Cast Iron Properties

Development of Microstructure in Cast Irons

Main difference between the molten cast irons and steels is that the liquid cast iron is a micro heterogeneous system which is not in equilibrium. It consists of carbon saturated iron and microgroups of carbon. An iron may solidify as grey, white or mottled depending upon the eutectic value and the rate of cooling.

Composition has effect on eutectic value. To simplify the effect of composition, a concept of carbon equivalent value has been developed. (CE). A commonly accepted definition of CE is Total Carbon % + (Si% / 3 + P% / 3). This carbon equivalent value tells whether the iron will be hypo eutectic, or hyper eutectic. In general, the lower the C value, the greater is the tendency for an iron to solidify white or mottled.

• Hypo eutectic irons do not solidify with the formation of graphite. Instead the solidification takes place by formation of austenite dendrites. The interdendritic areas remain high in carbon and solidify as a eutectic of iron carbide and austenite - called as ledeburite.

• Hyper eutectic irons solidify by the direct formation of graphite from the melt in the form of KISH generally appearing as long straight flakes. Graphite phase continues to solidify till the eutectic temperature. The eutectic graphite as a general rule solidifies as flake graphite.

ASTM A247 defines the graphite shape, size and distribution. ISO 945 has also accepted these definitions with extensions of additional graphite forms.

Type C : Kish, straight, thick chunkey flakes.
Type A : Normal Flakes –
Type D/E : Undercooled – is normally associated with rapid cooling, and is particularly common in thin sections. It can also be produced by other means like addition of titanium.
Type B : Rosettes type - is typical of slightly less rapid cooling.
Both Rosette and Under cooled graphites are frequently associated with ferrite, since the larger surface area of graphite flakes reduces the distance required to be traveled by carbon atoms during gamma to alpha cooling.

Other forms of graphite : Trace elements like lead (above 0.0007%) results in Widmannstatten graphite. Tellurium results in Mesh type graphite. Both these types of structures result in drastic reduction in strength of Cast Iron.

Manganese has its chief function to neutralise the sulphur content by formation of Manganese sulphide and to prevent formation of more harmful iron sulphide. This is possible if manganese contents equals 1.7 x sulphur percent + 0.3 %. When sulphur content is not balanced by the manganese content, anomolous inverse chilling effect may occur, indicated by free carbide in the center of an other wise grey section.

Mechanical Properties.

Although the structural constituents of steel can all be present in cast iron, there are two important constituents not normally present in steel which are responsible for major characteristics of cast iron - graphite and phosphorus. Effect of graphite on purely mechanical properties is essentially that of a void. Phosphorus forms a low melting point eutectic forming a brittle network affecting shock resistance and hydraulic soundness.

The structure and strength of grey iron castings are mainly governed by the amount and form of graphite - ie not only on the composition but also on the section thickness of the casting or the rate of cooling. Thus the metal cast from the same ladle may produce -a chilled white iron, a grey iron with chilled edge, a sound grey iron or a week open grained structure - depending upon the section thickness of the casting. For this reasons the standards specify the grey irons by reference to its strength not by composition, nor by the strength of the casting, but by the strength when cast into test bars of fixed diameter.

Tensile strength : Earlier standards used to specify tensile strength for different as cast test bar diameters. The definition of grade was based on tensile strength of 30 mm diameter test bar. Larger test bars showed less strength, and vice versa. It should be understood that the tensile strength obtained from the test bar does not indicate the strength of the casting, but the quality of the melting process and the molten metal. Strength of a particular section in a casting is dependent upon the cooling rate of the particular portion, and can be roughly correlated to the tensile strength results.

Transverse test : In other words Bend test to determine the transverse rupture stress. This is another way for melt quality and the results show good correlation with the tensile strength results.

The compression test : On specimen L/D 2:1. The compression strength is generally 3 to 4 times its tensile strength. Grey iron actually fractures at its maximum compression strength. In this respect it differs from the steel, malleable iron and other metals which deform plastically.

Hardness Test : Brinell hardness test is most commonly accepted test. It should be remembered that the Brinell hardness number is not independent of the test load.
• The diameter of the impression should be within 0.25 to 0.5 of the ball diameter. This is ensured by selecting ball diameter and test load (3000 Kf and 10 mm diameter ball, or 750 Kg and 5 mm ball - p/d2)
• Above 450 BHN, the ball tends to distort appreciably.

There is no clear relation between the hardness and tensile strength value for grey cast iron, as there is for steels.

Similarly there is no clear relationship between hardness and wear resistance.

Modulus of Elasticity : is measured by the slope of the stress-strain curve. For cast iron it is not a straight line as steels. Graphite flakes or nodules give dispersed discontinuities modifying stress strain response. The broken matrix carries a complex stress with high local stress at graphite flakes because of which the recoverable total strain does not follow a straight line.

Normal graphite structures show higher tensile, lower hardness, lower Eo values and higher total strain at failure than undercooled graphite structures.

Higher phosphorus irons show higher tensile, higher hardness, lower total strain at failure and little change in Eo values.

Annealing to ferritic state lowers tensile, hardness and Eo values

Dimensional stability under stress : For large castings, where maintenance of the higher dimensional accuracy is required – as in machine tools – initial deformation can be virtually eliminated by prestressing for a suitable period of time to a stress higher than that to be subsequently imposed.

Stress relieving is another method.

Damping Capacity : To absorb vibrational stresses – amount of energy absorbed per oscillation.
Increasing carbon equivalent increases damping capacity
Increasing cooling rate, causing refinement of matrix and graphite structures decreases damping capacity.
Annealed ferritic structures, and hardened structures have higher damping capacities than as cast structures.

Sliding Lubricated Wear –

Cast iron is an extremely good bearing material which can be used for applications from textile spindles (low load at 10000 rpm) to heavy planing machines (heavy load at few strokes per minute).

It is well known that under sliding lubricated wear against hardened steel surface, the greater wear may occur in hardened steel than in ordinary cast iron.

The presence of graphite contributes to the initial running in period to avoid seizing and scoring. The amount of graphite on the running surface is very small, and its effect is therefore likely to be confined to the initial period only.

Fully pearlitic matrix is normally preferable to one containing mainly free ferrite. Irons containing a high phosphorus content (> 0.7%) are more resistant under marginal lubricated wear. Iron containing such high phosphorus levels as tolerate 5- 10 % of free ferrite associated with moderately coarse flake graphite.

Surface hardness is only one of the factors affecting cylinder bore wear.

Machinability

Main criteria is chip cutting and removal. The coefficient of friction between the chip and the tool governs the character of chip.
For ductile materials, high coefficient of friction leads to a discontinuous chip.
Low coefficient of friction leads to a continuous chip and built up of metal on the tool.

For cast irons, which falls under brittle materials category, the discontinuous bricks will form for all coefficients of friction. Actual machinability depends on the different constituents of microstructure.

Graphite
• Provides dicsontinuities to facilitate chip breakage
• Provides anti-welding lubricant
• Form of graphite is less significant, than the quantity
• Coarse graphite machines better than fine undercooled graphite – but cannot produce better surface finish.

Ferrite
• Low hardness, however low ductility due to silicon content
• Increases machinability
Pearlite
• In general increasing amount of pearlite decreases machinability
• Coarse pearlite ismore readily machinable than fine pearlite

Phosphide eutietic
• the machinability reduces slightly above 0.5 % Phosphorus.
• Upto 1.4% phosphorus the castings are quite freely machinable, provided the ternary eutestic is absent. Carbides drastically reduce the machinability.

Carbides
• Free carbide markedly reduce the machinability and decrease tool life.

Typical points to check for machinability problems

Poor machinability – without hardspots

• Excessive machining allowance – leading to increased depth of cuts
• Higher strength and hardness
• Imperfect tools and speed / feeds
• Raw material sources

Hard Spots

• Too low carbon equivalent
• Variation in metal composition
• Unbalanced sulphur / manganese ratio – leading to hard center of casting. First metal out from cupola is likely to have high sulphur.
• Segregation of phosphorus/carbide complex – typically in thick castings where carbide forming elements are added.
• Hard spots due to trace elements is raw materials

Hard skin

• Typically for heavy castings
• Burnt on sand
• Deep oxidation of metal
• Sometimes a soft skin formed due to decarburisation also can produce a graphite free pearlite skin leading to heavy tool wear.