INTRODUCTION

Iron – carbide diagram is not a true equilibrium phase diagram because iron carbide is not a stable phase. These iron carbon decomposes into iron and carbon. Even at elevated temperatures it will take several years to decompose. This diagram generally represents the equilibrium changes happening in the material.

Carbon being a very small element gets into austenite / ferrite to form a solid solution. Ferrous metals are broadly classified into three main categories

Iron (C content 0.008%wt)
Steel (Fe-C alloy 0.008% to 2.11% C)
Cast iron (Fe-C alloy 2.11% to 6.7% C)

PURE IRON

Cooling curve for pure iron

The solubility of carbon is always greater in FCC when compared to BCC. The solubility of carbon in different phases are mentioned below.

α – Ferrite (BCC)

Max solubility of carbon is 0.022wt%

γ – Austenite (FCC)

Max solubility of C is 2.14wt%

δ – Ferrite (BCC)

Stable only at high temperature
Max solubility of C is 0.10wt%

Cementite (Iron – Carbide or Fe3C)

Orthorhombic structure
Intermetallic
Brittle

STEELS

Steel can be basically classified into three categories

Low Carbon steel (C < 0.3%)
Medium Carbon Steel (0.3% < C < 0.6%)
High Carbon steel (C > 0.6%)

Low CarbonSteel:

Mild steel is the most common material which is used is most of the industries. The material property of this material is suitable for most of the applications. Comparatively the price of Mild steel is also very low. The low carbon steel contains around 0.05 – 0.15% of Carbon and Mild steel contains of 0.16 – 0.30% of carbon. And the maximum amount of carbon content which can be present is only up to 0.3%. The surface of low-carbon steels at temperatures generally between 850 and 950°C (1560 and 1740°F), at which austenite, with its high solubility for carbon, is the stable crystal structure. These steels are malleable and ductile with a relatively low tensile strength. The surface hardness of the material can be increased through carburizing. These are used in manufacturing nuts, bolts, tin plate, wire product, tubes, girders screws, body panels, etc.

MediumCarbon Steel:

Medium carbon steels has a carbon content ranging from 0.3% to 0.6%. This material is ductile, has a very good strength and has a good wear resistance. They are even harder and have a greater tensile strength when compared to low carbon steel. These materials are used in manufacturing shafts, gears, crank shaft, connecting rods, spindles, rail axle, rail wheel, couplings, etc. The main properties are it has a better tensile strength, it’s harder and has very good wear resistance.

High Carbon Steel:

From 0.60 to 1.70% of carbon, they have higher tensile strength and harder than other plain carbon steels. They also readily respond to heat treatment. These steels can be tempered to great hardness. Used for special purposes like (non-industrial-purpose) knives, axles or punches. Most of these steels with more than 1.2% carbon content are made using powder metallurgy. Properties: Tough rather than hard, and fairly ductile Uses :- Used for making hand tools such as wrenches, chisels, punches, files, cutting tools such as drills, wood working tools, rail road wheels, springs, high strength wires etc.

In steels the general reaction which occur is,

Eutectoid Reaction:

It describes the phase transformation of one solid into two different solids. In the Fe-C system, there is a eutectoid point at approximately 0.8wt% C, 723°C. The phase just above the eutectoid temperature for plain carbon steels is known as austenite or gamma. We now consider what happens as this phase is cooled through the eutectoid temperature (723°C). The Eutectoid composition (Fe, 0.8wt %) then at all temperature above 723˚C is austenite (γ). Eutectoid point is a point where three phases are in equilibrium the composition of two new phases are given by the ends of the line through the eutectoid point. At eutectoid temperature 723˚C the microstructure transforms to pearlite, which is two phase microstructure of ferrite and cementite. This transformation occurs by nucleation and growth. Below the eutectoid temperature the microstructure is 100% pearlite. Pearlite is a mixture of both cementite and ferrite.

The phase diagram for eutectoid reaction is shown below:

CAST IRON

Cast iron can be defined as a group of iron carbon alloys where the carbon content of the material greater than 2% and a maximum up to 6.5%. Usually cast iron consist of 2.5 – 4% of carbon and 1 – 3% of silicon. The melting temperature of cast iron varies from a range of 1150 - 1200˚C. The melting point of pure iron is very high when compared to cast iron where the cast iron is 300˚C lesser. These are the main alloying elements of cast iron. The iron with less carbon content is referred to as steel. The solidification process of the cast iron can be easily understood by iron carbon diagram because the compositions are around eutectic point (lowest liquid point).

The cast iron can be classified into three different types.

· Grey Cast iron

· White Cast iron

· Ductile Cast iron

· Malleable Cast iron

Cast iron are generally brittle except malleable cast iron. These cast irons have very low melting point, good fluidity, cast ability, machinability and a very good wear resistance. Cast iron are widely used in manufacturing pipes, automotive components, cylinder box, gear box, etc.

Grey Cast Iron:

Grey Cast iron is named because the fractured surface on the iron is visible grey in color. They contain 2.5 – 4% of carbon and 1- 3% of silicon. The microstructure of the graphite flakes is in ferrite or pearlite matrix. Grey cast iron has less tensile strength which makes it weak and brittle and less shock resistance when compared to steel but its compressive strength high which makes it stronger and is comparable to low and medium carbon steel. The damping capacity and wear resistance is also good. The mechanical properties depends on the size and the shape of graphite flakes present inside.

White Cast iron:

White cast iron displays white fractured surfaces due to the presence of an iron carbide precipitate called cementite. With a lower silicon content (graphitizing agent) and faster cooling rate, the carbon in white cast iron precipitates out of the melt as the metastable phase cementite, Fe₃C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles. As the iron carbide precipitates out, it withdraws carbon from the original melt, moving the mixture toward one that is closer to eutectic, and the remaining phase is the lower iron-carbon austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide the benefit of what is called precipitation hardening (as in some steels, where much smaller cementite precipitates might inhibit plastic deformation by impeding the movement of dislocations through the pure iron ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer hardness at the expense of toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as acermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller and volute) of slurry pumps, shell liners and lifter bars in ball mills and autogenous grinding mills, balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).

Malleable Cast iron:

Malleable iron starts as a white iron casting which is heat treated for a day or two at about 950 °C (1,740 °F) and then cooled over a day or two. As a result, the carbon in iron carbide transforms into graphite and ferrite plus carbon (austenite). Surface tension is allowed by the slow processes to form the graphite into spheroidal particles rather than flakes. The spheroids are relatively short and far from one another, and lower cross section due to their lower aspect ratio. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems found in grey cast iron. In general, the properties of malleable cast iron are equal to that of mild steel. There is a limit to how large a part can be cast in malleable iron, as it is made from white cast iron.

Ductile Cast iron:

The ductile cast iron has its graphite in the form of very tiny nodules with the graphite in the form of concentric layers forming the nodules. As a result, the properties of ductile cast iron are that of a spongy steel without the stress concentration effects that flakes of graphite would produce. Tiny amounts of 0.02 to 0.1% magnesium, and only 0.02 to 0.04% cerium added to these alloys slow the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections.