Demystifying Cast Iron: Everything You Need to Know About it

Introduction

Cast press is a fabric with a rich heritage and admirable applications. It is used for pattern cutting and thus has formed the building blocks of designing and construction for a long time. Beside its flexibility, quality and one-off features, it has risen to be one of the key materials in all industries, from manufacturing to culinary expressions. In this article, we throw ourselves into the cast press realm, seeking to understand its constructs, category,properties and most importantly its enduring fame.

Understanding Cast Iron

Cast iron is an alloy of iron and carbon. The region of cast iron in the iron-carbon equilibrium diagram is shown in Fig 1. The diagram shows that there is a eutectic at 4.3 % (point C) which has solidification temperature 1130° C. Alloys within carbon range of 2% to 4.3% are referred to as hypo-eutectic irons, since their carbon content is below the eutectic composition. Alloys within carbon range of 4.3 % to 6.67 % are called hyper-eutectic irons, since their carbon content is above the eutectic composition.

Figure 1

With such a high carbon content, cast iron is very brittle and has low ductility. The general properties of cast iron are:

Cheap material
Good casting properties, e.g. high fluidity, low shrinkage, sound casting,etc.
High compressive strength.
Good machinability
High abrasion resistance
Good damping characteristic.
High corrosion resistance

Composition of Cast Iron

Cast iron contains different elements in addition to iron. The composition of cast iron is given below :

Carbon = 2 – 6.67 %
Silicon = 0.5 – 1.0 %
Sulphur = 0.06 – 0.12 %
Phosphorus = 0.10 – 0.30 %
Manganese = 0.10 – 1.0 %
Allaying elements : Ni, Cr, Mo, Mg, Cu, V.

Classification of cast Iron

The primary types of cast iron are described below.

1.Gray cast Iron

When the molten C.I is cooled, its final structure will depend upon the form in which the carbon solidifies, which in turn depend upon the cooling rates as well as composition. The control will be affected by the total C and Si (also P) and their total effect is expressed by the ‘carbon equivalent’. (C.E.)

`C.E.(%)=C%=frac{SI%+P%}3`

At relatively high C.E. and slower cooling rates, the solidified cementite, being unstable, breaks up into austenite and graphite flakes.The process is called as “graphitization”. The presence of certain elements, of which Si is most important, promotes graphitization. The graphite flakes give the cast iron the gray appearance when fractured,hence the name “gray” C.I.
Gray C.I. is the most widely used of all cast irons. In fact, it is common to speak of gray cast iron just as cast iron. It contains 2.50 to 3.75% C and upto 2.5% Si.
Gray cast iron is soft, easily machined and only moderately brittle. Its main advantages are : low cost, low melting point, fluidity and good damping capacity. Another good property it possesses is that the free graphite in its structure acts as a lubricant and when very large machine slides are made of it, a very free-working action is obtained.
Product Applications. Due to its low cost, gray C.I. is preferred in all fields where ductility and high strength are not required, for example,weights ; frames ; motor, gear and pump housings ; sanitary wares ; pipe fittings and gas and water pipes for underground purposes etc.
Due to its high compressive strength and good damping characteristics,gray C.I. is used for machine tool bases and supports for structures.Again, due to its fluidity and excellent wear properties, gray C.I. is used extensively in the manufacture of engine blocks, brake drums, sliding surfaces of machines, gearing, gear housings, piston rings, and so on.Gray C.I. has also been used for the manufacture of engine crankshafts,because of its good damping properties, high torsional shear strength and low notch sensitivity. Other product applications of gray C.I. are :Household appliances, manhole covers, cylinder heads for engines and in rolling mill and general machinery parts.

2.White Cast Iron

At low values of C.E. < 3 (C upto 2.5 % and Si < 1.5 %) and rapid cooling, the cementite will not have sufficient time to break into graphite and austenite. As a result, the total carbon will be exclusively in the combined form of Iron Carbide, Fe3 C (Cementite). It is a very hard and brittle metal with the entire cross-section having a white microstructure. Due to this, the metal is virtually unmachinable except by grinding and so has very limited applications.
Product Applications. The use of white C.I. is limited for wear resistant parts such as grinding ball, liners for ore-crushing mills and cement mixes, extrusion dies and some agricultural machinery. However, it is widely used in the manufacture of wrought iron and for making malleable iron castings.

3.Malleable Cast Iron

Malleable castings are first made from white cast iron and then malleabilized by two methods : “Black heart method” and “white heart method”.

Black heart method. In this method, the white iron castings are annealed by heating them for a prolonged period of serval days (to a temperature of 850 - 1000° C) in air tight pots filled with inert
material such as ferrous silicate scale (or iron oxide) or slag. The action of heat and iron oxide partially removes the carbon and remainder from combined state to a globular from of free carbon, so that after a slow cooling, a strong soft and somewhat ductile casting is obtained.This method, which is also called “decomposition” is used in U.S.A. and the material is Ferritic malleable iron.
White heart method. In this method (decarburisation), the castings are placed in pots packed with an oxidising material. The oxygen combines with carbon in the castings, reducing its amount to less
than 1%. This method is used in Europe, and the material is pearlitic malleable iron.

The difference between gray C.I. and malleable C.I. is the form in which the free carbon occurs. In gray C.I., the free carbon occurs in the form of flat or plate like particles, whereas in malleable C.I., the graphite is in form of irregularly shaped spherical particles which are much more desirable from a strength point of view than flakes.
Malleable C.I. is stronger and tougher, ductile, resistant to impact and easily machinable (due to the presence of graphite). Since the carbon change reaches only to a depth of about 10 mm, this process is not suitable for heavy castings. The application of malleable C.I. is considerably more limited than of gray C.I., because it is more expensive to produce and better mechanical properties are not required in most cases.
The use of malleable cast iron usually involves parts of complex shape that often need considerable machining to meet the specifications, such as, : in automotive and agricultural equipment industries (housings, yokes, wheel hubs), hinges, door-keys, spanners, mountings of all sorts, cranks, levers, thin walled components of sewing machines, textile machines and others, brake pedals in cars, spring hangers and so on.

4.Ductile Cast Iron

Ductile cast iron, which is also called as “Nodular Cast iron” and “Spheroidal cast iron” is of higher grade in comparison to malleable cast iron, because, the carbon is precipitated as spherical nodules of graphite which are more perfect spheres than those found in malleable cast iron. To produce ductile cast iron, the molten metal is first completely desulphurised. Then small amounts of special alloys containing magnesium or cerium are added to the molten iron in the ladle causing it, during solidification, to precipitate graphite as small spherical nodules.
Ductile cast iron possesses high fluidity which permits the casting of intricate shapes with excellent combination of strength and ductility, Ductile cast iron can be produced in thicker pieces than those produced by malleable cast iron.
Ductile cast iron is stronger, more ductile, tougher and less porous than gray cast iron. So, it is used in parts where density and pressure tightness is a highly desirable quality. These parts include : hydraulic cylinders, valves, pipes and pipe fittings, cylinder heads for compressors and diesel engines. Ductile cast iron is also used to make rolls for rolling mills, many centrifugally cast parts, pulleys, forming dies, pump housings and, in general, for parts subjected to impact loading or requiring a high elastic modulus.

5.Chilled Cast Iron

Quick cooling is called chilling and the iron so produced is chilled iron. It is made by placing “metal chills” inside the mould but near its surface. The molten metal, when poured into the mould, cools rapidly to produce a hard wear - resistant surface (of 1 to 2 mm thick) consisting of white cast iron. Below this surface the material is gray cast iron. Chilled cast iron can only be machined by grinding and is used in making stamping dies, mill and crushing rolls, railway wheels, car wheels, cam followers and so on.

6.Alloy Cast Iron

Alloying elements are added intentionally to cast irons to overcome certain inherent deficiencies in ordinary cast irons to give the required qualities for special purposes. By controlling the rate of graphitization, these elements develop special capabilities, such as better mechanical properties, improved resistance to heat, corrosion,wear, or brittle fracture. Also, alloying can improve both the castability and machinability properties of cast iron. Common alloying elements are : nickel, copper, chromium, molybdenum, vanadium and boron.

7.Meehanite Cast Iron

Mechanite cast iron, produced under patent protection, is made with the addition of a calcium - silicon alloy. Calcium silicide acts as a graphitizer and produces a fine graphite structure giving a casting of excellent mechanical properties. The basic gray cast iron used to obtain Mechanite iron is low in silicon and moderately low in carbon (about 2.5 to 3 %). Various grades of Mechanite are produced to meet special requirements.
All Mechanite irons have high strength, toughness, ductility and easy machinability. These irons also respond to heat treatment. Mechanite cast iron is ideally suited for machine tool castings.

8.Mottled cast iron

It is mixture of grey and white cast irons in which the outer layers have the structure of white cast iron and the core, that of grey cast iron. It is obtained by heating cast iron to red hot with powdered red hematite in an oven. This cast iron possesses increased toughness.

Below table gives the chemical composition of main types of cast irons (excluding alloy cast irons).

Metal	C	Si	Mn	S	P
row1 col 1	3.00 – 4.00	0.50 – 3.00	0.10 – 1.00	0.02 – 0.10	0.03 – 2.00
row2 col 1	2.50 – 3.75	1.00 – 2.50	0.40 – 1.00	0.06 – 0.12	0.10 – 1.00
Malleable cast iron	2.20 – 3.60	0.40 – 1.10	0.10 – 0.40	0.03 – 0.30	0.10 – 0.20
White Cast iron	1.75 – 2.30	0.85 – 1.20	0.10 – 0.40	0.12 – 0.35	0.05 – 0.20

The mechanical properties and applications of cast iron are give in the below table.

-	Cast iron	Composition wt. %	Condition	Structure	U.T.S. MPa	Y.S. MPa	Elong %	Typical Application
Gray cast iron	Ferritic	3.4 C, 2.2 Si, 0.7 Mn	Annealed	Ferrite matrix	180	-	-	Cylinder blocks and head clutch plates
	Pearlitic	3.2 C, 2.0 Si, 0.7 Mn	As-cast	Pearlite matrix	250	-	-	Truck and tractor cylinder blocks, gear box
	Pearlitic	3.3 C, 2.2 Si, 0.7 Mn	As-cast	Pearlitic matrix	290	-	-	Diesel engine castings
Malleable cast iron	Ferritic	2.2 C, 1.2 Si, 0.75 Mn	Annealed	Temper carbon and ferrite	345	224	10	General Engineering Service Machinability
	Pearlitic	2.4 C, 1.4 Si, 0.75 Mn	Annealed	Temper carbon and pearlite	440	310	8	General service with dimensional tolerance
	Martensitic	2.4 C, 1.4 Si, 0.75 Mn	Quenched and tempered	Tempered martensite	620	438	2	High strength parts, connecting rods, yokes for universal joints
Ductile cast iron	Ferritic	3.5 C, 2.2 Si	Annealed	Ferrite	415	275	10	Pressure casting as valve and pump bodies
	Pearlitic	3.5 C, 2.2 Si	As-cast	Ferritic Pearlitic	550	380	6	Crank shafts, gears and rollers
	Martensitic	3.5 C, 2.2 Si	Quenched and tempered	Martensitic	830	620	2	Pinions, gears, rollers and slides.

Wrought Iron

The word “Wrought” means that the metal possesses sufficient ductility to permit hot and,or cold plastic deformation.
Wrought iron is a mixture of pure iron and 1 – 3% slag. It also contains traces of carbon,silicon, mangenese, sulphur and phosphorus. It is made in this manner : First of all, all the elements in the iron (C, Si, Mn, S, P) are removed, leaving almost pure iron. The molten slag from the open-hearth is then intentionally added into vessels containing pure iron and thoroughly mixed into it.
The final mix is then squeezed in a press to remove excess slag and reduced into billets by a rolling mill. The material will consist of fibres of pure iron separated by thin layers of slag material. These layers of glass like slag material acts as barriers to corrosion which may attempt to penetrate the iron. The billets can be reheated to form bars, tubing, plates, structural shapes, pipe, forgings, bolts and nuts, nails, rivets, chains, crane hooks, railway couplings, barbed wire, boiler tubes, fittings,and so on.
Wrought iron is ductile and soft and is most readily forged and forge welded. It can withstand sudden and excessive loads. It can neither be hardened nor tempered like steel. The strength
of wrought iron can be increased by alloying, typically with nickel (1.5 - 3.5%). The ultimate
strength of wrought iron can be increased by cold working and subsequent aging.
A typical chemical composition of wrought iron is as given below:-

C : 0.02 – 0.08 %, Si : 0.01 – 0.20 %, Mn : 0.02 – 0.10 %
S : 0.02 – 0.04 %, P : 0.05 – 0.20 %

Effect of Impurities on Iron

The effects of impurities on iron are discussed below:

1. Sulphur

Sulphur is generally considered harmful in C.I. In gray cast iron, it counteracts the graphitizing effect of silicon, lowers fluidity during pouring, decreases strength and makes the metal more brittle. So, it should be kept as low as possible, preferably below 0.1 %.

2. Manganese

It encourages the formation of carbide and so, tends to whiten and harden cast iron. But it helps to control the harmful effects of sulphur. It has greater affinity for sulphur than for iron and it combines with sulphur to produce manganese sulphide, which is not objectionable. It is often kept below 0.75%.

3. Phosphorus

Phosphorus increases the flowability of gray cast iron. Phosphoric irons are useful for casting of intricate designs and for many light engineering castings when cheapness is essential.Phosphorous induces brittleness in cast iron and it is rarely allowed to exceed 1.0 %.

4. Silicon

It is the important graphitizer for cast irons, which makes the cast irons soft and easily machinable. It also produces sound castings free from blow holes because of its affinity for oxygen. It is present in cast irons upto 2.5 %.

Effect of Alloying Elements on Cast Iron

1. Nickel

Nickel is used in cast irons to refine grain structure, increase strength and toughness and increase resistance to corrosion. It has no effect on ductility. It also acts as a graphitizer but is only half as effective as silicon. It, thus, promotes the machinability of cast irons. In low alloy cast irons, its amount is from 0.25 to 5.0%. This alloy is used in steam and hydraulic machinery,compressors and I.C. engine parts. In heat and corrosion resistant cast irons as much as 35 % nickel is used.

2. Chromium

Chromium also refines grain structure and increases strength, hardness and resistance to corrosion. It also increases the wear resistance and heat resistance property of cast iron. Gray cast irons which will be subjected to severe wear conditions, often contain chromium (upto } 8 %). This alloy is used in pumps of all types. The alloy used in higher resistance parts may contain chromium from 10 to 30 %. However, chromium tends to prevent graphitization.

3. Copper

Copper is added to cast iron in amounts upto about 1.0%. It increases fluidity for improved mould filling ability, imparts corrosion resistance, and improves mechanical properties,notably toughness and hardness. Machinability of cast iron is also slightly improved, because copper promotes formation of graphite.

4. Molybdenum

The presence of molybdenum in cast iron produces fine and highly dispersed particles of graphite and good uniform structure. This increases the strength and toughness and improves high temperature strength of cast iron. Its amount ranges from 0.25 to 1.25 %. Molybdenum is frequently used in cast iron in combination with nickel or chromium or nickel and chromium.

5. Vanadium

It is added to cast iron in amounts of 0.10 to 0.50 %. It promotes grain refinement, increases strength and increases resistance to fatigue stresses. However, it tends to reduce graphitization.

6. Boron

Until quite recently, boron received little recognition as an addition to regular gray cast iron. 0.05% boron, 3.5 % carbon and 1.0 % silicon in cast iron help to increase surface hardness and refine structure. This alloy is used for rolls in rolling mills.

Properties of Cast Iron

High Strength: Cast iron provides high compressive strength, which suits load bearing structures in the construction as well as in the machinery.
Excellent Wear Resistance: Specific types of cast iron, for instance gray cast iron and white cast iron, have the best resistance to wear and they work excellently for the components which are exposed to abrasive wear.
Good Machinability: Gray cast iron is almost unparalleled in machinability, making complex shapes and complex components quite easy to produce.
Thermal Conductivity: Cast iron has excellent thermal conductivity and therefore can be used in those areas where heat transfer is of great importance, say the engine blocks and cast iron cookware.
Damping Capacity: Cast iron has an incredible damping capability. It absorbs vibrations and makes noise as a result of the use of equipment and machinery.

Conclusion

Cast iron is a kind of metallurgy that is a symbol of the inventiveness and versatility of this field, it offers a blend of wear resistance, machinability, and strength. Among everything, from processing the hand-shaped blocks of the first cars' engines to the simple frying pans in our kitchens, cast iron is still the backbone of our world. Knowing how metals are grouped and what qualities they possess, gives us an insight into their immortality and utilization over different areas of industries.