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New Developments in High Quality Grey Cast Irons

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New Developments in High Quality Grey Cast Irons Celebrating the th Anniversary 2004-2014 Special Report 10 Vol.11 No.4 July 2014 CHINA FOUNDRY New developments in high quality grey cast irons *Iulian Riposan, Mihai Chisamera, and Stelian Stan POLITEHNICA University of Bucharest, Romania Abstract: The paper reviews original data obtained by the present authors, revealed in recent separate publications, describing specific procedures for high quality grey irons, and reflecting the forecast needs of the worldwide iron foundry industry. High power, medium frequency coreless induction furnaces are commonly used in electric melting grey iron foundries. This has resulted in low sulphur (<0.05wt.%) and aluminium (<0.005wt.%) contents in the iron, with a potential for higher superheating (>1,500 °C), contributing to unfavourable conditions for graphite nucleation. Thin wall castings are increasingly produced by these electric melt shops with a risk of greater eutectic undercooling during solidification. The paper focused on two groups of grey cast irons and their specific problems: carbides and graphite morphology control in lower carbon equivalent high strength irons (CE=3.4%-3.8%), and austenite dendrite promotion in eutectic and slightly hypereutectic irons (CE=4.1%-4.5%), in order to increase *Iulian Riposan their strength characteristics. There are 3 stages and 3 steps involving Born in 1948, Ph.D, Professor. graphite formation, iron chemistry and iron processing that appear Major research areas: lamellar, to be important. The concept in the present paper sustains a three- nodular, compacted/vermicular and stage model for nucleating flake graphite [(Mn,X)S type nuclei]. coral graphite irons-processing There are three important groups of elements (deoxidizer, Mn/S, and and complex characterization; inoculant) and three technological stages in electric melting of iron austempered castingirons with LG, (superheat, pre-conditioning of base iron, final inoculation). Attention NG, CG or coral graphite; cast iron is drawn to a control factor (%Mn) x (%S) ensuring it equals to 0.03 matrix composited; new modifying techniques for cast irons; cupola – 0.06, accompanied by 0.005wt.%–0.010wt.% Al and/or Zr content and electric furnaces operations. in inoculated irons. It was found that iron powder addition promotes Publications: over 250 published austenite dendrite formation in eutectic and slightly eutectic, acting papers; three books (CG Iron – 1984, as reinforcement for the eutectic cells. But, there is an accompanying White Irons – 1985 and Bainitic Irons possible negative influence on the characteristics of the (Mn,X)S - 1989) and one textbook (Cast Irons type graphite nuclei (change the morphology of nuclei from polygonal - 1985); 35 Romanian patents. He compact to irregular polygonal, and therefore promote chill tendency is now the President of Romanian in treated irons). A double addition (iron powder + inoculant) appears Foundry Technical Association to be an effective treatment to benefit both austenite and graphite (ATTR). nucleation, with positive effects on the final structure and chill tendency. Key words: grey iron; S; Al; Zr; Ti; electric melting; furnace superheating; preconditioning; inoculation; graphite nucleation; graphite morphology; carbides; dendritic austenite; iron powder CLC numbers: TG143.2 E-mail: [email protected] Document code: A Received: 2014-04-02 Article ID: 1672-6421(2014)04-351-14 Accepted: 2014-05-16 351 Sepecial Report Celebrating the th Anniversary 2004-2014 CHINA FOUNDRY Vol.11 No.4 July 2014 10 n 2012, a year after world casting volume exceeded pre- strength, most engineering grey irons are hypoeutectic I2008 levels, global castings production increased to more compositions. The highest strength is obtained with the lower than 100 million metric tons (Fig. 1) [1]. Grey cast iron is still carbon equivalent levels (CE=3.4%-3.8%), maintaining the most used foundry metallic material, at 46% of this total reasonable thermal conductivity, vibration damping capacity world castings production. Grey iron is used to advantage in and machinability (Fig. 2a) [2,3]. The most important challenge the automotive industry, because of its excellent properties for these iron castings, especially for high cooling rates such as machinability, heat conductivity and vibration damping associated with thin wall castings, is the increased tendency to capacity. These properties in combination with reasonable carbide formation, in metastable solidification conditions. The strength are why grey iron has maintained its position as the promotion of type-A graphite morphology also requires more primary choice for the production of vital engine components process management. for vehicles. In this respect, emphasis on thin wall castings Near eutectic to slightly hypereutectic composition irons can be seen as the current trend in automotive engineering (CE = 4.1%-4.5%) show the best combination of castability, applications. tensile strength and thermal properties for thin wall castings In order to satisfy the designer’s demands for component (Fig. 2b) [2,3]. A critical problem for these irons is the limited 120 50 (a) (b) Gray iron 110 40 80 tons 30 60 tons Ductile iron 20 Million 40 Million Aluminum 20 10 Steel 0 1 0 2004 2005 2006 2007 2008 2009 2010 2011 2012 19981999200020012002 2003 2004 2005 2006 2007 2008 2009 2010 201 2012 Years Years (c) Production rate/Years (d) ) 70 Grey lron Ductile lron Steel Non ferrous (% 60 17% nonferrous: 17.1 million tons 50 rate 40 30 11% steel: 46% gray iron: 46 million tons 20 11.3 million tons Production 10 0 25% ductile iron: 9 1 25.2 million tons 1998 19992000 20012002 2003 2004 2005 2006 2007 2008 200 2010 201 2012 Years Fig. 1: Foundry metallic materials evolution worldwide (a, b,c) and 2012 structure (d) [1] 500 - - 160 (a) Hypo eutectic irons Hyper (b) Hypo-eutectic irons eutectic 140 400 irons R 120 m 300 100 ρ HB * 80 d * * * 200 * * E 60 λ 40 Hyper- 100 δ 20 eutectic irons 0 . 0 3334 35 36 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 CE (%) 33. 34. 35. 36. 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 CE (%) Fig. 2: Influence of CE on mechanical (a) and physical characteristics (b) of grey cast irons [Rm - Tensile Strength (MPa); E - Elastic Modulus (GPa); HB - Brinell Hardness; λ - Thermal Conductivity (W·m-1·k-1); d - Density (g·cm-3 × 10); ρ-Electrical Resistivity (Ω·mm2·m-1 × 102); δ - Vibration Damping Capacity (10-4)] 352 Celebrating the th Anniversary 2004-2014 Special Report 10 Vol.11 No.4 July 2014 CHINA FOUNDRY tensile strength, due to the low amount of austenite dendrite residuals (Cr, Mo, Pb, Sn, N etc.), at high superheat (>1,500 formation during primary solidification and the formation of a oC) for thin wall castings. The combination of such unsuitable pearlitic matrix during eutectoid transformation, respectively. conditions for solidification with low eutectic undercooling The investigation found that the optimum tensile strength leads to an exaggerated risk of carbides and undercooled could be obtained in as-cast grey iron by avoiding carbides, graphite morphologies forming, especially if the carbon increasing the amount of primary austenite dendrites, refining equivalent is well below the eutectic value. the eutectic cell size, maximizing the percentage of medium The objective of this paper is to review original data sized type A graphite and establishing an entirely fine pearlitic obtained by the present authors, to respond to the observed matrix structure, at pearlite spacing less than 0.8 mm. important worldwide changes in grey iron melting technology, Important changes in worldwide grey iron castings with effective metallurgical treatments to compensate for production have been observed: these conditions. The findings affect carbides, eutectic cells (1) Historically: cupola melted irons are characterized by a and graphite morphology control in hypoeutectic irons chemistry, including > 0.5wt.% Mn, > 0.05wt.% S, > 0.005wt.% and austenite dendrite promotion in eutectic and slightly Al, (%Mn) × (%S) > 0.03, providing favourable conditions for hypereutectic irons. MnS-type compounds, which are found to be major nucleation sites for flake graphite and correspondingly low chill tendency in grey cast irons. 1 Hypoeutectic grey cast irons (2) Small and/or inefficient cupolas have been replaced by a Hypoeutectic grey cast irons have traditionally been produced new generation of acid lined coreless induction furnaces [200 because of their greater mechanical properties due to the -1000 Hz frequency, > 250 kW/ton specific power]. These higher austenite dendrite/eutectic cells ratio. They can be furnaces became successful replacements for the smaller produced without carbides, while having a refined Type A cupolas because they have a high melting rate, do not require graphite morphology, with a finer eutectic cell size (increased a molten heel, can melt high steel charges with reasonable cell count). stirring capacity, and with the ability to reach superheating In commercial grey irons, solidified in non-equilibrium o temperatures > 1,500 C. The tapped iron chemistry is conditions, typical for foundry mould media, an as-cast characterized by > 0.5wt.% Mn, < 0.05wt.% S, < 0.005wt.% structure is generally heterogeneous, including different Al, (%Mn) × (%S) < 0.03, which does not support suitable amounts of carbides, different graphite morphologies and MnS-type compound formation. As a result, high chill and/or variable pearlite/ferrite ratios [Fig. 3(a)]. undercooled graphite formation tendencies increase. Final casting structure could be totally changed to the most (3) Producing grey, ductile and compacted graphite irons in favourable parameters such as a homogeneous structure, the same foundry with the same furnace or from the same heat including Type-A graphite, a pearlite matrix with very limited increasingly represents actual industry practice, with important or no carbides [Fig. 3(b)], by applying effective molten iron and inarguable advantages [a single low sulphur base iron, inoculation. o using a single inoculant, > 1,500 C furnace bath temperature].
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