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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

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.

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

COPPER-SILVER EUTECTIC ALLOYS

CONCLUSION

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