Rosalia Rementeria

Effect of Prior Austenite Grain Size on Pearlite Transformation in a Hypoeuctectoid Fe-C-Mn Steel

The aim of this work is to evaluate the influence of the prior austenite grain size (AGS) on the austenite-to-pearlite isothermal decomposition in a Fe-C-Mn hypoeutectoid steel. Due to the strong influence, grain boundaries have on pearlite transformation kinetics, morphological aspects of pearlite from two conditions with very different AGS were studied and characterized. Results allow us to conclude that the formation of pearlite and ferrite are favored for small AGS values, whereas a larger AGS led to an increase in the total amount of pearlite volume fraction. Furthermore, the average size of pearlitic colonies increased with increasing AGS, and it appears that the interlamellar spacing of the pearlite does not depend on AGS, but instead, is controlled by the isothermal decomposition temperature. Finally, it was observed that the ratio between lamellar thickness of ferrite and cementite depended on AGS.

A Constitutive Relationship between Fatigue Limit and Microstructure in Nanostructured Bainitic Steels

The recently developed nanobainitic steels show high strength as well as high ductility. Although this combination seems to be promising for fatigue design, fatigue properties of nanostructured bainitic steels are often surprisingly low. To improve the fatigue behavior, an understanding of the correlation between the nanobainitic microstructure and the fatigue limit is fundamental. Therefore, our hypothesis to predict the fatigue limit was that the main function of the microstructure is not necessarily totally avoiding the initiation of a fatigue crack, but the microstructure has to increase the ability to decelerate or to stop a growing fatigue crack. Thus, the key to understanding the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features that could act as barriers for growing fatigue cracks. To prove this hypothesis, we carried out fatigue tests, crack growth experiments, and correlated these results to the size of microstructural features gained from microstructural analysis by light optical microscope and EBSD-measurements. Finally, we were able to identify microstructural features that influence the fatigue crack growth and the fatigue limit of nanostructured bainitic steels.

Improving wear resistance of steels through nanocrystalline structures obtained by bainitic transformation

Nanocrystalline materials are structurally characterised by a large density of grain boundaries which significantly alter their mechanical properties in comparison with conventional coarse-grained materials. Specifically, nanocrystallisation has been proven to be an effective method to improve wear resistance of steels. The recent development of the first bulk nanocrystalline steel, which can be produced on a large scale with large dimensions solely by bainitic transformation at low temperature, has stimulated research in quest of its in-service properties and potential applications. This paper outlines the work devoted in the last years to assess and understand the wear response of those nanocrystalline bainitic steels.

Can Pearlite form Outside of the Hultgren Extrapolation of the Ae3 and Acm Phase Boundaries

It is usually assumed that ferrous pearlite can form only when the average austenite carbon concentration C0 lies between the extrapolated Ae3 (γ/α) and Acm (γ/θ) phase boundaries (the “Hultgren extrapolation”). This “mutual supersaturation” criterion for cooperative lamellar nucleation and growth is critically examined from a historical perspective and in light of recent experiments on coarse-grained hypoeutectoid steels which show pearlite formation outside the Hultgren extrapolation. This criterion, at least as interpreted in terms of the average austenite composition, is shown to be unnecessarily restrictive. The carbon fluxes evaluated from Brandt’s solution are sufficient to allow pearlite growth both inside and outside the Hultgren Extrapolation. As for the feasibility of the nucleation events leading to pearlite, the only criterion is that there are some local regions of austenite inside the Hultgren Extrapolation, even if the average austenite composition is outside.

Complex Nano-Scale Structures for Unprecedented Properties in Steels

Processing bulk nanoscrystalline materials for structural applications still poses a significant challenge, particularly in achieving an industrially viable process. In this context, recent work has proved that complex nanoscale steel structures can be formed by solid reaction at low temperatures. These nanocrystalline bainitic steels present the highest strength ever recorded, unprecedented ductility, fatigue on par with commercial bearing steels and exceptional rolling-sliding wear performances. A description of the characteristics and significance of these remarkable structures in the context of the atomic mechanism of transformation is provided.

Comparison of Ductile-to-Brittle Transition Behavior in Two Similar Ferritic Oxide Dispersion Strengthened Alloys

The ductile-to-brittle transition (DBT) behavior of two similar Fe-Cr-Al oxide dispersion strengthened (ODS) stainless steels was analyzed following the Cottrell–Petch model. Both alloys were manufactured by mechanical alloying (MA) but by different forming routes. One was manufactured as hot rolled tube, and the other in the form of hot extruded bar. The two hot forming routes considered do not significantly influence the microstructure, but cause differences in the texture and the distribution of oxide particles. These have little influence on tensile properties; however, the DBT temperature and the upper shelf energy (USE) are significantly affected because of delamination orientation with regard to the notch plane. Whereas in hot rolled material the delaminations are parallel to the rolling surface, in the hot extruded material, they are randomly oriented because the material is transversally isotropic.

Correlation of Fatigue Limit and Crack Growth Threshold Value to the Nanobainitic Microstructure

The recently developed nanobainitic steels show high ultimate tensile strength (UTS) as well as high ductility. Although this combination seems to be desirable for fatigue design, fatigue limit of nanostructured bainite is often disappointingly low. To improve fatigue properties we tried to earn a fundamental understanding of the microstructural parameters governing fatigue behavior.Therefore our hypothesis to improve the fatigue behavior was not necessarily avoiding the initiation of a fatigue crack – which could lead to failure of the material – but to improve the ability of the present microstructure to slow down or stop growing cracks. Thus, the key to understand the fatigue behavior of nanostructured bainite is to understand the role of the microstructural features which could act as barriers for growing cracks.We tried to correlate our results of fatigue tests and analysis of fracture surfaces to the size of microstructural features like bainitic ferrite plates, crystallographic bainite blocks and packets or prior austenite grains, as well as cracks induced at nonmetallic inclusions. Thereby we found that the crystallographic bainite block size governs fatigue behavior. Additionally, threshold values were determined from crack growth experiments and related to the characteristic microstructural features.

The non-steady state growth of pearlite outside the Hultgren extrapolation

The goal of this paper is to analyse the effect of adding Al on the non-steady pearlite growth occurring in a Fe–C–Mn system. The results are discussed in terms of the partitioning of elements across the austenite/ferrite and austenite/cementite interfaces, and the modification of the pearlite driving force related to the change in carbon activity in austenite.

New Insights into Carbon Distribution in Bainitic Ferrite

Abstract A number of studies on bainite transformation at low temperature have revealed that bainitic ferrite formed super-saturated in carbon. The most sensible explanation to this is the increased solubility of a tetragonal ferrite lattice, by virtue of synchrotron radiation and X-ray diffraction results, as well as ab-initio calculations. The question is if this increased tetragonality in bainitic ferrite is the result of a disordered distribution of carbon atoms in ferrite or the result of local carbon clustering (ordering) in association with a locally increased tetragonality. This development of carbon-enriched and carbon-depleted zones that leads gradually to the formation of a modulated structure was reported in the early stages of decomposition of martensite. In the present work, new experimental and theoretical results on the distribution of carbon in bainitic ferrite will be shown trying to shed new light on the nature of bainite transformation.