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Casts Irons The cast irons are made and used by many industries including the automotive industry, farming industry (e.g., tractors), construction industry, etc to make the housings of crank shafts, piston rods, heads, intake and exhaust manifolds, transmission housing, starter motor housing, gear shafts and assembles, wheels, etc. A hard, good wear-resistant material with a reasonable amount of toughness is required. Cast iron is cheaply made using a blast furnace from Pig Iron since it contains a high carbon and Si content. Thus, the cast irons are not an iron-carbon alloy but an iron-carbon-silicon alloy of high C content between 3-3.75% C and high Si content of 1.5-3% Si.
Casts Irons • The main factors determining the type of cast iron are: • 1) The solidification rate • 2) The composition Solidification Rate • The stable Fe-C system is iron-graphite – rather than iron-cementite • So slow solidification from the melt favours the iron-graphite system while rapid solidification will favour the Fe-Fe3C system. • In order to obtain some areas of Fe-Fe3C within a Fe-graphite casting, to make surface hard areas in a casting, nails with large heads (called chills) are inserted at the mould face to form local spots of cementite – instead of graphite.
Casts Irons Composition Silicon and carbon in solution in the melt – both enhance the formation of graphite eutectic transformation at 1154oC. These elements are both naturally present in relatively high concentrations in pig iron, which is the major raw material – mixed with iron and steel scrap – for making cast iron. Typical Composition of Pig Iron C 3.00 – 3.75 S 0.05-0.06 Si 1.5 – 3.00 P 0.3-1.5 Mn 0.1 – 1.0
Iron-Graphite Phase Diagram Note: Changes in eutectic and eutectoid compositions and temperatures
The Need for Silicon • The eutectic of graphite occurs at 1154 oC, • This eutectic reaction is given by • L4.26%C g2.08%C + Cgraphite • The eutectic graphite reaction is competing with the eutectic reaction of Fe3C at 1148 oC. • In the cast irons, Si is used to control the formation of the graphite phase. • The eutectic reaction does not actually occur at 1154 oC nor 1148 oC nor 4.3 %C because of the addition of Si.
The Need for Silicon • Cast irons with a carbon equivalent, C.E., less than 4.3 wt% C are hypoeutectic. • Cast irons with C.E. more than 4.3 wt% C are hypereutectic. • C.E. close to 4.3 wt% induces the graphite reaction producing gray irons. • Hypoeutecticcast irons will tend to have Fe3C and form whitecast irons.
The Need for Silicon • In order to induce the eutectic graphite reaction to go to completion, a certain amount of Si is used to complete the formation of graphite from the available carbon. • The addition of Si retards the Fe3C formation, which allows more timeto form graphite. • Si stabilizes the formation of graphite structures. • It does this by increasing the undercooling of the eutectoid reaction. • This encourages the formation of the ferrite and graphite phases from austenite. • This is seen in the (Fe-2%Si)-C phase diagram. Do you recall what undercooling is?
(Fe-2%Si)-C Phase Diagram 3.66%C 1.5%C undercooling 0.05%C
(Fe-2%Si)-C Phase Diagram Cast irons contain 1.15 – 2.85% Si – and are thus conveniently described in terms of a pseudo-binary (Fe+2%Si)-C phase diagram. • The g-field is severely constricted – compared to the Fe-C diagram. • The maximum solubility of C in g-Fe is 1.5%C – instead of 2.11% C. • The eutectic is 3.66%C – instead of 4.26%C • Thus, the eutectic is given by L3.66%C g1.5%C + Cgraphite The eutectic temperature is increased to 1154 – from 1148 oC.
(Fe-2%Si)-C Phase Diagram The eutectoid • The eutectoid is 0.60%C – instead of 0.77%C. • The eutectoid temperature is increased to 765 – from 727 oC. • The maximum solubility of C in a-Fe is increased to 0.05% - from 0.02%C. • The eutectoid reaction is given by: g0.60%C a0.05%C + Cgraphite
The Need for Silicon The Si is converted to an equivalent carbon and the desired total carbon content is about 4.3 wt% C . This is expressed as: This equation is taken from the ranges of carbon and Si in ferrous alloys for different types of cast irons.
The Need for Silicon If Phosphorus is added to the cast iron, the C.E. is:
Effect of Other Elements on the Fe-C Eutectic • Beside Si and C – the g-Fe-Graphite eutectic is also promoted by the following elements: • Ni – Mg – Al – Ti – Zr – Cu • which are expensive so not generally added. • Carbide forming elements – such as Cr & Mo – stabilize the carbide phase and thus promote the (g-Fe + Fe3C) eutectic. • Mn is also a strong carbide (Fe3C) forming solute – but its effect on the eutectic depends on the presence – or absence – of Sulphur. • Sulphur is present in cast irons – from the coke used to smelt the pig iron. • It does not actually form a carbide – but strongly promotes carbide formation, e.g., 0.01%S can offset the graphitizing effect of 0.15% Si.
Effect of Other Elements on the Fe-C Eutectic • S has a similar affinity for Mn – forming MnS, which itself is neutral - but the first additions of S remove some of the carbide-stabilizing Mn – and thus can cause the amount of graphite to increase. • Similarly – the first additions of Mn can remove the S from solution – and again increase the amount of graphite. • Phosphorus can act in two ways: • 1) Physically – it forms a phosphide eutectic with a lower melting point than FeC (graphite) – which increases the time available for Si to promote graphite. • 2) Chemically – it promotes carbide formation – so large amounts increase carbide formation. The various alloying elements must thus be carefully balanced to obtain a desired grey or white cast iron.
The Need for Silicon ForSi contents > 2% • The eutectic is composed of (g–Fe + C (graphite)) when the alloy is cooled slowly. • These Fe-C alloys are the grey cast irons. • The eutectic is composed of (g–Fe + Fe3C) when the alloy is chill cast. • These alloys are the white cast irons.
Casts Irons • When a sample of cast iron is fractured – the exposed surface may be grey – white- or a mottled grey/white mixture. • A sooty grey fracture indicates that the microstructure is composed of graphite flakes in a ferrite matrix. • This graphite is relatively weak – as the fracture goes from flake to flake – so the grey fracture is mostly exposed graphite. • Gray cast iron contains small, interconnected graphite flakes in an alpha iron matrix. • It has low strength and low ductility. • Gray cast iron is the most commonly used cast iron for engine blocks.
Casts Irons • A white fracture means that the microstructure consists of cementite and ferrite – with the fracture going along – or through – the brittle white areas of cementite. • White cast iron contains massive amounts of cementite. • When it fractures it’s surface appears white, hence the name. • White cast irons are very hard and resistant to wear. • A mottled colour means that graphite flakes are present in some areas, while cementite is present in others.
Casts Irons Schematic drawings of five types of cast irons a) gray iron, b) white iron, c) malleable iron, d) ductile iron and e) compacted graphite iron. We will discuss these in detail.
Casts Irons Sketch in a) and photograph in b) of the flake structure of graphite in gray cast iron.
Time Temperature Transformation (TTT) Diagram of Cast Iron Later we will discuss TTT diagrams in detail. In fact we’ll use TTT diagrams to make steel alloys.
Forms of Graphite Flakes • Many forms of cast iron exist depending on its solidification. • The iron-carbon eutectic can solidify by one or other of the two reactions: • liquid austenite + graphite • liquid austenite + cementite • If a cast iron is superheated to destroy the graphite nuclei – undercooling will result – and the lower temperature (g-Fe+Fe3C) will form. • However – if there is sufficient Si and/or C present – the cementite will break down to austenite plus graphite – the sequence is thus: • liquid austenite + cementite – then cementite austenite + graphite • This secondary graphite is quite distinct from the eutectic graphite and forms between the austenite dendrites. What does superheated mean?
Dendritic Growth Mechanism Liquid temperature ahead of growing dendrite is cooler than dendrite, which promotes formation of protuberances and nodules. Heat of fusion, DHf heats liquid metal ahead of dendrite, slowing its growth. Images of dendrites
What is inoculation? A B D C • Eutectic Graphite • Uniform flakes – random orientation • Rosette graphite – by inoculation • Non-uniform flakes – random orientation • Secondary Graphite • D. Interdendritic – random orientation • E. Interdendritic – preferred orientation E
Sizes of Graphite Flakes • Rapid solidification results in finer graphite flakes. • But too rapid solidification will result in cementite unless there is a very high Si and C concentration. • Graphite flakes are classified in sizes – like ASTM grain sizes – according to the maximum length observed at a magnification of 100x. • No. 1 Longest flakes > 4 in (100 mm) • No. 2 Longest flakes 2 - 4 in (50 – 100 mm) • No. 3 Longest flakes 1 - 2 in (25 – 50 mm) • No. 4 Longest flakes 0.5 – 1 in (12.5 – 25 mm) • No. 5 Longest flakes (0.25 – 0.5 in (6.25 – 12.5) • No. 6 Longest flakes (0.125 – 0.25 in (3.125 – 6.25) • Etc. What does ASTM stand for?
What does BHN And UTS stand for? Note: C-Eq. (carbon equivalent)
Graphitization in the “Solid State” • Both grey and white cast irons contain austenite. • In hypereutectic irons – the austenite is only in the eutectic. • In hypoeutectic irons – the austenite is the pro-eutectic constituent – as well as being a constituent of the eutectic – so the structure consist of austenite dendrites surrounded by interdendritic eutectic. • This helps us to understand the structures we see. • During slow cooling from the eutectic to the eutectoid temperature – the proeutectic austenitewill precipitate carbon. • The graphite – or carbide – in the eutectic exerts a strong nucleation effect – so that the precipitation of excess carbon from the proeutectic austenite results in the growth of the eutectic graphite – or cementite.
Graphitization in the Solid State During slow cooling from the eutectic to the eutectoid temperature – the proeutectic austenite will precipitate carbon.
The Eutectoid Transformation in White Cast Irons On cooling through the eutectoid temperature – the austenite transforms by: austenite ferrite + graphite or austenite ferrite + cementite (i.e., pearlite) In a hypoeutectic white cast iron – the pearlite reaction will normally occur – but austenite in the eutectic will precipitate carbide on to the cementite formed in the eutectic – instead of forming pearlite between these carbide plates. That is, the pre-existing cementite acts as a nucleation site for the precipitation of cementite from austenite. (see next slide)
The Eutectoid Transformation in White Cast Irons Nucleation sites pearlite Interdendritic carbide
The Eutectoid Transformation in Grey Cast Irons In a grey cast iron – the type of eutectoid reaction will depend on the Si and C – and on the rate of cooling. For a given composition – With slow cooling the eutectic will be austenite + coarse graphite – and on passing through the eutectoid – this austenite will transform to ferrite + graphite – with the graphite depositing on the existing flakes. With rapid cooling the eutectic structure will be austenite + fine graphite – but on passing through the eutectoid – the austenite will transform to a regular pearlite structure. (see next slide)
Malleable Cast Irons It is possible to have a composition that – would form a grey cast iron on extremely slow cooling but form a white cast iron on regular cooling Reheating a regularly cooled sample – and holding at high temperature – will cause decomposition of the carbide – to form graphite + austenite. The graphite formed by this process is quite different from eutectic graphite – it grows in the form of compact aggregates – rather than flakes. On slow cooling to room temperature – the austenite will decompose to ferrite + graphite – which will deposit on the previously formed aggregates – so the alloy will no longer exhibit brittleness. (see next slide)
Nodular or Spherulitic Cast Iron Cast irons with tensile ductility can be formed – by adding Mg (or Ce Ca Li or Na)to a very low S alloy (0.01%) – just before casting. These additives cause the graphite to form as tiny balls of spherules rather than flakes – during the eutectic solidification of grey irons. Since Mg is above its boiling point at casting temperatures of 1400 – 1500 oC, it is necessary to add it in the form of a Mg-Ni alloy. Alternatively – the cooling of a white cast iron can be arrested to permit the eutectic carbide to decompose to austenite + aggregates of graphite – (like the malleable irons) – an then rapidly cool through the eutectoid temperature – to transform the austenite to pearlite. (see next slide)
Nodular or Spherulitic Cast Iron The pearlitic nodular irons have a greater strength than the ferritic nodular irons – but still have reasonable ductility. pearlite graphite
