Cast Iron Foundry Practices 1. The family of cast irons

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1 MME 345, Lecture 34 Cast Iron Foundry Practices 1. The family of cast irons Ref: [1] Heine, Loper and Rosenthal. Principles of Metal Casting, Tata McGraw-Hill, 1967 [2] S H Avner. Introduction to Physical Metallurgy, 2nd Ed., McGraw-Hill. Topics to discuss today 1. Introduction 2. Classifications of cast irons 3. Iron carbon silicon phase diagram 4. Chemical composition 5. Solidification process 6. Properties and applications

2 Introduction Cast irons are the largest tonnage of cast-to-shape products of foundry industry in the world found everywhere where manufacturing occurs. Of course, the largest tonnage of a cast material is steel, which is then mechanically formed into wrought products Have tremendous range of properties at low cost Strength and hardness Wear and abrasion resistance Machinability Corrosion resistance Have excellent foundry properties Fluidity Ease of production Yield Casting soundness Shrinkage 3/18 what are cast irons? Complex alloys of iron, carbon (up to ~ 4.0%) and silicon (up to ~ 3.5%) where carbon present in either graphitic form (pure C, hexagonal structure) or as an intermetallic iron carbide compound (the most common one being cementite or Fe 3 C, an orthorhombic crystal structure) Ductility is very low and cannot be rolled, drawn or worked at room temperature; not usually malleable as cast Solidifies with a eutectic; casting is the only suitable way to produce object using these alloys. Wide variation in types and properties are achieved by varying the balance between C and Si alloying with various metallic and non-metallic elements varying the melting and casting conditions (e.g., cooling rate, inoculations) and heat treatment practices after casting 4/18

3 Classification of cast irons based on the form of carbon Grey cast iron a large portion of its carbon is distributed throughout the casting as free or graphitic carbon in flake form. grey iron always presents a grey sooty surface when fractured. Grey iron showing graphite flakes. Unetched. Grey iron showing graphite flakes in steel matrix. Etched White cast iron all of its carbon is present in a chemically combined form as cementite (iron carbide). white iron presents a white crystalline surface when fractured. White iron showing pearlite dendrites and ledeburite in cementite. Etched 5/18 Malleable iron carbon is present as irregular nodular-shaped aggregates of graphite, called tempered carbon produced by heat-treating (malleableizing) white iron of suitable composition to impart ductility, or malleability in the casting Malleable iron showing tempered carbon. Unetched Ferritic malleable iron showing tempered carbon in ferrite matrix. Etched Pearlitic malleable iron showing tempered carbon in ferrite-pearlitic matrix. Etched 6/18

4 Nodular cast iron also known as ductile cast iron, or spheroidal graphite cast iron a large proportion of its carbon to occur as regular spheroids or nodules. small percentage of magnesium, cerium, or other agent is added in a specially prepared liquid iron (spherodizing) to form graphite spheroids ductility is the highest in these types of cast irons presents a bright steely surface when fractured Ductile iron showing graphite nodules. Unetched. Ferritic ductile iron showing graphite nodules in ferrite matrix. Etched. Pearlitic ductile iron showing graphite nodules surrounded by ferrites in ferrite-pearlite matrix. Etched. 7/18 Mottled irons an iron of intermediate composition which freezes partly as a white iron and partly as a gray iron under prevailing cooling conditions Chilled cast irons an iron with such composition that it would normally freeze as a gray iron but which is caused to freeze white in some locations by rapid cooling during solidification, i.e., chilling fractured surfaces of chilled irons show areas of white iron where freezing was rapid and other areas of gray iron where the cooling rate was normal Compacted graphite irons also known as vermicular graphite iron, has characteristics midway between ductile and gray iron and combines many of the properties of both here both graphite flakes and nodules are present; the flakes are shorter and thicker than that obtained in grey irons applications include brake disc for high-speed rail trains, diesel engine cylinder blokes, turbo housings, exhaust manifolds 8/18

5 9/18 Iron carbon phase diagram lower carbon level for A cm line and eutectic point austenite dendrites eutectic (ledeburite) higher eutectic and eutectoid temperatures austenite dendrites plus ledeburite pearlite dendrites plus pearlite plus cementite Addition of silicon iron carbide system reduces the eutectic and eutectoid carbon levels (e.g., for 2% Si, eutectic = 3.6%C, eutectoid = 0.6%C) increases austenite instability and reduces austenite zone; widen ferrite area iron graphite system 10/18

6 carbon Content, % Chemical composition of cast irons influence of chemical composition on properties and uses of cast irons is largely related to the two alloying elements, carbon and silicon, and their influence on the process of graphitization. Fe 3 C = Fe + 3C cementite ferrite graphite presence of C, especially above 2.0%, increases the graphitization process presence of Si causes cementite to become less stable (thus promotes graphite formation) other elements helping graphitization process (graphite stabilizers): Cu, Ni elements inhibiting graphitizing process (carbide stabilizers): Mn, S, P, Cr, V, Mo elements having dual effects: Al, Ti (actual effect depends upon composition) elements other than C and Si (e.g., Mn, Cu, Ni, Cr, Mo) added mainly to control the matrix microstructure; have small effects on solidification microstructures 11/ grey irons ductile irons composition of cast irons are described in terms of both carbon and silicon percentages malleable irons white irons Thus the term carbon equivalent, CE, has been adopted where: CE = %C + %Si/3 CE > 4.3% hypereutectic alloy (ductile irons) CE < 4.3% hypoeutectic alloy (grey, white, malleable irons) 1.0 steels silicon Content, % Type of iron Composition, % C Si Mn S P Grey White Malleable Ductile max 0.10 max Compacted max 0.10 max 12/18

7 per cent carbon grey white per cent silicon combined effects of carbon and silicon on the type of cast iron formed 13/18 besides composition, other important variable affecting graphitization processes is the cooling rate or section size in casting. slower cooling rates (or, heavy castings) shifts the lines on Fe-G phase diagram to the left rapid cooling rates (or, thin castings) shifts the lines on Fe-G phase diagram to the right Consequences grey iron piston rings (thin sections) are high in C and Si content grey iron heavy machine tool castings are low in C and Si content white irons used for making malleable irons are even lower in C and Si content 14/18

8 Solidification process liquid to austenite (A) dendrites transformation begins eutectic transformation begins L E (A + G) grey/nodular iron L E (A + C) white iron eutectic transformation ends end of solidification structures: A + G grey/nodular iron A + C white iron between 3 and 4, ppt. of carbon (as G or C) from austenite takes place composition factor, %C + %Si/3 solid state transformation begins very complex; some examples are: A F + G ferritic GI or NI A P + F + G pearlitic GI or NI A P white iron simplified Fe-C-Si ternary diagram showing approx. temperature ranges for solidification and graphitization in cast irons solid state transformation end 15/18 cast iron microstructures type of cast iron (white / mottled / chilled / grey) is largely established during the freezing process. the room-temperature microstructure reflects the entire freezing and cooling of the iron. Commercial designation Carbon-rich phase Matrix (a) Fracture Final structure resulted after Grey iron Lamellar graphite (Gr) P Grey Solidification Ductile iron Spheroidal graphite F, P, A Silver-grey Solidification White iron Cementite (Fe 3 C) P, M White Solidification and H/T Mottled iron Lamellar Gr + Fe 3 C P Mottled Solidification Malleable iron Temper graphite F, P Silver-grey Heat treatment (a) F, ferrite; P, pearlite; A, austenite; M, martensite 16/18

9 Properties and applications of cast irons wide ranges in properties properties dictated by microstructures each family is considered as a series of alloys Type of Iron Hardness (BHN) UTS (psi) Common Uses Grey ,000 60,000 White / Chill 350 >550 50,000 Malleable , ,000 Ductile , ,000 Motor blocks, piston ring, heavy machine tool bed, engine crankshaft, municipal and water works, dies, crankshafts, high-pressure cylinders, large gears production of malleable iron, grinding media, pulverizing mill plates; car wheel, rolls Automotive components, pipe fittings, railway inserts, agricultural equipment Heavy-duty machinery (gears, dies, rolls), pressure castings (valves, pump bodies, shock resisting parts), ductile iron pipes 17/18 Next Class MME 345, Lecture 35 Cast Iron Foundry Practices 2. Melting of cast irons in cupola