1. Process of making SG iron

Transcription

1 Researches on an Advanced Engineering Material for Automobile Industry: SG iron with better Mechanical Properties and higher friction obtained by post innoculation of tin and casting in vacuum ( depression ) mould By *Prof. Dr. Pervaiz Habibullah Spheroidal (nodular or ductile) graphite iron; SG iron, has an as-cast structure containing graphite particles in the form of small rounded, spheroidal, globular or nodular particles in a ductile metallic matrix. It has been long established that all of the mechanical and physical properties of SG iron are a result of the graphite in the spheroidal/nodular shape. Late additions in SG iron of innoculants are nowadays well established in almost all the ferrous foundries. Industrial products of SG iron are ductile, durable, resistant to wear and fracture and their mechanical properties may further be altered, as per requirement, by post innoculation, with different elements. We have innoculated the SG iron with a minute amount of Sn, cast in vacuum ( depression ) mould and have studied the behaviour of tin in SG iron. 1. Process of making SG iron 1.1 Melting Return of SG iron scrap, cast iron having low sulphur and phosphorus and manganese is melting in a ladle of 60kg capacity in a gas fired furnace for 3-4 hours and ladle is taken out of the furnace and following ingredients are added: Mg.FeSi = 1.3lg Steel scrap = 3kg Desulphurizer = 0.2kg Fe.Si = 1kg Charge Composition of the charge is as follows: Carbon = 3.36% Silicon = 2.3 Manganese = 0.48 Phosphorous = Sulphur = Sn = 0.2 Fe = Balance *Former G.M. Coord (Tech), Regional Headquarters (North), AIIP, CAA, Pakistan

2 1.2 Process Spherdizing methods (Fe.Si) i) Ladle transfer method (sandwich method) ii) Covered ladle method iii) Treatment with porous plug or other stirring method The magnesium master alloy is placed on the bottom of the empty treatment ladle and liq. iron is poured over it. A popular variation is the sandwich process where, small steel pieces, about 2% of the total melt, are placed over the master magnesium alloy. During heating, melting and superheating approx 2% steel is consumed. We utilized sandwich process which is very much common for production of SG iron and uses pouring of hot molten metal at about o C in ladle which contains the nodulant under the innoculants coverage. The reaction with the molten metal is violent. Magnesium acts as a deoxidizer (MgO) and desulphurizer (MgS). Si acts as graphitizer and promotes nucleation sites for nodules. The phenomenon for which SG iron (graphite instead of becoming flakes become nodule) is still unknown and various theories have been put forth but none has sustained upto this time. Desulpherization These are the compounds that remove sulphur and are added in the melt whenever required: 1. Caustic soda (NaOH) 2. Soda Ash (NA2CO 3 ) 3. Burnt lime (CaO) 4. Limestone (CaCO 3 ) 5. Calcium carbide (CaC 2 ) 6. Calcium Cyanide (CaCN 2 ) Post innoculation (Sn): Soon after the spherdizing treatment, when ladle is transfer to the mould line, 0.2% Sn is added in the ladle before pouring. Before addition the tin should be heated to eject moisture and air from its pores and it should be red hot. Tin will flare and silvery flames will appear. The liq metal then poured in the mould.

3 1.3 Moulding An automobile part, Differential gear box (see fig. 1), was cast in the specially designed mould box on which vacuum ( depression ) can be imposed. Specially designed mould box shown in fig. 2, was used. This type of moulding box is identical to the cope and drag commonly used in foundries except that on its one side a hole (1in. dia) is drilled which is connected on its exterior side to vacuum pump (-1 kgf/cm 2 ) and on the interior side it is connected with ½ dia perforated pipe, which is, made rectangular, giving sufficient space for pattern and encircling the mould cavity all around (see fig-2). The gates and risers of proper size are provided. Soon as the liq. SG iron was poured in the sand, the vacuum pump was turned on. In this way, the vacuum (depression or negative pressure) is imposed on the sand mould. It absorbs all the gas formed at the metal-mould interface mould and cores and reduces the temp. of the mould by 20 to 30%. In this way, the casting produced is free of blowholes and pinholes. 1.4 Product SG iron of the following composition is obtained. Carbon = 3.4% Silicon = 2.28% Manganese = 0.3% Magnesium = 0.05% Phosphorous = 0.04% Sulphur = 0.02% Fe = Balance 2. Metallography and Mech. Properties* The microstructure of SG iron produced is given in fig. 3 to SG iron without any post innaculation or alloying Microstructure Microstructure reveals well-formed SG iron nodules uniformly distributed throughout the section thickness of the given sample. Nodule count ranges between * Pakistan Standards & Quality control Authority (TSC) Lahore - Pakistan

4 Fig. 1 Differential gear housing of SG iron innoculated with Sn cast in vacuum (depression) mould

5 New casting strategy for imposing vacuum depression in green sand mould Mould box Moulding sand ½ dia pipe with orifices (for imposing vacuum) vacuum pump Casting Fig.2 Specially designed mould box for imposing vacuum depression in green sand mould

6 Fig. 3 Microstructure of well formed SG iron, nodules uniformly distributed through the section thickness (nodule counts nodules /mm 2, average nodule size 20 ), with ferritic matrix

7 Fig. 4 Microstructure reveals well formed and evenly distributed graphite nodules. The nodule count ranges nodules/mm 2, nodule size ranges between microns. Etched 200 x Fig. 5 Microstructure of SG iron, post innoculated by Sn = 0.2%. Reveals fine pearlitic matrix with 10-20% ferrite dominantly around nodules. (bull eye structure)

8 nodules/mm 2, whereas average nodule size is about 20microns. Nodularization is almost complete. Etched specimen shows all ferritic grains matrix (Fig.3). Hardness = 128 BHN Tensile strength = 70 ksi Yield strength = 50 ksi Elongation = 6% 2.2 SG iron innoculated with 0.2% Sn Unetched: Microstructure reveals well formed and evenly distributed graphite nodules. The nodule count ranges nodules/mm 2, nodule size ranges between microns (fig. 4). Etched with 2% Nital: Microstructure reveals fine pearlitic matrix with 10-20% ferrite dominantly around the nodules i.e., bull eye structure (Fig.5) Hardness : 234 BHN Tensile strength: 58.2 ksi Yield Strength: 48 ksi Elongation: 1.419% Comparison of cast iron, SG iron and SG iron post innoculated is given in the table Electron Microscopy - SEM i) Fig. 6: Scanning electron micrograph: as-cast specimen of SG iron graphite morphology 3D (fully spheroidal; x 600, 20 m) ii) Fig. 7: Scanning electron micrograph: as-cast specimen of SG iron graphite morphology 3D (fairly spheroidal; x 600, 20 m) iii) Fig.7: Scanning electron micrograph: as-cast specimen of SG iron graphite morphology 3D (compacted / vermicular; x 200, 20 m) (Courtesy by B.I. Imasogie and U. Wendz, 2004)

9 3. Discussion- Effect of post innoculants on SG iron[10-16] and prevention of defects Lanthanum Post innoculation or alloying of some elements are nowadays practised in most of the SG iron foundries. Lanthanum element promotes the Equi axial solidification behaviour by (i) developing higher number of nucleation sites, thus providing more solidification sites within the molten metal (ii) increasing viscosity of the liq. metal by stirring motions within the molten iron (iii) restricting the growth of columnar grains. At a given solidification stage, when lanthanum is added, the equi axial solidification is developed. The thickness of the columnar solidification growth zone is reduced leaving large free flowing passages for maintaining liq iron to travel within the channels feeding casting areas. This phenomenon minimizes the shrinkage defect. Tin & Copper Effect of Tin additions upto 0.2% is SG iron were studied in the present research work. These levels of additions were sufficient to produce pearlitic structure in casting samples. Our researches have shown that tin additions promote pearlite as cast structure, however the ultimate tensile strength and elongation drops when tin is added beyond the point required to obtain a fully pearlitic matrix. However, hardness is increases (see table-1). The sensitivity of tin additions is somewhat reduced on normalizing. Heat treatment of SG iron increases the tensile strength of copper containing SG iron [10]. Further, tin and copper slow the response to ferritize annealing, when copper / tin ratio is greater than 6 or 7, annealing response is substantially improved reflecting, copper-tin interaction. Austinizing before annealing can improve mechanical properties similarly normalizing can significantly raise hardness, strength and impact resistance of castings containing tin and copper, with slight loss of elongation. Strength and hardness decrease with decrease in austinizing temp. from 940 to 816 o C; while elongation and impact resistance increase. Copper increases hardness and corrosion resistance of SG iron. In case of SG iron having more than 0.5% copper, increase in tin addition brings about increase in hardness [11-12].

10 Chromium Chromium raises strength of ferritize- annealed casting and reduces elongation and impact resistance. When SG iron containing 0.09% Cr is normalized, its strength is reduced, while normalizing 0.18% Cr brings about increase in strength and reduction in elongation, impact resistance. Lead (Pb) Pb has low melting point and transforms into fumes just after addition in the molten SG iron. However it may effect the properties. With % tin and % copper, the addition of lead upto 200ppm reduces strength and elongation; max. at ppm lead. With no tin, impact resistance decreases above 100ppm lead. With varied content of tin, copper and chromium, wide varieties of SG iron can be produced for various as cast or heat treated conditions. Other elements A modified thermal analysis was presented in ref [7] for observing the effect of alloy elements. Surface areas of nodular graphite on austenite decomposition of SG iron during isothermal holding o C completes in three stages: Developing ferrite plate in austenite matrix were observed on the basis of the calculated heat evolution of phase transformation i.e. nucleation and early growth, sidewise growth and branching of ferrite plates. Silicon addition increases the surface area of nodular graphite. However Mo, Cu, Ni, and Mn suppress ferrite formation in the austempered ductile iron. Mo is the most effecting alloying element in suppressing the decomposition of austenite during isothermal holding. Ductile iron containing V and Co when heat treated, the microstructure of the alloyed iron consisted of graphite nodules in ferrite matrix with fine dispersion. (20 80nm in dia ). These carbide particles improve strength and refine the grain size of ferrite, resulting in an iron of intermediate strength and high ductility. Investigation on ductile iron produced in the commercial foundries with varying contents of Si, Mn, Cu, Ni, Mo, and P have also concluded that tensile properties, hardness and microstructure impact, toughness are correlated with the composition and content of elements added.

11 Defects in SG Iron Compacted graphite within the structure The most common cause of appearance of compact graphite within the structure of SG iron, is the failure of complete nodulisation process. Use of unsuitable noduliser or while using correct noduliser, it has been added in an inadequate quantity, are main causes of this defect. Low Nodule count Nodule count depends upon the quantity of the innoculant. Avoiding long holding times in the furnace and prolonged pouring time, post-innoculation will result in consistent nodule counts. Nodule count depends upon the additions of innoculant in a proper quantity. Exploded graphite The exploded graphite is apparently appears similar to a nodule in SG iron but it is split and blown apart (fig. 9). Some rare earth, cerium, lanthanum, neodymium, praseodymium etc are beneficial in that they neutralize the effects of some detrimental trap elements such as Pb, Bi, Sb, Ti etc. Rare earth elements are also good nudularisers and promote the nudularization. However, they may not be utilized in excess because otherwise, they are an energetic source of exploded graphite. This is more especially when high purity charges are used which are low in impurities. Exploded graphite is normally found in thicker section castings with slow cooling rates or at very high carbon equivalent levels. Shrinkage The casting defect, shrinkage, in SG iron is mainly caused by sand systems and feeding & gating systems. Some shrinkage defects are contributed by the metallurgical factors such as composition, casting temp. innoculation and high magnesium residuals Gas porosity In the present experiment, we have cast differential gear box of SG iron innoculated with Sn, in the sand mould on which the vacuum was imposed (see text). Depression or negative pressure imposed on the sand mould absorbs all the mould gas formed at the metal-mould interface, mould and cores and reduces the temp. of the mould by 20 to 30%. In this way, the casting produced is free of blowholes and pinholes.

12 4. Conclusion 1. The results obtained showed clearly that the properties of the SG irons not only depend largely on the form and/or morphology of graphite precipitated in the casting but also on the alloying elements. However, the common defining characteristics of this group of materials is the morphological structure of graphite 2. Pearlite and ferrite ratio affects the tensile strength, yield strength and elongation of SG iron. The ferrite and pearlite ratio can be controlled through alloying, shake out temp. controls or post-casting heat treatment. 3. Effect of tin additions (upto 2%) was studied. This level of addition was sufficient to produce pearlitic structure in casting samples. Our researches have shown that tin additions promote pearlite as cast structure, however the ultimate tensile strength and elongation drops when tin is added beyond the point required to obtain a fully pearlitic matrix. The sensitivity of tin additions is somewhat reduced on normalizing. 4. Further, tin and copper slow the response to ferritize annealing, when copper / tin ratio is greater than 6 or 7, annealing response is substantially improved reflecting, copper-tin interaction. Austinizing before annealing can improve mechanical properties similarly normalizing can significantly raise hardness, strength and impact resistance of castings containing tin and copper, with slight loss of elongation. 5. Ductile iron containing V and Co when heat treated, the microstructure of the alloyed iron consisted of graphite nodules in ferrite matrix with fine dispersion. (20 80nm in dia ). These carbide particles produce dispersion strength and refine the grain size of ferrite, resulting in an iron of intermediate strength and high ductility. 6. Rare earth lanthanum element promotes the Equi axial solidification behaviour by (i)developing higher nucleation power, thus producing more solidification sites within the molten metal (ii) modifying the molten iron viscosity favouring stirring motions within the molten iron (iii) restricting the growth of columnar grains. 7. SG irons, post innoculated by 0.2% Sn and some other elements, such as Cu and V, were first time carried out in Pakistan, at WMZA Foundry, Lahore the leading

13 specialist in founding cast iron grades 14, 17, SG iron and non ferrous metals. The alloy made by post inoculation of SG iron with Sn was applied in the automobile industry and deferential gear box and diff. housings were cast and fitted with the heavy vehicles for their testing. As reported by the WMZA Foundry, no such complaint of failure of said parts, was reported. Acknowledgement Author is grateful to Ch. Ikram, Dy. Director, Pakistan Standards & Quality control Authority (TSC) Lahore, for his valuable cooperation in studying the metallography of newly proposed SG iron. In the same token, I am grateful to Mr. Waheed, Manager WMZA Foundry for extended the facility for using his ferrous foundry for casting experiments.

14 Bibliography 1. B.I. Imasogie and U.Wendt, Characterization of graphite particle shape in spheriodal graphite iron using a computer-based image analyzer, V Journal of Min. & Mat. Characterization & Engg. Vol. 3 No. 1, pp 1-12, ductile iron society ( 3. Harding R.A., Campbell J., Saunders N.J. The inoculation of ductile iron: a review of current understanding. Conference; solidification processing Sheffied July. 4. Imasogie, B.I., 2003, Optimum Ca-CaC 2 -Mg Master alloy Conc. Requirements in graphite nudularising treatment of Cast iron Mat. Engg. Vol. 14, No.1, pp L.A. Neumeier, B. A Betts in AFS Transaction (1976), Ductile Iron Containign Tin, Coipper and Other Contaminant. 6. Masato Tsujkawa, Norkazu Matsmoto, Koji Nakamoto, Yoshisada Michiura in Key Engineering Materials (1011), Pearlite Stabilization by Copper on Ductile Cast Iron. 7. N.K Datta, N.N. Engel in AFS Transaction (1963), Influence of Copper on Properties of Ductile Iron. 8. R Siclari, T Margaria, E Berthelet, J Fourmann, Keith Milllis Symposium on Ducitle Cast Iron (2003) France Micro-shrinkage in ductile iron / mechanism & solution). 9. Ruxanda R., Beltran0Sanchez I., Masson J. and Stefanescu D.M., AFS Transactions , Smith, William F.; Hashemi, Javad (2006) Foundations of Mat. Sci. and Engg. (4 th Ed. ) McGraw Hill, ISBN *Soforni L., I.I. Riposant I. Casting in Vacuum Mould (translation from Romanian Language) 12. *Solidmetal.net- SG Iron by Admin on Jan. 16, T. Levin, P.C Rosenthal, C.R. Loper Jr. R.W. Heine Tin and Copper in Ductile Iron, T.C. Rooney, C.C. Wang, P.C. Rosenthal, C.R. Loper Jr. R.W. Heine in AFS Transactions (1971), Tin and Copper in Gray Cast Iron. 15. T.R. Baruch, A. J. Stone, H.W. Lowine JR in AFS Transaction (1963), Influence of Copper on Properties of Ductile Iron 16. *wikipedia, ductile iron 17. Sofroni, L., Habibullah P, and others Improvement of surface quality of castings by creating vacuum in sand mould during pouring of liquid alloys. 52nd International Foundry Congress, Melbourne, Australia, Habibullah, P., Virk A Designing by computer modeling and casting of an antique of early Islamic period, in vacuum mould 69 th World Foundry Congress, China, Oct , 2010.

15 Fig. 6 Scanning electron micrograph of as cast specimen of SG iron graphite morphology 3D (fully spheroidal; x 600, 20 m) (B.I. Imasogie and U. Wendz, 2004) Fig. 7 Scanning electron micrograph of as cast specimen of SG iron graphite morphology 3D (fairly spheroidal; x 600, 20 m) (B.I. Imasogie and U. Wendz, 2004)

16 Fig. 8 Scanning electron micrograph of as cast specimen of SG iron graphite morphology 3D (compacted / vermicular; x 200, 20 m) (B.I. Imasogie and U. Wendz, 2004) Fig. 9 Exploded graphite unetched x 100

17 Table 1 Comparison of Mechanical properties of cast iron, SG iron and innoculated with tin (0.2% Sn) Type of iorn alloy Chemical Comp Ferritic C = 3.4% Si = 2.2% Mn = 0.7% Pearlitic C = 3.2% Si = 2.0% Mn = 0.7% Pearlitic C = 3.3% Si = 2.2% Mn = 0.7% Ferritic C = 3.5% Si = 2.2% Pearlitic C = 3.5% Si = 2.2% Martensitic C = 3.5% ( ) Si = 2.2% Pearlitic C = 3.34% Si = 2.2% Mn=0.3% Sn =0.2% Condition Microstructure Tensile strength Yield strength Gray iron Annealed Ferritic matrix 26ksi (179MPa) As cast As cast pearlitic matrix Pearlitic matrix 36ksi (252MPa) 42ksi (293MPa SG Irons Annealed Ferritic 60ksi 40 ksi (414MPa) (276MPa) As cast Ferritic 80ksi 55 ksi Pearlitic (552MPa) (379MPa) Martensitic Quenched & 120ksi 90 ksi tempered (828MPa) (621MPa) SG iron post inoculated with Sn* As cast Pearlitic 58.2 ksi 48kN matrix 10- ( N/mm 2 ) (48ksi) 20% ferrite Elongation (%) Applications Cylinder blocks, heavy gear box diesel engine casting do do- 18 Pressure castings such as valves and pump 6 -do- 2 -do Differential gear box, diff. housing Ref: Data of cast iron and SG iron by courtesy of Foundations of Mat. Sci. and Engg. Ed. II by Smith W.F. p 492 * Result of the present researches have been incorporated in the table.