Phase Diagram for Iron Carbon Alloys | Steel | Metallurgy | Industries

A phase diagram is a chart which shows the number and nature of phases that are present in a given alloy at any temperature and composition under equilibrium condition.  Fig. 1.13 shows the different constituents of steel.

There are four main phases of steels, viz., ferrite, cementite austenite and pearlite. It has been noticed that various other elements, besides carbon and iron, which are present in steels as impurities or as alloying elements, have no appreciable effect on the iron carbon constitution diagram shown in Fig. 1.13. However, by their presence the position of the boundary lines are changed.

These four phases are discussed below:

1. Ferrite:

It is a solid solution of upto 0.025% carbon in the solvent a-iron. This phase is indicated in the diagram by GSP. ϒ phase is converted to ferrite due to slow cooling of the solid alloys. Ferrite generally contains no carbon but many other elements such as Mn, Si, Cr in the solid solution.

Ferrite is soft, weak and ductile.

The hardness of the ferrite is as low as 50 to 100 Brinell. Ferrite is most prominent by its high ductility.

2. Cementite:

It is an intermittent compound consisting of a definite lattice arrangement of iron and carbon atoms, the relative number of each of the atoms present in a given sample being in accordance with the formula Fe₃C. This phase is formed due to slow cooling of solid alloys within the area ESK. In chemical composition it is a compound of carbon and iron carbide, Fe₃C.

Other elements which may be present in steel are also in the form of their carbides. In such a steel, cementite is very hard and the hardness is of the order of 1400 Brinell. In the annealed steel, (that is, in the steel which has been cooled in a controlled manner) cementite is found as spheroids (rounded particles) or parallel plates (lamellar layers) or as a covering over the pearlite grains. Cementite is quite brittle, but hard and strong.

Ferrite and cementite tend to form a laminated structure called pearlite.

The amount of pearlite in plain carbon steel (carbon 0.1 to 1.5%) increases with increase in carbon content. When carbon content is 0.83%, all the grains will be pearlitic. Carbon is excess of 0.83% forms free cementite at the grain boundaries, causing a reduction of the tensile strength, due to the localised brittleness.

3. Austenite:

It is a solid solution of upto 1.7% carbon in gamma iron. Austenite is obtained by heating carbon-iron- steel above the range GSF. The formation of austenite takes place due to interface reaction of ferrite and cementite. At first, nuclei of austenite are formed and then they go on growing by the further reaction of ferrite and cementite. The speed of formation of austenite is increased by the increase of temperature.

The iron-carbon diagram tells which of the three steel phases are preset, at a given temperature and carbon concentration, when the alloy is cooled or heated slowly enough so that it remains in a state of equilibrium.

4. Pearlite:

The iron at room temperature has a body-centred-cubic lattice (a-iron), and on heating above 910°C, it adopts face-centred-cubic arrangement (ϒ-iron also called austenite). These two allotropic forms of iron have different physical properties like coefficients of expansion).

The change from one allotropic form to other is called transformation or reaction. Energy is expended in the change from ϒ to α iron, and the cooling curve shows a thermal arrest at the temperature at which the event occurs.

For pure iron the transformation occurs at 910°C but with addition of carbon it occurs over a range of temperatures, the limits being known as critical points — (A₃ the higher temperature, i.e. the one at which transformation starts during cooling, and A₁ — the lower point which is 723° for all types of iron/steel). At 0.8% carbon A₃ merges with A₁ and rises at lower as well as upper carbon levels.

Carbon has different solubility in austenite and ferrite. At 1130°C, austenite can hold upto 1.7% carbon when all the carbon is in solution. The carbon is dissolved interstitially i.e. the carbon atom does not replace an iron atom to form part of the cube but nestles between iron atoms. In the case of body-centred-cubic pattern (for ferrite), the space between the iron atoms is inadequate and the carbon atom is ejected from its site. This is what reaction occurs at A₃ point.

At A₃ point, the b.c.c. ferrite grain starts nucleating at the edge of an austenite grain, setting up a boundary between the two constituents. As the temperature is lowered between A₃ and A₁ limits, the boundary advances further and further into the austenite grain. When the carbon is rejected from the ferrite lattice, it diffuses into the body of the austenite grain and is retained in solution.

The untransformed austenite thus gets progressively richer in carbon upto a limit of 0.8%. On further cooling at A₁ limit, the remaining austenite is completely changed to ferrite and virtually all the carbon is rejected from the solution to form layers of cementite which are sandwiched between ferrite.

If a piece of plain carbon steel (carbon upto 1.5% and no alloying elements) is heated at a uniform rate in a furnace, its temperature will rise at a uniform rate until a temperature is reached at which this rise is halted for a short time, or temperature may even fall, although the temperature of the furnace continues to rise. At this point, the heat is being used to cause a rearrangement of the iron atoms, which in turn cause the formation of a solid solution called austenite.

The temperature at which the pearlite is transformed into austenite is called lower critical heating point (around 723°C). The ferrite or cementite get converted into austenite at a higher temperature known as upper critical heating point (which depends upon the carbon content of the steel).

Similarly during uniform cooling, temperature falls at a uniform rate until a temperature is reached at which it starts to cool less rapidly (known as upper critical cooling temperature and is 30°C less than the upper critical heating point).

At this point, the austenite starts to break down to produce either ferrite or cementite, according to the carbon content of the steel. This change continues until the lower critical cooling point (695°C) is reached, when any remaining austenite is transformed into pearlite.

The iron-carbon diagram does not tell anything about the state of aggregation of the phases present. In other words, the information like the relative sizes or shapes of the ferrite and cementite cannot be had when both of these are present in a certain temperature region. This information can be had only by the microscopic examination of a polished and etched surface of the metal.

The iron-carbon diagram also helps in determining the phase transformations which occur when a steel specimen of certain composition is cooled slowly from a high temperature. Let us study the case of a steel specimen of composition 0.5% carbon. In the region GSEN the metal is all austenite and will remain in this state until it is cooled and crosses line GS when ferrite will start to form along the grain boundaries of the austenite.

The amount of ferrite will increase as the temperature falls from line GS to line PS. At temperature of 723°C all of the remaining austenite will have a carbon content of 0.80% and will transform at constant temperature into alternate plates of ferrite and cementite (this plate like lamellar structure being known as pearlite). At all temperatures below 723°C we still have mixture of ferrite and pearlite.

Next let us examine what changes occur when an eutectoid steel (steel of 0.8% carbon) is cooled. In this case nothing happens till a temperature of 723°C is reached when all of the metal transforms at constant temperature into pearlite.

If a steel piece having carbon content of 1.2% be cooled, then cementite first precipitates along the grain boundaries at temperature corresponding to line SE. This continues until temperature of 723°C is reached when all of the remaining material transforms into pearlite. There is no further change in structure below temperature of 723°C.

The ability of steel to harden depends upon the difference in carbon solubility of austenite and ferrite and the tendency for the excess carbon to precipitate in the form of cementite when austenite transforms to ferrite. The spacing of the cementite particles depends upon the rate of cooling and their shape.

If rate of cooling of steel, while it is crossing 723°C temperature line, is quick then particles of cementite precipitated will be very small and closely spaced, and if it is slow then particles of cementite precipitated will be larger and more widely spaced.

The shape of the cementite particles may be of lamellar, spheroidal or acicular type. The lamellar structure known as pearlite consists of alternate plates of cementite and ferrite. Whether the pearlite is coarse or fine depends upon the relative spacing of the cementite plates.

The spheroidal structure consists of roughly spherical globules of cementite in a matrix of ferrite. This type of structure is known as spheroidite in which the globules are relatively large. The acicular structure, also sometimes known as Widmanstatten structure consists of a cross- hatched needle-like structure of ferrite needles in very fine pearlite.

What happens when a 0.5% carbon steel specimen is cooled from above 723°C temperature line is shown below:

The more complex changes which occur on very rapid cooling of austenite leading to very hard structures can be explained by a time-temperature-transformation (3T) diagram, or S curves, so called, due to their shape (also known as isothermal transformations).

It is pointed out that when steel is heated above line GS, the iron carbide in the iron decomposes; all of the iron will transform to the y iron and the carbon will all go into the solution (the resulting structure being known as austenite). When austenite steel is suddenly cooled to a temperature below 723°C and held at that temperature for varying lengths of time then different types of structures of different hardness are formed.

The transformation for different temperatures starts and ends at different times. The S-curves or ST-curves are the plots on semi-log paper which show the time at which the resulting transformation starts and ends when the austenitic steel is quenched quickly to a particular temperature. Fig. 1.15 shows such a curve.

It may be noted that if transformation takes place near temperature of 723°C (A₁), the resulting structure will be coarse pearlite and steel will be relatively soft. As the transformation temperature is decreased, the pearlite becomes finer and the steel becomes harder.

This trend continues till temperature of about 565°C (A₀) is reached, below which upto a temperature of 150°C (A₀’) a new structure known as bainite is obtained. It consists of a feathery combination of iron carbide and ferrite. The structure near the temperature A₀ resembles pearlite and that near A₀‘ is finer and harder and has a more acicular structure of the Widmanstatten type.

If the transformation temperature is below 150°C then a structure known as martensite is formed. The austenite mainly consists of delta iron and is of face-centred cubic form whereas iron at low temperature is of body-centred cubic form.

Thus the ordinary transformation of delta iron to alpha iron upon cooling requires a significant readjustment of the iron atoms which requires time to achieve. If material be quenched at very fast rate then metal has no time to rearrange and super-cooled delta iron is formed.

The super-cooling of iron from austenitic form renders the atoms less mobile and increases the time required for the normal delta to alpha rearrangement to occur and subjects the material to very large internal stresses due to the excess carbon present. The net result is formation of a distorted form of ferrite with tetragonal martensite. The resulting super-saturated solid solution of carbon in body- centred tetragonal distorted ferrite is very hard and brittle and is known as martensite.

Fig. 1.16 shows the formation of ferrite, pearlite and cementite for carbon variation upto 1.2% and the variation of mechanical properties.