Microcapillary Features in Silicon Alloyed High-Strength Cast Iron1

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Microcapillary Features in Silicon Alloyed High-Strength Cast Iron1 R.K. Hasanli1,a, S.N. Namazov2,b 1 – Associated professor, Dr., Azerbaijan Technical University, Baku, Azerbaijan 2 – Professor, Dr., Azerbaijan Technical University, Baku, Azerbaijan a – hasanli_dr@mail.ru b – subhan_namazov@daad-alumni.de DOI 10.2412/mmse.89.99.501 provided by Seo4U.link

Keywords: alloyed, high-strength cast iron, metal form, segregation, structure.

ABSTRACT. Present study explores features of silicon micro capillary in alloyed high-strength cast iron with nodular graphite (ductile iron) produced in metal molds. It identified the nature and mechanism of micro liquation of silicon in a ductile iron alloyed with Nickel and copper, and demonstrated significant change of structural-quality characteristics. It was concluded that the matrix of alloyed ductile iron has a heterogeneous structure with cross reinforcement and highsilicon excrement areas.

Introduction. High quality of iron castings depends largely on the structure and properties, including the nature of the distribution of silicon. However, the structure of high-strength cast iron with nodular graphite (ductile iron) produced in the metallic form, and the nature of silicon micro liquation in it has not been studied. Therefore, research in this direction represents both scientific and practical interest.

Analyses of the Microcapillary Features in Silicon Alloyed High-Strength Cast Irons. Method of etching in an alkaline solution of sodium picrate was used to study microrespirometry of silicon in cast iron. The study of micro liquation of silicon in ductile iron, cast in the mold, showed that there is a significant difference from micro liquation in ductile iron, cast in sand form. Analysis of ductile iron chill in the cast state showed that the iron acquires the structure of a white cast iron consisting of perlite and ledeburite (Fig.1).

Fig. 1. The microstructure of unalloyed ductile iron, x300.

1

© 2017 The Authors. Published by Magnolithe GmbH. This is an open access article under the CC BY-NC-ND license http://creativecommons.org/licenses/by-nc-nd/4.0/

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Ductile iron, cast in the mold, unalloyed Nickel and copper are significantly different from unalloyed, cast in the mold (Fig.2). Introduction of iron Nickel in an amount of from 1.0 to 2.0% increases the chill cast iron and refines eutectic grains (Fig.3).

Fig. 2. The microstructure of ductile iron, alloyed with 1.0% Ni and 0.5% Cu, x300.

Fig. 3. The microstructure of ductile iron, alloyed with 1.0% Ni, x300.

It is established that the characteristic distribution of silicon in the alloy, chill cast irons due to their cast structure and has a stable character which does not change after annealing, normalizing and tempering. The area of metal, representing annealing of cementite, retains their chemical composition with low content of silicon. It was determined that the copper and Nickel with temperatures of crystallization increase the activity of carbon (like silicon) and therefore increase its distribution in the crystal lattice of iron.

The distribution of silicon in the Nickel-copper cast irons, cast into the metal mold and sand mold, close to equilibrium. Only a small region along the boundaries of austenitic grains has a lower content of silicon. However, in this case, the contrast in the color of the cone is negligible.

In the Nickel-irons non-uniform distribution of silicon was observed, a reflective cast of austenite ledeburite structure. The size of the striped areas (concentration of Si) decreases with increasing Nickel content in the alloy. However, such heterogeneity does not cause deterioration of properties that would be expected in accordance with existing views on the impact of micro liquation silicon on notch toughness (COP) at low temperatures.

It is found that in chill alloyed and unalloyed cast iron with partial chill the most enriched silicon are at the boundaries of the areas occupied by stable and metastable eutectics. The high

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

concentration of silicon is observed between colonies of plastic ledeburite. Graphite is observed around the highest concentration of this element.

Unlike half-iron, with the cross-cutting chill, silicon is concentrated mainly in high-silica areas, namely between the eutectic colonies in complex eutectics, one of the phases in which the silicocarbide (BCC) or carbide in eutectoid (C+SC), acanthophyllum cementite. In primary and eutectic austenite (pearlite) content of silicon is lowered.

In works [1], [2], [3], [4], [5] it was proposed that the annealing leads to equalization of chemical composition upon exposure of the alloy in the austenite region. However, there is another point of view according to which crystallization occurred when the heterogeneity of the chemical composition is very stable and fundamentally cannot be eliminated by heat treatment and, in particular, annealing [6], [7], [8], [9].

When etching of annealed cast iron in picrate of sodium, we found a very peculiar picture of micro capillary silicon. There is a clear alternation of the areas enriched and depleted in silica, the first of which are arranged around graphite inclusions, the second - in places where before annealing the cementite existed.

Mutual arrangement, configuration and size of the combined silicon regions coincide with the location and shape of the plates of cementite or ledeburite colonies. At the edges of the casting is observed banded pattern characteristic of directional solidification, and in the center - homogeneous composition of the zone close to equated.

Thus, it is established that during annealing there is a redistribution of silicon and this is the most enriched region adjacent to the graphite. However, it is clear that the redistribution occurs only within a former permitting (austenitic crystallization) regions.

In the locations of the eutectic of cementite, the content of silicon is reduced and after austenitization of the metal substrate, as well as the final thermal treatment – normalization. This result is consistent with the view stating that when crystallization occurs chemical polarization, caused by different affinity of the elements to carbon [10], [11], [12].

It is established that carbide-forming elements (Mn, etc.) during the crystallization of the concentrate in the cementite promote graphitization (Si, Ni, Cu, etc.) in the austenite and this polarization is very stable. Given the suggested it can be argued that the revealed micro liquation in the picture reflects an inhomogeneous distribution of not only silicon, but also other elements.

Thus, high-strength cast iron can be considered as a composite material having a heterogeneous structure with a cross reinforcement consisting of alternating high-silicon regions having a high hardness and brittleness, and almost excrement, more plastic granules, probably doped with manganese. Such arrangement of the matrix and reinforcing phases in combination with the presence of solid lubricants – graphite meets the Sharpie rule and may be one of the prospective directions in creation of wear-resistant cast iron with a heterogeneous structure.

Detected feature in the distribution of silicon can certainly affect the Mechanical properties of the material. It was established that higher mechanical properties are observed at full, or a significant chill (exceeding 50%) alloyed cast irons in the cast state. During subsequent thermal treatments bleached and half-alloy cast irons are mainly pearlite structure with spheroidal graphite.

This structure iron is the most desirable to improve its properties. In addition, the size of the homogeneous micro regions and graphite inclusions with the increase in the rate of supercoiling of the alloy crystallizing decreases, which also positively affects the properties of cast iron.

All this makes it possible to express an opinion of the relative influence of micro heterogeneity of chemical composition on the properties of cast iron. Meanwhile, the generally accepted view that segregation in cast iron is undesirable as it decreases the ductility of the material. Moreover, even after annealing to achieve complete homogeneity of the cast iron chemical composition is not possible.

Mechanics, Materials Science & Engineering, July 2017 – ISSN 2412-5954

Therefore, in our opinion, one should strive to create such a structure of iron, in which a homogeneous chemical composition in micro-area would be very fine and brittle and ductile zones efficiently combined, forming a relatively micro heterogeneous structure. Such a picture is obtained by chill casting magnesium cast irons subjected to annealing, which increases their properties compared with the cast irons of similar composition, obtained by casting in sand mold.

Summary. The study of the processes of structure formation in ductile iron casting in the mold confirms the accuracy of the choice of complex dopants, in an amount of 1.0% Ni and 0.5% Cu that are responsible for the development of cast metal parts applied in conditions of friction, wear and elevated mechanical loads. These working conditions are typical for parts of oilfield equipment.

Based on the research of micro capillary silicon and other alloying elements it was proposed that ductile iron can be considered as a composite material having a heterogeneous structure with alternation of regions with different silicon content.

References [1] I.P. Bunin, Y.N. Malinochka, B.P. Taran. Fundamentals of metallography of cast iron. Moscow, Metallurgy, 1998, 413 p.

[2] V.A. Ilyinsky, A.A. Zhukov and others. New in the theory of graphitization. The relationship between primary and secondary crystallization graffitists iron-carbon alloys // Metallography and heat treatment of metals, 2001, No.10. P.10-16 [3] High-strength cast iron with nodular graphite. Theory, production technology, properties and applications / ed. by M.V. Voloshchenko. Kiev: Sciences. Dumka, 2004, 203 p.

[4] R.K. Hasanli. Structure and properties of ductile iron. Baku, Science, 2013, 252 p.

[5] R.K. Hasanli Peculiarities of structure and phase composition of heat-treated high-strength cast irons with nodular graphite // Journal of mechanical engineering, 2013, No. 10, pp. 31-33 [6] A.I. Belyakov and others. Production of castings from high-strength nodular cast iron. M., Mechanical Engineering, 2010, p. 712 [7] R.K.Hasanli. High-strength cast iron with nodular graphite. Baku: Science, 1998, 203 p.

[8] V.V. Dubrov and others, The use of high-strength cast iron in valve. In proc. High-strength cast iron with nodular graphite. Kiyev, Naukova Dumka, 1998, pp. 78-81.

[9] E.A. Silva, L.F.V.M. Fernandes, N.A.S. Sampaio, R.B. Ribeiro, J.W.J. Silva, M.S.Pereira (2016), A Comparison between Dual Phase Steel and Interstitial Free Steel Due To the Springback Effect. Mechanics, Materials Science & Engineering Journal Vol.4, Magnolithe GmbH, DOI: 10.13140/RG.2.1.3749.7205 [10] L. I. Éfron, D. A. Litvinenko (1994), Obtaining high-strength weldable steels with bainite structure using thermomechanical treatment, Metal Science and Heat Treatment, Vol. 36, Is. 10, Springer, DOI 10.1007/BF01398082 [11] I.N. Bogachev, R.I. Mints Cavitation-erosion fracture of cast iron. Sat. Theory and practice of foundry production, Ural Polytechnic Institute, vol. 89, 1999, pp. 71-78.

[12] L.P. Ushakov Wear-resistant cast iron with spheroidal graphite. M., Mechanical engineering, 2005, 153 p., DOI 10.1007/BF01398082