S.M.C. van Bohemen

Bainite and martensite start temperature calculated with exponential carbon dependence

AbstractAnalysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0·1–1·9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and Ms temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the Ms temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter αm controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of Ms and αm, the volume fraction of retained austenite at room temperature has been calculated for Fe–C a...Abstract Analysis of published data demonstrates that the start temperatures of bainite (Bs) and martensite (Ms) formation exhibit an exponential carbon dependence. Empirical models are proposed to describe this specific carbon dependence. The models are relatively simple and sufficiently accurate for conventional steels with 0·1–1·9 wt-% carbon and less than 7 wt-% in total of other alloying elements. Predictions of the Bs and Ms temperatures show a better accuracy than those obtained with equations from literature. An improved prediction of the Ms temperature is important to accurately determine of the amount of martensite at a certain arrest temperature using the Koistinen and Marburger (KM) equation. Predictions of the volume fraction martensite are also influenced by the rate parameter αm controlling the kinetics of martensite formation. Based on the improved models for the composition dependence of Ms and αm, the volume fraction of retained austenite at room temperature has been calculated for Fe–C alloys. The calculated fraction of retained austenite as a function of carbon content is found to be in good agreement with published data, which gives confidence in the proposed models for Ms and αm.

Kinetics of martensite formation in plain carbon steels: critical assessment of possible influence of austenite grain boundaries and autocatalysis

Abstract The kinetics of the martensitic transformation in Fe–0·80C has been determined from dilatometry data and shows no significant variation when the cooling rate is changed by two orders of magnitude. All kinetic data can be adequately simulated by the Koistinen and Marburger (KM) equation using a specific start temperature TKM and rate parameter αm. This finding supports the suggestion that the transformation is athermal, and moreover, the absence of a time dependence strongly indicates that autocatalytic nucleation does not contribute to the transformation kinetics in plain carbon steels on measurable time scales. Furthermore, dilatometry experiments with different austenitising conditions were conducted to examine the effect of the prior austenite grain size on the overall kinetics of martensite formation. The present results indicate that the progress of martensite formation beyond a fraction fu200a=u200a0·15 is independent of the prior austenitising treatment. It is therefore concluded that austenite–austenite grain boundaries have no significant effect on the overall nucleation and growth of athermal martensite, which is consistent with a model proposed by Ansell and co-workers.

Autocatalytic nature of the bainitic transformation in steels: a new hypothesis

To ensure improvements in predicting the kinetics of bainite formation, it is important to understand the autocatalytic nature of the transformation so that this accelerating effect can be rigorously incorporated in kinetic models. In the present paper, it is assumed that the broad faces of bainitic plates in particular provide new potential nucleation sites for autocatalytic nucleation. The dislocations in the austenite near a bainitic plate are thought to stimulate autocatalysis because carbon is assumed to pile up at these regions and thereby other austenite–bainite interface regions may contain less carbon which promotes nucleation. Based on these assumptions, it is derived that the autocatalytic contribution is proportional to the volume fraction of as-formed bainite, which is consistent with the dependence proposed by Entwisle [V. Raghavan and A.R. Entwisle, Special Report No. 93, The Iron and Steel Institute, London, 1965, p.30] on the basis of empirical knowledge. In addition, it is assumed that autocatalytic nucleation can also depend on the morphology of bainite due to the associated difference in cementite precipitation. This new hypothesis for autocatalysis offers a viable explanation for the irregular variation in kinetics associated with the transition from upper to lower bainite measured for an alloy with eutectoid composition. Furthermore, comparison with experimental data of a Si-rich steel demonstrates that the isothermal kinetics of bainite formation can only be satisfactorily described when the autocatalytic factor is inversely proportional to the thickness of bainitic plates, which is consistent with the model proposed.To ensure improvements in predicting the kinetics of bainite formation, it is important to understand the autocatalytic nature of the transformation so that this accelerating effect can be rigorously incorporated in kinetic models. In the present paper, it is assumed that the broad faces of bainitic plates in particular provide new potential nucleation sites for autocatalytic nucleation. The dislocations in the austenite near a bainitic plate are thought to stimulate autocatalysis because carbon is assumed to pile up at these regions and thereby other austenite–bainite interface regions may contain less carbon which promotes nucleation. Based on these assumptions, it is derived that the autocatalytic contribution is proportional to the volume fraction of as-formed bainite, which is consistent with the dependence proposed by Entwisle [V. Raghavan and A.R. Entwisle, Special Report No. 93, The Iron and Steel Institute, London, 1965, p.30] on the basis of empirical knowledge. In addition, it is assumed that a...

Critical Assessment 10: Tensile elongation of strong automotive steels as function of testpiece geometry

Abstract Total elongation as measured in tensile tests on strong steels is adopted in product specifications and development goals, and sometimes assumed to relate to formability. The quantity, however, is measured in a variety of ways in the reported literature and is often used to make unjustified comparisons. Using purposely generated experimental data, the effect of specimen geometry on the measured total elongation is critically assessed.

Relationship between acoustic emission energy and the kinetics of martensite formation in plain carbon steels

The kinetics of the martensitic transformation in Fe-0.80C determined on the basis of dilatometry data is compared to the acoustic emission (AE) energy accompanying the transformation in the same steel reported in a previous study. The discrepancy between the AE energy and the volume fraction of martensite indicates that the mechanism for the generation of AE during the martensitic transformation is not solely dependent on the kinetics and the associated moving interfaces as suggested in previous studies. During the growth of martensite, slip takes place in order to relieve internal stresses, and dislocations are thought to be mainly introduced in the relatively soft austenite matrix. The quantitative analysis in this study demonstrates that the AE energy generated per unit time is a function of both the transformation kinetics and the volume fraction of remaining austenite. This strongly indicates that the moving dislocations associated with the plastic deformation of the austenite surrounding the as-for...The kinetics of the martensitic transformation in Fe-0.80C determined on the basis of dilatometry data is compared to the acoustic emission (AE) energy accompanying the transformation in the same steel reported in a previous study. The discrepancy between the AE energy and the volume fraction of martensite indicates that the mechanism for the generation of AE during the martensitic transformation is not solely dependent on the kinetics and the associated moving interfaces as suggested in previous studies. During the growth of martensite, slip takes place in order to relieve internal stresses, and dislocations are thought to be mainly introduced in the relatively soft austenite matrix. The quantitative analysis in this study demonstrates that the AE energy generated per unit time is a function of both the transformation kinetics and the volume fraction of remaining austenite. This strongly indicates that the moving dislocations associated with the plastic deformation of the austenite surrounding the as-formed martensite are the dominant sources of the generated acoustic waves. This improved AE source model is consistent with the well-accepted mechanism of AE during conventional plastic deformation due to an external load.

Influence of the Processing Variables on the Microstructure Evolution of a Bainitic Carbide-Free Steel

New trends focused on achieving higher performance steels has led to a so-called 3rd Generation Advanced High Strength Steels (AHSS), in which the typical polygonal ferrite found in TRIP steels as a matrix phase is replaced by harder phases as Carbide-Free Bainite (CFB) and/or (tempered) martensite. Besides, large volume fractions of retained austenite (R.A.) with adequate stability are aimed for to improve the formability of the steels. Si containing steels are regarded as the most suitable to retard cementite formation and consequently reach high volume fractions of RA. In this work, CFB annealing schedules were applied to dilatometer samples of Fe-0.22C-2.0Mn-1.3Si. The overaging temperature TB was varied between 390 oC and 480 oC, and other processing variables investigated were the austenitizing temperature Taus, and the overaging holding time tB. The annealed samples analyzed with LOM, FEG-SEM, EBSD and X-ray diffraction techniques show that markedly different complex microstructures made up of bainite, ferrite, MA phase and retained austenite (R.A) are accomplished depending on the specific thermal cycle. These results are described in detail and discussed in relation to the dilatometry measurements.