goal of this work was to analyze the effects of varying quenching and partitioning temperatures on the phase evolution, on the retained austenite phase fraction and on the mechanical properties. The phase evolution during heat treatment was analyzed by dilatometer and the microstructure after processing was characterized by light microscopy in combination with color etching according to Klemm. The austenite phase fraction and its carbon content were determined by X-Ray diffraction and the mechanical properties were evaluated by tensile tests. It is shown that the quenching temperature has a minor impact on the phase fraction of austenite as well as its stabilization by carbon and thus, also the mechanical properties. Furthermore, austenite transformation during the partitioning treatment is illustrated.
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