Area Ingenieria

Durango, Spain

Area Ingenieria

Durango, Spain

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Alonso G.,Area Ingenieria | Stefanescu D.M.,Ohio State University | Stefanescu D.M.,University of Alabama | Larranaga P.,Area Ingenieria | And 3 more authors.
TMS Annual Meeting | Year: 2015

The tensile strength of near-eutectic gray iron can be increased from 230-300 to 300-345MPa, without a significant increase in hardness, through 0.3-0.4%Ti addition to low sulfur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfine-interdendritic-graphite (SIG), characterized by a fine (10-20μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analyzed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on graphite substrates at the austenite/liquid interface because of carbon supersaturation.


Alonso G.,Area Ingenieria | Stefanescu D.M.,Ohio State University | Stefanescu D.M.,University of Alabama | Larranaga P.,Area Ingenieria | Suarez R.,Veigalan Estudio 2010 SLU
International Journal of Cast Metals Research | Year: 2016

While the manufacture of compacted graphite (CG) iron castings has seen significant expansion over the recent years, the growth of CG during iron solidification is still not fully understood. In this work, effort was expanded to experimentally reveal the evolution of graphite shape during early solidification and its relationship to the solid fraction. To this purpose, interrupted solidification experiments were carried out on hypereutectic irons with three magnesium levels. The graphite shape factors were measured and analysed as a function of chemical composition and solid fraction. Scanning electron microscopy was carried out to establish the fraction of solid at which the transition from spheroidal graphite (SG) to CG occurs. It was confirmed that solidification started with the development of SG for all CG irons. The SG-to-CG transition was considered to occur when the SG developed a tail (tadpole graphite). The findings were integrated in previous knowledge to attempt an understanding of the solidification of CG iron. © 2016 Informa UK Limited, trading as Taylor & Francis Group.


Alonso G.,Area Ingenieria | Stefanescu D.M.,Ohio State University | Larranaga P.,Area Ingenieria | De la Fuente E.,Area Ingenieria | And 2 more authors.
International Journal of Cast Metals Research | Year: 2016

The tensile strength of near-eutectic grey iron can be increased from 230–300 to 300–345 MPa, without a significant increase in hardness, through 0.3–0.4%Ti addition to low sulphur (<0.01%S) iron. This is due to the combination of higher primary austenite/eutectic ratio and the precipitation of superfine-interdendritic-graphite (SIG), characterised by a fine (10–20 μm) and highly branched fibrous structure. To reveal the influence of the %Ti on graphite shape evolution during solidification and its relationship to the solid fraction, quenching experiments at successive solidification stages were carried out on hypoeutectic alloys with 0.18% and 0.32%Ti. The graphite shape factors were measured, and their evolution as a function of the titanium content and the solid fraction was analysed. SEM was used to evaluate the change in graphite shape during early solidification, as well as its nucleation and growth. The correlation between the oxygen in the melt and SIG formation was also explored. It was concluded that nucleation of graphite in SIG irons occurs on carbon rich regions at the austenite/liquid interface and, sometimes, on titanium carbides. Solidification velocity-undercooling curves were used to explain the transition from lamellar (type-A), to interdendritic (type-D), to SIG, and then to coral graphite. © 2016 Informa UK Limited, trading as Taylor & Francis Group


Stefanescu D.M.,Ohio State University | Stefanescu D.M.,University of Alabama | Alonso G.,Area Ingenieria | Larranaga P.,Area Ingenieria | And 2 more authors.
Acta Materialia | Year: 2016

Extensive SEM work was carried out on deep etched specimens to reveal the evolution of graphite shape in Fe-C-Si alloys of industrial composition during early solidification and at room temperature. The samples had various magnesium and titanium levels designed to produce graphite morphologies ranging from coarse lamellar to interdendritic lamellar to mixed compacted - spheroidal. The present findings were then integrated in previous knowledge to produce an understanding of the crystallization of lamellar, compacted and spheroidal graphite. It was demonstrated that most forms of graphite grow radially from a common center, most of the time as foliated dendrites (see Figure). The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets with nanometer height and micrometer width. During solidification, thickening of the platelets occurs through growth of additional graphene layers nucleated at the ledges of the graphite prism. In the magnesium-free irons that graphite platelets assemble into foliated crystals and dendrites, forming graphite plates that grow along the a-axis. In the magnesium-modified melts the graphite platelets stack along the c-axis, producing clusters with random orientation. The clusters are then assembled into quasi-cylindrical shapes connected to more or less curved walls to form tadpole graphite, compacted graphite, or chunky graphite. If enough magnesium is added, conical sectors made of platelets stacked in the c-direction grow from the same nucleus. The conical sectors may occupy the whole volume of the sphere forming a graphite spheroid, or only part of it like in chunky graphite. The large number of cavities observed between the platelets is consistent with growth of foliated dendrites. © 2016 Acta Materialia Inc.


Stefanescu D.M.,Ohio State University | Stefanescu D.M.,University of Alabama | Alonso G.,Area Ingenieria | Larranaga P.,Area Ingenieria | Suarez R.,Area Ingenieria
Acta Materialia | Year: 2016

Extensive effort was expanded to elucidate the growth and morphology of the stable eutectic grains during early solidification of continuous cooled Fe-C-Si alloys. To this purpose, quenching experiments at successive stages during solidification have been carried out on five cast irons with various magnesium and titanium levels designed to produce graphite morphologies ranging from lamellar to mixed compacted-spheroidal. The graphite shape factors were measured on the metallographic samples, and their evolution as a function of the chemical composition and the solid fraction was analyzed. Extensive scanning electron microscopy was carried on to evaluate the change in graphite shape during early solidification, to establish the fraction of solid at which the transition from spheroidal-to-compacted-to-lamellar graphite occurs, and to outline the early morphology of the eutectic grains. It was confirmed that solidification of Mg containing irons started with the development of spheroidal graphite even at Mg levels as low as 0.013 mass%. Then, as solidification proceeds, when some spheroids developed one or more tails (tadpole graphite), the spheroidal-to-compacted graphite transition occurs. The new findings were then integrated in previous knowledge to produce an understanding of the eutectic solidification of these materials. It was concluded that in hypoeutectic lamellar graphite iron austenite/graphite eutectic grains can nucleate at the austenite/liquid interface or in the bulk of the liquid, depending on the sulfur content and on the cooling rate. When graphite nucleation occurs on the primary austenite, several eutectic grains can nucleate and grow on the same dendrite. The primary austenite continues growing as eutectic austenite and therefore the two have the same crystallographic orientation. Thus, a final austenite grain may include several eutectic grains. In eutectic irons the eutectic grains nucleate and grow mostly in the liquid. The eutectic austenite has different crystallographic orientation than the primary one. The solidification of the austenite/spheroidal graphite eutectic is divorced, with graphite spheroids growing on primary austenite dendrites. The eutectic austenite grows on the primary austenite and has the same crystallographic orientation. The result is large austenite (primary and eutectic) dendrites that incorporate numerous graphite spheroids. A eutectic grain cannot be defined. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.


Stefanescu D.M.,Ohio State University | Stefanescu D.M.,University of Alabama | Huff R.,Caterpillar Inc. | Alonso G.,Area Ingenieria | And 3 more authors.
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2016

Extensive SEM work was carried out on deep-etched specimens to reveal the evolution of compacted and chunky graphite in magnesium-modified multicomponent Fe-C-Si alloys during early solidification and at room temperature. The findings of this research were then integrated in the current body of knowledge to produce an understanding of the crystallization of compacted and chunky graphite. It was confirmed that growth from the liquid for both compacted and chunky graphite occurs radially from a nucleus, as foliated crystals and dendrites. The basic building blocks of the graphite aggregates are hexagonal faceted graphite platelets with nanometer height and micrometer width. Thickening of the platelets occurs through growth of additional graphene layers nucleated at the ledges of the graphite prism. Additional thickening resulting in complete joining of the platelets may occur from the recrystallization of the amorphous carbon that has diffused from the liquid through the austenite, once the graphite aggregate is enveloped in austenite. With increasing magnesium levels, the foliated graphite platelets progressively aggregate along the c-axis forming clusters. The clusters that have random orientation, eventually produce blocky graphite, as the spaces between the parallel platelets disappear. This is typical for compacted graphite irons and tadpole graphite. The chunky graphite aggregates investigated are conical sectors of graphite platelets stacked along the c-axis. The foliated dendrites that originally develop radially from a common nucleus may aggregate along the c-axis forming blocky graphite that sometimes exhibits helical growth. The large number of defects (cavities) observed in all graphite aggregates supports the mechanism of graphite growth as foliated crystals and dendrites. © 2016 The Minerals, Metals & Materials Society and ASM International

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