J. Cornide

Influence of bainite morphology on impact toughness of continuously cooled cementite free bainitic steels

Abstract 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.

Distribution of dislocations in nanostructured bainite

The dislocation density in ferrite and austenite of a bainitic microstructure obtained by transformation at very low temperature (300 °C) has been determined using transmission electron microscopy. Observations revealed that bainitic ferrite plates consist of two distinctive regions with different substructures. A central region in the ferrite plate is observed with dislocations that may result from lattice-invariant deformation at the earlier stage of bainite growth. As plastic deformation occurs in the surrounding austenite to accommodate the transformation strain as growth progresses, the Ferrite/Austenite interface has also a very distinctive dislocation profile. In addition, atom-probe tomography suggested that dislocation tangles observed in the vicinity of the ferrite/austenite interface might trap higher amount of carbon than single dislocations inside the bainitic ferrite plate.

Composition Design of Nanocrystalline Bainitic Steels by Diffusionless Solid Reaction

NANOBAIN is the term used to refer to a new generation of advanced steels capable of producing by isothermal transformation at low homologous temperatures, T/Tm∼0.25 where Tm is the absolute melting temperature, a nanocrystalline microstructure, composed exclusively of two phases, thin plates of bainitic ferrite separated by C enriched austenite. Such alloys are exclusively designed on the basis of bainitic transformation theory and some physical metallurgy principles. In this work, by designing a new set of alloys capable of producing such microstructure, a further step toward the industrialization of NANOBAIN is taken. Some important industrial requirements, including circumventing the inclusion of expensive alloying elements and the need for faster transformations, are also considered. For all the alloys, the experimental results validate the design procedure and they illustrate that the NANOBAIN concept is a step closer to industrialization, probing that it is possible to obtain nanocristalline bainite in simpler alloy systems and in shorter times than those reported previously.

Toughness of Advanced High Strength Bainitic Steels

Carbide free bainite has achieved the highest strength and toughness combinations to date for bainitic steels in as-rolled conditions. By alloying designing and with the help of phase transformation theory, it was possible to improve simultaneously the strength and toughness because of the ultra-fine grain size of the bainitic ferrite plates. Ultimate tensile strengths ranging from 1600 MPa to 1800 MPa were achieved while keeping a total elongation higher than 10 %. Their toughness at room temperature matches tempered martensitic steels, known to be the best-in-class regarding this property. However, it has been observed that the presence of coalesced bainite leads to a dramatic deterioration in toughness in these novel high strength bainitic steels.

Structural and Microstructural Characterization of CoCrFeNiPd High Entropy Alloys

According to literature, a High Entropy Alloy (HEA) has close to equimolar composition and forms mostly fcc and/or bcc phases as well as solid solutions, i.e. the elements take random occupations on available lattice sites. In this paper we report studies on HEAs of the CoCrFeNiPd system. All alloys have been found to, contrary to what has been reported earlier in literature, consist of four different phases, three of them of fcc type. The relative amounts of the different phases depend on Pd concentration. The different phases seem to be fully interconnected.

Structure and Properties of Some CoCrFeNi-Based High Entropy Alloys

The understanding of the structure and the stability of high entropy alloys is still incomplete and the mechanism behind the composition-property relationship is unclear. One reason is that few systematic and accurate determinations of the composition-dependent structure on the atomic level and of the physical properties have been made. In this paper we report on the structure and physical properties of CoCrFeNi and CoCrFeNi-X, (X=Pd, Sn, Ru) alloys of equimolar composition using different experimental techniques (microscopy, neutron and X-ray diffraction, atom probe tomography, Mossbauer spectroscopy, calorimetry). The results show that i) the alloys are not completely homogeneous as is generally suggested in existing literature; ii) they do not form a perfect solid solution; iii) their structure is not single phase, even not either fcc or bcc.

Evolution of Microstructure and Mechanical Properties during Tempering of Continuously Cooled Bainitic Steels

With the increasing demand for high performance engine or suspension components, bainitic steels are receiving interest as potential replacement of their quench and tempered counterparts. Indeed, for a number of mechanical components, ferrite pearlite microstructures are no longer sufficient in terms of mechanical properties. Bainitic steel grades allow production of hot-rolled bars or forged components exhibiting a homogeneous bainitic microstructure and achieving UTS up to 1200 MPa without the need for additional heat-treatments [1]. During tempering, these V-microalloyed bainitic steels exhibit unusual yield strength variations, with a very pronounced increase around 250-300 °C followed by the better known secondary hardening peak for temperatures around 600-650 °C. Indeed, after tempering at 250-300 °C, some of these steels exhibit an increase in yield strength of up to 200 MPa, concurrent with an increase in impact toughness of up to 25%. This, however, goes unnoticed if hardness measurements are used to characterize tempering. In the following, results are presented for three different bainitic steel grades, and the origins of the changes in mechanical properties are discussed.