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Guo L.,University of Cambridge | Roelofs H.,Swiss Steel AG | Lembke M.I.,Steeltec AG | Bhadeshia H.K.D.H.,University of Cambridge
Materials Science and Technology (United Kingdom) | Year: 2017

A model for recalescence has been established by integrating a model for the decomposition of austenite and one dealing with heat transfer with latent heat release taken into account. The effects of recalescence on each individual austenite transformation product have been studied. It was found that Widmanstätten ferrite and pearlite reactions are most affected. The calculated cooling curve, as affected by the recalescence, has also been verified with a commercial steel subjected to two different environment conditions. © 2017 Institute of Materials, Minerals and Mining. Published by Taylor & Francis on behalf of the Institute

Swiss Steel AG, ArcelorMittal and Instytut Metalurgii Zelaza | Date: 2010-06-23

A rolled steel wire or rod suitable for the production of cold headed products without the application of heat treating operations has a chemical composition, by weight, of:0.04% C 0.1%1.8% Mn 2.0%0.15% Si 0.30%S 0.025%P 0.025%Cr 0.50%Mo 0.08%Ni+Cu 0.30%0.01% Al 0.05%V 0.05%10 ppm B 30 ppmN 100 ppmTi 0.06%the remainder being iron and impurities resulting from the production process. The steel has a cementite-free microstructure comprising a predominant phase, a minor phase and MX-precipitations distributed within the predominant phase, the predominant phase consisting of bainitic ferrite, the minor phase comprising retained austenite and optionally martensite with a size of 2 to 3 m, the relative amount of the minor phase amounting to < 20% by volume.

Guo L.,Charles University | Roelofs H.,Swiss Steel AG | Lembke M.I.,Steeltec AG | Bhadeshia H.K.D.H.,Charles University
Materials Science and Technology (United Kingdom) | Year: 2016

A model for estimating the upper to lower bainite transition has been developed for the iron–carbon–manganese–chromium–silicon alloy system by comparing the time required to decarburise a supersaturated bainitic ferrite platelet and that needed for the start of cementite precipitation in the ferrite. The problem is treated as a competition between the decarburisation time and the kinetics of cementite precipitation. Lower bainite is induced when the latter process is faster. The time for forming a volume fraction of 0.01 of the equilibrium amount of cementite is taken as the precipitation start time. The model was calibrated using experimental data from the iron–carbon system, and verified with the iron–carbon–manganese–molybdenum system and an experimental steel currently being developed for high-strength applications. © 2016 Institute of Materials, Minerals and Mining

Morales-Rivas L.,CSIC - National Center for Metallurgical Research | Roelofs H.,Swiss Steel AG | Hasler S.,Swiss Steel AG | Garcia-Mateo C.,CSIC - National Center for Metallurgical Research | Caballero F.G.,CSIC - National Center for Metallurgical Research
Advanced Materials Research | Year: 2014

Chemical segregation of alloying elements during solidification of steel grades leads to development of a banded microstructure, causing a degree of anisotropy that can be detrimental to the mechanical behavior under service conditions. It is well-known that the presence of strongly orientated martensite bands in carbide-free bainitic microstructures, associated to inhomogeneous Mn redistribution during solidification, leads to a remarkable deterioration in toughness in advanced high strength bainitic steels. In this study, while bands were clearly visible on light optical micrographs of continuously cooled carbide-free bainitic steels, scanning electron microscopy examination revealed only a gradual transition between matrix and bands, both with a granular bainitic structure. Electron backscatter diffraction was used to quantify the bainitic packet size and volume fraction of martensite/austenite constituent between and within the bands, after a process of optimization of the analysis settings in order to minimize the inherent difficulties linked to submicrometric and minority phase indexation. The quantitative microstructural results showed negligible morphological differences between bainitic structure bands and matrix, only solute segregation of Cr and Mo was detected by energy-dispersive X-ray spectroscopy within bands, which must be responsible for a stronger resistance against metallographic etching in those regions. © (2014) Trans Tech Publications, Switzerland.

Morales-Rivas L.,CSIC - National Center for Metallurgical Research | Roelofs H.,Swiss Steel AG | Hasler S.,Swiss Steel AG | Garcia-Mateo C.,CSIC - National Center for Metallurgical Research | Caballero F.G.,CSIC - National Center for Metallurgical Research
Journal of Mining and Metallurgy, Section B: Metallurgy | Year: 2015

The presence of banding in a hot-rolled air-cooled bainitic steel has been thoroughly characterized. The banded microstructure was observed due to distinctive sensitivities to the etching agent. Microstructural and crystallographic studies by means of Scanning Electron Microscopy and Electron Backscatter Diffraction did not reveal any substantial differences between bands and matrix. However, solute segregation of some alloying elements was detected by Wavelength- Dispersive Spectroscopy, it being found that bands are enriched in Cr, Mo and Si and depleted in Mn. Finally, a set of theoretical calculations suggested that the solidification mode is responsible for such unusual partitioning behavior, which is beneficial in terms of the bainitic transformation.

Caballero F.G.,CSIC - National Center for Metallurgical Research | Roelofs H.,Swiss Steel AG | Hasler St.,Swiss Steel AG | Capdevila C.,CSIC - National Center for Metallurgical Research | And 3 more authors.
Materials Science and Technology | Year: 2012

The influence of bainite morphology on the impact toughness behaviour of continuously cooled cementite free low carbon bainitic steels has been examined. In these steels, bainitic microstructures formed mainly by lath-like upper bainite, consisting of thin and long parallel ferrite laths, were shown to exhibit higher impact toughness values than those with a granular bainite, consisting of equiaxed ferrite structure and discrete island of martensite/austenite constituent. Results suggest that the mechanism of brittle fracture of cementite free bainitic steels involves the nucleation of microcracks in martensite/austenite islands but is controlled by the bainite packet size. © 2012 Institute of Materials, Minerals and Mining.

Smolenicki D.,ETH Zurich | Boos J.,ETH Zurich | Kuster F.,ETH Zurich | Roelofs H.,Swiss Steel AG | Wyen C.F.,Rieter Machine Works Ltd.
CIRP Annals - Manufacturing Technology | Year: 2014

In order to represent actual cutting process conditions, an in-process tribometer is examined to measure friction during orthogonal turning process at cutting speeds up to 300 m/min. The tribometer consists of a spring preloaded tungsten carbide pin with spherical tip mounted behind the cutting edge and rubbing on the freshly generated workpiece surface. The pin preload is set according to feed force. A 3D-force measuring device in the fixation of the pin allows evaluating friction coefficient from tangential and normal forces. Experiments show strongly different results when contacting fresh and oxidized surfaces and decreasing friction coefficient with increasing cutting speed. © 2014 CIRP.

In order to achieve the goals for a sustainable development, concrete with recycled aggregates (concrete and mixtures from concrete and masonry, e.g. clay bricks and calcium silicate blocks etc.) is going to be used more and more in the future. Due to its thermal insulating properties, the demand for concrete with light-weight aggregates (e.g. foam glass) will also increase. As an essential element of this development, an increasing amount of cements with a reduced clinker factor as well as of mineral additions such as fly ash and ground granulated blast furnace slag is used for concrete production. Therefore, as a general tendency, the carbonation resistance of concrete mixes decreases while the risk of corrosion damages increases. The goal of the investigations described in this paper was to evaluate and to assess the corrosion resistance of a stainless rebar (Top12, composition corresponds approx. to steel grade 1.4003) in various carbonated concrete mixes and to compare the results with common rebars. The results lead to the conclusion that, in contrast to common rebars, Top12 is durable in all investigated concrete mixes, including strongly carbonated recycling concretes. Copyright © 2010 Ernst & Sohn Verlag für Architektur und technische Wissenschaften GmbH & Co. KG, Berlin.

Lembke M.I.,Steeltec AG | Olschewski G.,Steeltec AG | Roelofs H.,Swiss Steel AG | Klumper-Westkamp H.,IWT - Foundation Institute of Materials Engineering
HTM - Journal of Heat Treatment and Materials | Year: 2014

Modern bainitic long products with ultimate tensile strength values in the range 800-1400 MPa provide new property profiles and permit the manufacturing of complex components without subsequent quench and temper (Q&T) treatment. Abrasion resistance and fatigue limit can be enhanced by surface hardening or coating. The IWT in Bremen systematically evaluated the behaviour of bainitic steel grades 7MnB8 and 18MnCrSiMoS641 in nitriding and nitrocarburizing processes in comparison to conventional (ferritic-pearlitic) steels 44SMn282 and 16MnCrS5Pb+HH3. The results were analyzed based on metallographic analysis, hardness measurement and element concentration profiles. Regular and compact layers of approx. 8 μm in thickness were obtained after nitriding at rN = 2 bar-1/2 and a temperature of 520 °C during 10 h with the selected steel grades. The resulting surface hardness was in the range obtained for quenched and tempered steels. © Carl Hanser Verlag GmbH & Co. KG.

Stucklin S.,Swiss Steel AG
Praktische Metallographie/Practical Metallography | Year: 2016

Accredited metallography laboratories are required to estimate the uncertainty ot the result of every test carried out. This presents a particular challenge in evaluating the cleanliness of a metal as usually a particle distribution - log-normal at best and represented by a tiny sample size - is truncated and split into discrete classes. Therefore, neither indirect (top-down) nor direct (bottom-up) methods succeed in estimating the uncertainty of cleanliness evaluations. For these reasons, such evaluations should be viewed through a process surveillance lens, particularly when discussing results or setting acceptance thresholds. © Carl Hanser Verlag, München.

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