The microstructure of cast iron

ABSTRACT

THE MICROSTRUCTURE OF CAST IRON:

In the experiment, the microstructures of five samples of different cast iron forms were observed and investigated under the optical microscope and an iron-carbide phase was studied. The suitable drawings were made under different magnification of 100 and 200. Each constituent of the microstructure was identified and also other structural features of the sample provided were identified. The samples areBlackheart malleable cast iron, Ferritic spheroidal graphite iron, Pearlitic spheroidal graphite iron, White cast iron and Phosphoric grey cast iron. The differences in the microstructure were due to the difference in heat treatment, process of cooling and additives present.

COPPER SILVER EUTECTIC ALLOY:

The eutectic alloy formed between silver and copper was observed. The microstructure of all the four samples was drawn using the optical microscope with 200 magnifications. 90%Ag 10% Cu, 72%Ag 28% Cu, 50% Ag 50% Cu, 30% Ag 70% Cu are the samples provided. An equilibrium diagram was constructed for the copper-silver alloy system, the features of interest as well as the constituent of the structure was identified for all the samples.

INTRODUCTION

THE MICROSTRUCTURE OF CAST IRON

Cast irons are a class of ferrous alloys with a carbon content of between 2.0 – 4.5%; they contain sufficient carbon so that the eutectic reaction occurs during solidification. They are the most economical in terms of foundry cost which makes the useful even though they are quite brittle; they are fine for low stressed components like cylinder block. Their versatility makes them a high demand in the market. Cast iron contain contrasting amount of manganese, sulphur and phosphorus. They have varying strength and can resist wear and abrasion and corrosion and they can be easily machined. They are easily melted and cast making the good casting impression.

The carbon in a cast iron exists in two forms, as a free form of graphite or in a combination as a cementite which is unstable iron carbide. Iron is hard and difficult to machine due to how brittle the cementite is while graphite is soft making the iron softer and easy to machine. Graphite weakens metal due to its occurrence in flakes by breaking up its continuity. Because of the characteristics of these two carbon form, the relative amount, the shape and distribution in the cast iron produces different cast irons variety of properties.

Grey cast iron contains tiny interconnected flakes of graphite that allow low strength and ductility. It’s the mostly used cast iron and named after its grey colour on fracture surfaces. White cast iron produces more cementite than graphite during solidification, it is a hard brittle alloy containing massive amount of fe3c. Alloyed white cast iron is used due to their hardness and wear resistance for abrasive wear. The name was given due to white fractured surface.

Malleable cast iron is formed by the heat treatment of white cast iron, it has better ductility and they produce rounded clumps of graphite. It is very machinable and is made by heat treating unalloyed 3% carbon. A spherodite are micro constituent of coarse spheroidal graphite particles in a matrix of pearlite or ferrite, permitting excellent machining characteristics in high carbon steel.

The structure of cast iron is affected by a number of factors. The type of iron form is determined by the rate of solidification as slow cooling will produce grey iron and the rapid one will produce white iron structure. Whether graphite or cementite is formed and by what quantity is determined by the carbon content of the melt and presence of other element. For example nickel and silicon promote the formation of graphite in the iron structure. The structure is affected by the type of heat treatment, cementite will decompose to ferrite and graphite will produce a completely different structure.

COPPER-SILVER EUTECTIC ALLOYS:

There are three single phase regions on the phase diagram of binary alloys of silver and copper. The phase is a solid solution rich in copper which has silver as the solute and an FCC structure it also include pure copper and is considered to include pure copper. An eutectic region can be defined as a three phase invariant reaction in which one liquid phase solidifies to form two solid phases. Copper and silver form an eutectic at 72%Ag and 28%Cu at a temperature of 780oC.The temperature at which an alloy become totally liquid decreases as silver is added to copper which is also the same as the addition of copper to silver.

A microstructure may be defined as the structural feature of an alloy, its grain and phase structure that are subject to observation under microscope. Copper is a face centred cubic structured metal possessing good ductility, good thermal and electrical conductivity. It is often used as a constituent of various metal alloys. The melting point of pure copper is 1083oC while that of pure silver is 961oC.

Silver possesses one of the highest electrical and thermal conductivity of any metal. It has FCC structure and is sometimes produced as a by-product of copper. When the full liquid solubility is possible with complete solid insolubility or very limited solid solubility then an eutectic relationship exist. This exists in copper and silver but they are fully soluble in liquid state.

EXPERIMENTAL

THE MICROSTRUCTURE OF CAST IRON

Five prepared micro specimens were provided and the microstructure of each studied and drawn using a microscope.

The specimens provided were

• Blackheart malleable cast iron at magnification 100
• Ferritic spheroidal graphite iron at magnification 100
• Pearlitic spheroidal graphite iron at magnification 200
• White cast iron at magnification 100
• Phosphoric grey cast iron at magnification 200

Each constituent and other structural feature of importance in the microstructure of white cast iron was labelled on the drawing. The procedure was performed to all specimens.

COPPER-SILVER EUTECTIC ALLOYS

Four polished and etched micro sections of copper silver alloys were provided.

• 30% Ag 70% Cu
• 72%Ag 28% Cu
• 50% Ag 50% Cu
• 90%Ag 10% Cu

These alloys have already been melted in a gas fired furnace, deoxidised by polling with graphite rod and then cast in refractory moulds preheated to 500oC.

I placed the samples under the microscope at a magnification of 200; my observation was drawn with the help of the microscope. The constituent and structural features are drawn and labelled, I repeated the steps for the entire specimen and the equilibrium diagram was drawn from the data.

RESULTS

The results are compiled in the couple of pages attached to the next pages.

DISCUSSION

THE MICROSTRUCTURE OF CAST IRON

• The way a metal is cooled produces the different structures of cast iron. Two of these structures are white and grey cast iron. As the metal is cooled, the amount of austenite in the matrix increases. At the eutectic temperature of 1130°C the remaining liquid solidifies producing austenite in a eutectic matrix. A structure consisting of cementite, Fe3C and eutectoid iron starts to form as structure starts to decompose. The eutectoid contains area of pearlite and cementite and it’s also a mixture of cementite and ferrite. The pearlite is formed as a result of the decomposition of the austenite on cooling. This structure is that of white cast iron which is formed due to the rapid rate of cooling.

The cooling rate of white and grey cast iron affects the structure; fast cooling rate promotes white cast iron while slower cooling rate promotes grey cast iron. The carbide composition can have effect on the structure of the iron produced; high amount of chromium promotes white cast iron while low amount promotes grey cast iron. Section size can also determine iron structure obtained. Cool section cool faster and produce white cast iron while thick sections will cool slower promoting formation of grey cats iron.

A number of variables must be controlled in order to produce grey cast iron instead of white cast iron. For a grey cast iron to form, the rate of cooling must be made as slow as possible. High silicon content will also promote the formation of grey cast iron as silicon has strong graphitising
tendencies. Phosphoric grey iron is stronger, has a lower melting point and better fluidity than normal grey cast iron.

• The Blackheart malleablising process is the packing of white iron castings into pots with a neutral packing, such as sand or crushed slag, and heating them to 900°C for three days. After the three days they are cooled very slowly. The cementite in the white iron is being decomposed into ferrite and graphite is being precipitated in a smoothly dispersed form. The structure is composed entirely of ferrite and graphite. The graphite present in the structure is shown as ‘rosettes’ of carbon in the ferrite. After the process, the small amount of pearlite left has no effect on the properties of the casting. The steel produced in this process has good wear resistance and strength and reasonable toughness.
• Additives are used to produce spheroidal graphite iron rather than flake graphite iron. Magnesium amounting to 1-2% of the weight of the iron is added in the form of a nickel magnesium alloy of 10-20% magnesium. The alloy is used to prevent an extremely violent reaction from occurring. The presence of silicon also assists the formation of the nodules therefore Ferro-silicon is added. The sulphur level needs to be kept low in order to avoid removing the Mg as sulphide.
• In the production of a ferritic graphite iron, spheroids of graphite in pearlite matrix are heat treated to form spheroids of graphite in ferrite matrix. This is time dependent process and doesn’t go into completion therefore causing pearlite area to still be seen.

COPPER-SILVER EUTECTIC ALLOYS

• On drawing of the 30%Ag70%Cu, alpha particles can be seen and dark patches show primary dendrites outside eutectic.

The 50%Ag50%Cu alloy sample also shows primary alpha dendrites outside eutectics. In the 90%Ag10%Cu sample there is small amount of eutectic and light areas show beta particles while the dark areas show alpha particle.

On the equilibrium diagram ADB is the liquidus and ACDEB is the solidus. The area ACF represents the alpha phase of the silver in copper while BEG represent the condition of limited solid solution of copper in silver which is the beta phase. Below FCDEG the two phase’s alpha and beta exist side by side.

The 72%Ag28%Cu micro section has an all eutectic composition with dark areas also representing alpha particles and light areas showing beta particles the centre of the microstructure is the first to solidify then the outside area.

Composition of solid and liquid phases will vary with the temperature along the solidus and liquidus lines. The final liquid between composition C and E will always end at eutectic regardless of what the initial composition may be. As a result the solid will be composed of masses of A and B. At eutectic CW parts liquidus composition Y while WY parts composition C.

• During casting it’s virtually impossible to achieve equilibrium conditions. Coring is the non equilibrium cooling on microstructures. Coring explains how the varying primary dendrites allow lighter areas in the centre than in the outside. The properties of a cored structure are less than optimal, as a casting having a cored structure is reheated causing grain boundaries regions will meet first as long as they are richer in low melting components.

The liquid film that separate the grain gives an outcome of sudden loss of mechanical integrity. The melting may begin at a temperature below the equilibrium solidus temperature of the alloys. Homogeneous heat treatment can be used to remove coring at a temperature below the solidus point of the alloy composition. During the process, atomic diffusion occur producing compositionally homogeneous grain.

CONCLUSION

• Cast irons have many different structures each one caused by a different cooling rate, additives and different heat treatments.
• Formation of grey cast iron over white cast iron is promote by slow cooling rate and enormous silicon content
• Coring can be eliminated by a homogenising heat treatment.
• Magnesium and silicon help to produce spheroidal graphite iron rather than flake graphite iron
• To produce a ferritic spheroidal graphite iron from pearlitic spheroidal graphite iron the steel must be heated to just below the lower critical temperature.
• Phosphoric iron will enable to cast very fine details and in blackheart malleablising process, rosettes from graphite in ferrite matrix are produced from white cast iron.
• The composition of the copper silver eutectic alloys has a very large effect on the microstructure of the alloy, with different amounts of the phases being produced on cooling.