M. M. Aranda

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.

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.