Physical Metallurgy EBB222 Cast Irons

Overview of cast iron Cast iron - Ferrous alloys containing carbon above 2.14 wt%. (3 – 4.5 wt%) C + (1-3 wt.%) Si. Cheap. Low melting (liquid at temperatures between approximately 1150 °C and 1300 °C) Advantages - can be can produce complex parts quickly and easily through sand casting Disadvantage - BRITTLE so can’t be used for high stress or shock loading

Iron Carbon Phase Diagram Liquid Austenite a + Fe3C d g+ L a + g L + Fe3C 723˚C 910˚C 0% 0.8% ~2% ~4.3% a g + Fe3C Cast Iron Carbon Steel

Production of cast iron Pig iron, scrap steel, limestone and carbon (coke) Cupola Electric arc furnace Electric induction furnace Usually sand cast, but can be gravity die cast in reusable graphite moulds

Types of cast iron Grey cast iron - carbon as graphite White cast iron – carbides Ductile cast iron nodular, spheroidal graphite Malleable cast iron Compacted graphite cast iron CG or Vermicular Iron

Chemical composition of cast iron

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. The iron-carbon phase diagram showing the relationship between the stable iron-graphite equilibria (solid lines) and the metastable iron-cementite reactions (dashed lines).

Factors influencing structure of cast iron The rate of solidification - slow rates of solidification in a sand mould allow for graphite formation solidify as gray cast iron. - rapid solidification will tend to give white cast irons structures - metal chills are sometimes inserted into parts of sand moulds in those areas where a high surface hardness is required. Carbon content - higher carbon content of the iron, the greater will be the tendency for it to solidify as gray cast iron The presence of other elements - some elements promote the formation of graphite in an iron structure. - silicon and nickel have strong graphitising tendencies.

Factors influencing structure of cast iron – cont’d The effect of heat treatment - the prolonged heating of a white iron will cause graphitization to occur. - This phenomenon is used as the basis for the production of malleable irons The rate of cooling on the structure of cast iron. - high rate of cooling during solidification tends to prevent the decomposition of cementite. - slow cooling, would become graphitic

Composition and types of cast iron

Carbon and silicon composition ranges for various cast irons and silicon-containing steels

Effect of composition Carbon Equivalent, C.E. = % C + 1/3 % Si A CE over 4.3 (hypereutectic) leads to carbide or graphite solidifying first & promotes gray cast iron A CE less than 4.3 (hypoeutectic) leads to austenite solidifying first & promotes white cast iron

Cooling rate and type of cast iron The effect of cooling rate and composition on the structure of cast iron. Moltted cast iron is a mixture of the white and the grey cast iron structure

Effect of alloying elements in cast iron Silicon - To increase the amount of under-cooling required for the formation of cementite and promote the formation of graphite during solidification. - Influence fluidity. - Graphitizer agent. - Cooling rate control to decomposed carbide eutectic. - promotes the precipitation of secondary graphite on the primary graphite during the eutectoid transformation, which results in large areas of ferrite (commonly called “free ferrite”) around the graphite particles. Sulfur - residual impurity from the extraction process. - stabilising the cementite and preventing the formation of flake graphite thus harden the cast iron - high sulfur tend to reduce fluidity and it also causes embrittlement due to the formation of iron sulphide (FeS) at the grain boundaries.

Effect of alloying elements in cast iron Manganese - addition of this element in small quantities is essential as it combines with any residual sulphur present to form manganese sulphid MnS. - Unlike ferrous sulphide, manganese sulphide is insoluble in the molten iron and floats to the top of the melt to join the slag. - thus by removing the sulphur, the manganese indirectly softens the cast iron and also removes a source of embrittlement. Phosphorus - residual impurity from the extraction process. - it is present in cast iron as iron phosphide (Fe3P). - this phosphide forms a eutectic with ferrite in gray cast irons, and with ferrite and cementite in white cast irons. - causes embrittlement in the cast iron and the amount present must be kept to a minimum in castings.

Gray Cast Iron 2.5 – 4wt.% C, 1 – 3 wt.% Si. fractured surface, gray appearance – gives the name of gray cast iron. solidifies according to the equilibrium eutectic Fe-C. Si stabilises the Fe-C system, rather than Fe-Fe3C.

Gray Cast Iron so microstructure based on Fe + C (graphite) , where graphite in the form of flakes surrounded by an α-ferrite or pearlite matrix. slow cooling : at liquidus temperatures primary γ dendrites form; as temperature is decreased the eutectic temperature is reached (γ +graphite) eutectic form; as temperature decreases in γ + graphite, carbon is rejected from the γ which goes to the graphite. As continue to cool to the eutectoid temperature α + graphite formed. **gives a microstructure of graphite flakes in a ferrite matrix ferritic gray iron faster cooling takes places through eutectoid temperature, final microstructure pearlite + graphite pearlitic gray iron

Microstructure of gray cast iron Graphite α- ferrite Ferritic gray iron

Pearlitic gray cast iron (Fe 3.4wt%C 2.5wt%Si 0.01wt%P) Graphite Pearlite

Properties of gray cast iron among least expensive metallic materials high fluidity – easy to cast, especially complex shapes low shrinkage during casting good mechanical properties in compression BUT brittle due to shape of graphite flakes excellent machinability excellent thermal conductivity excellent bearing properties

Properties of gray cast iron – con’td excellent damping properties excellent wear resistance (b) (a) (c) Comparison of the relative vibrational damping capacities of (a) steel (b) ductile (c) gray cast iron.

Application gray cast iron base structure for machines and heavy equipment, damping plates for pianos, engine blocks, engine cylinders, flywheels, piston rings, brake discs and drums gears. Engine blocks Brake discs

Ductile cast iron Also known as spheroidal graphite (SG), and nodular graphite iron addition of Mg (~0.1%) /Ce (~0.2-0.4%) added to Fe-C in the gray iron composition range low levels of minor elements such as S and P greater strength and ductility than gray cast iron of similar composition graphite forms as spheres rather than flakes improved toughness microstructure : spheroidal graphite particles in ferrite or pearlite matrix

Microstructure of ductile cast iron Nodular graphite α- ferrite Ductile cast iron with a matrix that is predominantly ferrite. Original magnification: 250× Fe, C 3.2, Si 2.5, Mg 0.05 (wt%) Ferritic ductile cast iron

Pearlitic ductile or nodular iron (Fe 3.4wt%C 2.5wt%Si 0.01wt%P 0.03wt%Mg) Removing the graphite flakes improves the tensile strength, ductility and toughness.

Properties of ductile cast iron high ductility high strength high modulus elasticity excellent castability excellent machinability wear resistance poor corrosion Crankshaft

Properties of ductile cast iron- cont’d poor weldability BUT can be welded with nickel and iron electrodes excellent machinability is fair to excellent excellent castability good fatigue strength

Applications of ductile cast iron valves, pump bodies, gears, crankshafts, pistons, rolls for rolling mills, tubes and door hinges, automotive engine crankshafts, heavy duty gears, military and railroad vehicles. Crankshaft Differential housing

Applications of ductile cast irons

White cast iron 1.8 – 3.6 wt.% C, < 1 wt. % Si. A fracture surface of this alloy has a white appearance, and thus it is termed white cast iron. solidification occur according to Fe-Fe3C metasable phase diagram. microstructure based on Fe + Fe3C. Phase formation : primary γ dendrites γ + Fe3C (eutectic) at eutectic temperature, as temperature decrease below eutectic temperature, additional Fe3C form as C is rejected from γ during cooling, below eutectoid temperature pearlite formed Final microstructure consists of Fe3C in pearlite matrix.

Microstructure of white cast iron Fe3C pearlite

Fe-2.8wt%C-1.8wt%Si (White cast iron) The cementite makes white cast iron very hard and abrasion resistant.

Properties of white cast iron hard BUT brittle and almost impossible to machine. excellent wear resistance high compressive stress

Application of white cast iron rollers in rolling mills, connecting rods, transmission gears, pipe fittings, flanges. brake shoes, shot blasting nozzles, mill liners, crushers, pump impellers and other abrasion resistant parts.

Malleable cast Iron produced by heat treatment of white cast irons at temperature between 800 ° C and 1470 ° C for a prolonged time period (10-30 hr), in a neutral atmosphere (to prevent oxidation) causes decomposition of the cementite, forming graphite, Fe3C Fe + C which exists in the form of clusters or rosettes surrounded by a ferrite or pearlite matrix, depending on cooling rate

Microstructure of malleable cast Iron

Ferritic Malleable Cast Iron Microstructure of malleable cast Iron Graphite Ferrite Ferritic Malleable Cast Iron Graphite Pearlite Pearlitic Malleable Cast Iron

Properties of malleable cast iron Good ductility and machinability Good shock resistance properties Ferritic malleable cast irons are more ductile and less strong and hard, than pearlitic malleable cast irons.

Application of malleable cast iron parts of power train of vehicles, bearing caps, steering gear housings, agricultural equipment, railroad equipment, connecting rods, transmission gears, and differential cases for the automotive industry, and also flanges, pipe fittings, and valve parts, railroad, marine, and other heavy-duty services. Transmission gears Connecting rod

Compacted graphite (CG) cast iron Si (1.7 - 3.0) wt%, C (3.1 – 4.0 wt.%). Microstructure consist of graphite in the form of worm-like (or vermicular). The matrix phase either pearlite and/or ferrite depend on heat treatment. Microstructure is intermediate between gray iron and ductile (nodular) iron. Magnesium and/or cerium is also added in lower concentrations than for ductile iron. compositions of magnesium, cerium, and other additives must be controlled to produce a microstructure that consists of the worm-like graphite particles, while at the same time limiting the degree of graphite nodularity, and preventing the formation of graphite flakes.

Microstructure of CG cast iron graphite α- ferrite

Properties of CG cast iron Higher thermal conductivity Better resistance to thermal shock (i.e., fracture resulting from rapid temperature changes) Lower oxidation at elevated temperatures

Application of CG cast iron diesel engine blocks, exhaust manifolds, gearbox housings, brake discs for high-speed trains, and flywheels

Typical properties of cast iron

Summary of cast iron Types of cast iron Graphite Ductility Description White No Fast cooling rates Grey Flake Slow cooling rates Malleable Anneal : flake to nodule Yes white iron + annealing heat treatment Ductile/Nodular nodular additions of minor elements made, so nodules of graphite form instead of flakes CG cast iron Worm like additions of minor elements made, worm like form instead of flakes

Microstructure of cast iron Gf - flake graphite Gr - graphite rosettes Gn -graphite nodules P - pearlite α - ferrite.

Summary of cast iron Ductile cast iron