Critical assessment of models for predicting the Ms temperature of steels

Different approaches to predicting the Ms temperatures of steels are reviewed and discussed with the objective of summarising the main characteristics, advantages and difficulties of each method, mostly from a practical point of view. Empirical methods, and methods based on thermodynamics are then assessed against published data.NPL for provision of MTDATA and Neuromat for provision of the Model ManagerPeer reviewe

Preventing Abnormal Grain Growth of Austenite in Low Alloy Steels

International audienceno abstrac

Induced martensitic transformation during tensile test in nanostructured bainitic steels

Retained austenite in nanostructured bainite is able to undergo mechanically induced martensitic transformation. However, the link between transformation and deformation mechanisms involved makes difficult the understanding of the process. In this work, a model has been developed to assess the effect of the external stress itself on the martensite phase transformation. In addition, after a detailed initial microstructural characterization, the martensite fraction evolution during tensile deformation has been obtained by means of X-ray diffraction analyses after interrupted tensile tests in several nanostructured bainitic steels. Experimental results have been compared to the outputs of the model, as a reference. They suggests that stress partitioning between phases upon tensile deformation is promoted by isothermal transformation at lower temperatures.The authors gratefully acknowledge the support of the European Research Fund for Coal and Steel, the Spanish Ministry of Economy and Competitiveness and the Fondo Europeo de Desarrollo Regional (FEDER) for partially funding this research under the contracts RFSR-CT-2012-00017, RFSR-CT-2014-00016 and MAT2013-47460-C5-1−P respectively. LM-R also acknowledges this same Ministry for financial support with ref. FPI: BES-2011-044186.Peer Reviewe

Tensile behaviour of a nanocrystalline bainitic steel containing 3 wt% silicon

Much recent work has been devoted to characterize the microstructure and mechanical properties bainitic nanostructured steels. The microstructure is developed by isothermal heat treatment at temperatures as low as 125–350 °C and adapted steel grades typically contain high carbon contents to achieve sufficient depletion of the BS–MS temperature range, and above 1.5 Si wt.% to suppress carbide formation during isothermal holding. On the latter, most of the published literature agrees on a limit of around 1.2–1.5 wt.% to suppress cementite in high carbon steels. For this reason perhaps, additions of Si significantly above this limit have not been investigated systematically in the context of nanostructured bainitic steels. The present work is concerned with the effect of up to ∼3 Si wt.% in a steel grade adapted to low temperature bainitizing. Tensile properties as compared to similar grades, though with lower Si contents, exhibited unrivalled combinations of strength and ductility, with above 21% total elongation for a UTS above 2 GPa. An attempt is made to explain the mechanical properties of this microstructure in terms of some of its most relevant and unique morphological and microstructural featuressupport of the European Research Fund for Coal and Steel and the Spanish Ministry of Science and Innovation Plan Nacional de I+D+I (2008–2011) for funding this research under the contracts RFSR-CT-2008-00022, and MAT2010-15330,also acknowledges the Spanish Ministry of Science and Innovation for financial support in the form of a PhD research grant (FPI)Peer reviewe

Evaluation of potential of high Si high C steel nanostructured bainite for wear and fatigue applications

The present study is concerned with the potential of high carbon, high silicon steel grades isothermally transformed to bainite at low temperature (<300 C). The first part gives an overview of the design principles, allowing very high strength and ductility to be achieved while minimising transformation duration. Wear and fatigue properties are then investigated for over 10 variants of such materials, manufactured in the laboratory or industrially. The results are discussed against published data. Tensile strengths above 2 GPa are routinely achieved, with, in one case, an exceptional and unprecedented total elongation of over 20%. Bainite plate thickness and retained austenite content are shown to be important factors in controlling the yield strength, though additional, non-negligible parameters remain to be quantified. Rolling-sliding wear performances are found to be exceptional, with as little as 1% of the specific wear rate of conventional 100Cr6 isothermally transformed to bainite. It is suggested that this results from the decomposition of retained austenite in the worn layer, which considerably increases hardness and presumably introduces compressive residual stresses. Fatigue performance was slightly improved over 100Cr6 for one of the two industrially produced materials but significantly lower otherwise. Factors controlling fatigue resistance require further investigations. © 2013 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute.Peer Reviewe

Carbon content evolution in austenite during austenitization studied by in situ synchrotron X-ray diffraction of a hypoeutectoid steel

Using in situ high energy X-ray diffraction study of austenite formation in hypoeutectoid steel with three differ- ent initial microstructures (ferrite-pearlite, tempered martensite and bainite), the lattice parameters of ferrite, cementite and austenite are examined on heating at 0.25, 10 and 100 °C/s. The lattice parameters of ferrite, cementite and austenite do not vary linearly with the temperature, especially, in the temperature range where the austenitization takes place. For the austenite, it is suggested that the deviation from the linearity is mainly associated to the carbon content variation. Using Dyson and Holmes equation, the carbon content in austenite is evaluated for any moment of the austenite formation for each initial microstructure and all heating rates. For the ferrite-pearlite microstructure heated at 0.25 °C/s, the carbon content in austenite after complete cementite dissolution corresponds to that of pearlite. Moreover, a rapid decrease in carbon content in the austenite is observed during the first stage of the austenitization (simultaneous dissolution of ferrite and cementite) followed by a slow further decrease during the transformation of the remaining ferrite. The obtained results are discussed using thermodynamic calculations

A model for predicting the Ms temperatures of steels.

Using neural networks in a Bayesian framework, a model has been derived for the Ms temperature of steels over a wide range of compositions. By its design and by use of a more extensive database, this model improves over existing ones, by its accuracy and its ability to avoid wild predictions.NPL for provision of MTDATA and Neuromat for provision of the Model Manager.Peer reviewe

Critical assessment of models for predicting the Ms temperature of steels

Different approaches to predicting the Ms temperatures of steels are reviewed and discussed with the objective of summarising the main characteristics, advantages and difficulties of each method, mostly from a practical point of view. Empirical methods, and methods based on thermodynamics are then assessed against published data.NPL for provision of MTDATA and Neuromat for provision of the Model ManagerPeer reviewe

Simultaneous Precipitation Reactions in Creep-Resistant Austenitic Stainless Steels

Supporting materials for this thesis (computer programs) may be found on the website of the Materials Algorithms Project (MAP): http://www.msm.cam.ac.uk/map/mapmain.htm