The Iron-Carbon Phase Diagram

The basis for understanding the heat treatment of steels is the iron-carbon (Fe-C) phase diagram. The Fe-C diagram is really two diagrams in one, showing the equilibrium between cementite (iron carbide, or Fe3C) and the several phases of iron, as well as the equilibrium between graphite and the other phases. Steels are alloys of iron, carbon and other elements that contain less than 2% carbon (usually less than 1%), therefore the portion of the diagram below 2% C; that is, the iron-cementite (Fe-Fe3C) diagram, is more pertinent to steel heat treatment. In cast irons, high carbon content (1.75-4.0% C) and high silicon content promote graphite formation. Therefore, cast iron technology is based more on the Fe-graphite diagram.

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The Iron Carbon Equilibrium Diagram

Pure iron is a relatively soft, ductile low strength metal with few practical engineering applications. The addition of Carbon to pure iron increases strength and hardenability to useful levels. However it decreases ductility.

The addition of Carbon influences the allotropic changes discussed in the last section. Since mechanical behaviour is directly related to the phases present it is important to study these phases and how they are influenced by temperature. A study of the Iron- Carbon phase diagram is used for this purpose. Figure 1 is an Iron - Carbon diagram showing the phases present in any alloy containing up to 6% Carbon. As you can see a eutectic point also exists in this diagram

Figure1 The Iron Carbon Equilibrium Diagram

This phase diagram tells us the various phases a particular alloy of Iron and Carbon will go through when allowed to cool down to room temp. Click here to see the phases a 3% Carbon alloy would go through upon cooling from molten liquid. In general iron carbon alloys up to 2% are known as steels while from 2% upwards the alloys are identified as cast iron. Our study mainly deals with the alloys up to 2% carbon i.e the steels part of the diagram so we will move on to look at this section of the diagram. To see the next page click on this diagram anywhere up to 2% Carbon content.

The Steel Section of the Iron - Carbon Diagram.

Shown here is the steel part of the iron carbon diagram containing up to 2% Carbon. At the eutectoid point 0.83% Carbon, Austenite which is in a solid solution changes directly into a solid known as Pearlite which is a layered structure consisting of layers of Ferrite and Cementite.

The steel part of the iron- carbon diagram

In order to fully understand the changes that occur in these different alloys of steels we will look at individual microstructures of common steel alloys.

Here we see the various microstructures that exist in phases up to 2% Carbon content. (The red dots represent carbon dissolved in austenite, in the real structure these would be invisible).

Looks pretty confusing doesn't it. Well do not worry it's really very simple, the easiest way to work on these diagrams is to take an example. Let's take 3 steels containing Carbon as our examples shall we?

o 0.4 % Carbon o 0.83% Carbon o 1.2 % Carbon

• 0.4 % Carbon

Would you like to see these changes occuring? If so click here.

• 0.83 % Carbon

Would you like to see these changes occuring? If so click here.

• 1.2 % Carbon

Would you like to see these changes occuring? If so click here.

Q1. With reference to the given iron carbon diagram;

a) describe the phases present by A, B, C, and D; b) name the points E and F and state what they represent.

Phase "A" is

a) A liquid state consisting of iron and carbon. b) Represents a phase of pearlite with cementite around the grain boundaries. c) Represents ferrite containing a maxmium of 0.2 % Carbon dissolved in a solid solution and the atoms in a B.C.C. arrangement. 2. Phase "B" is a) A phase representing pure pearlite. b) Represents Austenite, which is a solid solution of carbon dissolved in a F.C.C structure c) Represents Austenite and ferrite in an F.C.C. structure 3. Phase "C" is a) Austenite + Liquid. b) Cementite + Liquid. c) Liquid iron and carbon. 4. Phase "D" is a) A phase representing pure pearlite. b) Represents a microstructure of pearlite with cementite around the grain boundaries. c) Represents Austenite and ferrite in an F.C.C. structure

5. Point "E" is a) Austenite + Liquid. b) Cementite + Liquid. c) Liquid iron and carbon. d) The Euctectoid point; At 723 degrees a steel containing 0.87% carbon will change from F.C.C austenite to a B.C.C pearlite structure. 6. Point "F" is a) Peritectic reaction. b) Cementite + Liquid. c) Liquid iron and carbon. d) The eutectic point; At this temperature, a transformation from liquid iron to solid austenite and cementite occurs..

carbon: iron-carbon equilibrium diagramBritannica Concise

Iron-carbon equilibrium diagram.

Referenced by:

Properties of steel > Effects of carbonIn its pure form, iron is soft and generally not useful as an engineering material; the principal method of strengthening it and converting it into steel is by adding small amounts of carbon. In solid steel, carbon is generally found in two forms. Either it is in solid solution in austenite and ferrite or it is found as a carbide. The carbide form can be iron carbide (Fe3C, known as cementite), or it can be a carbide of an alloying element such as titanium. (On the other hand, in gray iron, carbon appears as flakes or clusters of graphite, owing to the presence of silicon, which suppresses carbide formation.)

The effects of carbon are best illustrated by an iron-carbon equilibrium diagram (seefigure). The A-B-C line represents the liquidus points (i.e., the temperatures at which molten iron begins to solidify), and the H-J-E-C line represents the solidus points (at which solidification is completed). The A-B-C line indicates that solidification temperatures decrease as the carbon content of an iron melt is increased. (This explains why gray iron, which contains more than 2 percent carbon, is processed at much lower temperatures than steel.) Molten steel containing, for example, a carbon content of 0.77 percent (shown by the vertical dashed line in thefigure) begins to solidify at about 1,475° C (2,660° F) and is completely solid at about 1,400° C (2,550° F). From this point down, the iron crystals are all in an austenitic—i.e., fcc—arrangement and contain all of the carbon in solid solution. Cooling further, a dramatic change takes place at about 727° C (1,341° F) when the austenite crystals transform into a fine lamellar structure consisting of alternating platelets of ferrite and iron carbide. This microstructure is called pearlite, and the change is called the eutectoidic transformation. Pearlite has a diamond pyramid hardness (DPH) of approximately 200 kilograms-force per square millimetre (285,000 pounds per square inch), compared to a DPH of 70 kilograms-force per square millimetre for pure iron. Cooling steel with a lower carbon content (e.g., 0.25 percent) results in a microstructure containing about 50 percent pearlite and 50 percent ferrite; this is softer than pearlite, with a DPH of about 130. Steel with more than 0.77 percent carbon—for instance, 1.05 percent—contains in its microstructure pearlite and cementite; it is harder than pearlite and may have a DPH of 250.

Properties of steel > Effects of heat-treatingAdjusting the carbon content is the simplest way to change the mechanical properties of steel. Additional changes are made possible by heat-treating—for instance, by accelerating the rate of cooling through the austenite-to-ferrite transformation point, shown by the P-S-K line in thefigure. (This transformation is also called the Ar1 transformation, r standing for refroidissement, or “cooling.”) Increasing the cooling rate of pearlitic steel (0.77 percent carbon) to about 200° C per minute generates a DPH of about 300, and cooling at 400° C per minute raises the DPH to about 400. The reason for this increasing hardness is the formation of a finer pearlite and ferrite microstructure than can be obtained during slow cooling in ambient air. In principle, when steel cools quickly, there is less time for carbon atoms to move through the lattices and form larger carbides. Cooling even faster—for instance, by quenching the steel at about 1,000° C per minute—results in a complete depression of carbide formation and forces the undercooled ferrite to hold a large amount of carbon atoms in

solution for which it actually has no room. This generates a new microstructure, martensite. The DPH of martensite is about 1,000; it is the hardest and most brittle form of steel. Tempering martensitic steel—i.e., raising its temperature to a point such as 400° C and holding it for a time—decreases the hardness and brittleness and produces a strong and tough steel. Quench-and-temper heat treatments are applied at many different cooling rates, holding times, and temperatures; they constitute a very important means of controlling steel's properties. (See also below Treating of steel: Heat-treating.)

steel : The process (Encyclopædia Britannica) Making a heat begins with an inspection of the refractory lining, with the converter in a turned-down position. Sometimes a laser contour instrument is used to determine the remaining lining thickness. With the converter tilted at about 45°, scrap is then charged into the furnace by heavy cranes or ...

steel : Heat-treating (Encyclopædia Britannica) In principle, heat-treating already takes place when steel is hot-rolled at a particular temperature and cooled afterward at a certain rate, but there are also many heat-treating process facilities specifically designed to produce particular microstructures and properties. The simplest ...

steel : Annealing (Encyclopædia Britannica) To make steel ductile for subsequent forming operations, an annealing treatment is applied. In annealing, the steel is usually held for several hours at several degrees below Ar1 (shown by the P-S-K line in the ) and then slowly cooled. This precipitates and coagulates the carbides and results in ...

Iron-Iron Carbide Phase Diagram Example

Fig 1: Fe-Fe3C Phase Diagram (clickable), Materials Science and Metallurgy, 4th ed., Pollack, Prentice-Hall, 1988

Figure 1 shows the equilibrium diagram for combinations of carbon in a solid solution of iron. The diagram shows iron and carbons combined to form Fe-Fe3C at the 6.67%C end of the diagram. The left side of the diagram is pure iron combined with carbon, resulting in steel alloys. Three significant regions can be made relative to the steel portion of the diagram. They are the eutectoid E, the hypoeutectoid A, and the hypereutectoid B. The right side of the pure iron line is carbon in combination with various forms of iron called alpha iron (ferrite), gamma iron (austenite), and delta iron. The black dots mark clickable sections of the diagram. Allotropic changes take place when there is a change in crystal lattice structure. From 2802º-2552ºF the delta iron has a body-centered cubic lattice structure. At 2552ºF, the lattice changes from a body-centered cubic to a face-centered cubic lattice type. At 1400ºF, the curve shows a plateau but this does not signify an allotropic change. It is called the Curie temperature, where the metal changes its magnetic properties. Two very important phase changes take place at 0.83%C and at 4.3% C. At 0.83%C, the transformation is eutectoid, called pearlite. gamma (austenite) --> alpha + Fe3C (cementite) At 4.3% C and 2066ºF, the transformation is eutectic, called ledeburite. L(liquid) --> gamma (austenite) + Fe3C (cementite)Equilibrium Calculations 1. Given the Fe-Fe3C phase diagram, Fig. 1, calculate the phases present at the eutectoid composition line at:

a. T = 3000ºF b. T = 2200ºF c. T = 1333ºF d. T = 410ºF

2. Calculate the phases in the cast-iron portion of the diagram at the eutectic composition of 4.3% C in combination with 95.7% ferrite at:

a. T = 3000ºF b. T = 1670ºF c. T = 1333ºF

3. A eutectoid steel (about 0.8%C) is heated to 800ºC (1472ºF) and cooled slowly through the eutectoid temperature. Calculate the number of grams of carbide that form per 100g of steel.

4. Determine the amount of pearlite in a 99.5% Fe-0.5%C alloy that is cooled slowly from 870ºC given a basis of 100g of alloy.

Solutions

1. a. T = 3000ºF. Since the composition E is eutectoid, the carbon content is 0.83%. b. T = 2200ºF. At this temperature, austenite exists as a single-phase solid. c. T = 1333ºF. Two phases exist, ferrite and austenite. The percentages are determined by the lever rule: X ÷ (X+Y) = (Cy-C) ÷ (Cy-Cx).

proeutectoid ferrite = (0.83 - 0.18) / (0.83 - 0.025) x 100 = 80.7% austenite = (0.18 - 0.025) / (0.83 - 0.025) x 100 = 19.3%

d. T = 410ºF. A small amount of cementite will precipitate following the solubility line from 0.025% C at 1333ºF to 0.008% C at room temperature. The overall percentages of ferrite and cementite are:

ferrite = (6.67 - 0.18) / (6.67 - 0.01) x 100 = 97.4 cementite = (0.18 - 0.01) / (6.67 - 0.01) x 100 = 2.6%

2. a. T = 3000ºF. At this temperature, the eutectic is all liquid. b. T = 1670ºF. Since there is a change of the solubility line, there will be a change in the austenite composition, that will transform to the eutectoid at 1333ºF. The overall austenite and cementite composition will be:

austenite = (6.67 - 4.3) / (6.67 - 1.2) x 100 = 43.3% cementite = (4.3 - 1.2) / (6.67 - 1.2) x 100 = 56.7% c. T = 1333ºF. At this temperature the austenite has a eutectoid composition and will transform into pearlite. The compositions of proeutectoid cementite and austenite are: eutectoid austenite = (6.67 - 4.3) / (6.67 - 0.83) x 100 = 40.6% proeutectoid cementite = (4.3 - 0.83) / (6.67 - 0.83) x 100 = 59.4% The austenite has a eutectoid composition that is: eutectoid ferrite = (6.67 - 0.83) / (6.67 - 0.025) x 100 = 88% eutectoid cementite = (0.83 - 0.025) / (6.67 - 0.025) x 100 = 12%

3. Interpolate between alpha (0.02%C) and Fe3C (6.7%) at 1333ºF

Carbide = (0.8 - 0.02) / (6.7 - 0.02) x 100 = 12g

4. Since pearlite originates from austenite of eutectoid composition, determine the amount of gamma just prior to the eutectoid reaction.

From 870ºC - 780ºC : 100g austenite with 0.5% C From 780ºC - 727ºC : ferrite separates from austenite and the carbon content of the austenite increases to about 0.8% C At 727ºC (+) : proeutectoid ferrite: composition of ferrite = 0.02% C, amount of carbon = 38g Gamma that transforms to pearlite: compostion of austenite = 0.8% C. Amount of austenite = 62g. At 727ºC (-): Amount of pearlite = 62g

Plain Iron Carbon Steels

IntroductionThis page includes notes on iron and what happens when iron /carbon mixtures are cooled from liquid to solid. The notes are based on the Iron Phase Diagram (equilibrium diagram ). A "Phase" is a form of material having characteristic structure and properties. It is a form of the material which has identifiable composition, structure and boundaries separating it from other phases in the material volume. The diagram below shows shows the phases present when when Fe-C alloys (C up to 7%) are cooled from liquid to solid.

The left side of the diagram represents pure iron and the right hand of the diagram represents an alloy with 6,67% C. which result on cooling in the formation of Cementite. This is a intermetallic compound (iron carbide-Fe3 C) which although not 100% stable is but is to all practical purposes a stable phase. The phase diagram shown is therefore a metastable phase.

Note: Different reference sources indicate the Eutectoid point at 0,8% C and 0,77% C.

Iron Forms

Iron can exist in three forms:α... BCC crystal with crystal dimension a = 2,86 Angstrom exists at temperatures up to 910oC γ... FCC crystal with crystal dimension a = 3,65 Angstrom exists at temperature range 910oC to 1403oC δ... BCC crystal with crystal dimension a = 2,93 Angstrom exists at temperature range 1403oC to 1535oC

Solid PhasesThe phased diagram includes four solid phases

α Ferrite ..The solid solution of carbon in iron. At 0% C this is pure iron. BCC crystal structure. The maximum solubility of carbon in iron is 0,02% at 723oC. At 0 oC temperature the solubility falls to 0,008%. The carbon atoms are located in the crystal interstices.

AusteniteThe solid solution of carbon in γ iron is called austenite . This has a FCC crystal structure with a high solubility for carbon compared with α ferrite. The solubility reaches a maximum of 2,08% at 1148oC . The solubility decreases to 0,8% at 723 oC The carbon atoms are dissolved interstitially. The difference in solubility between the austenite and α Ferrite is the basis for the hardening of steels

CementiteThis is an intermetallic compound which contains 6,67% C and 93,3% Fe. Cementite is a hard brittle compound with and orthorhombic crystal structure each unit cell has 12 Fe atoms and 4 C atoms

δ Ferrite..This is a solid solution of carbon in iron and has a BCC crystal structure. The maximum solubility or C in Fe is 0,09% at 1495oC. This has no real practical significance in engineering.

Lever Rule The lever rule can be applied to any phase region an provides an indication of the proportions of the constituent parts at any point on the phase diagram

Applying the lever rule to the eutectoid point (0,80% C at 723oC )Wt% Ferrite = 100 (6,67 -0,8)/ 6,67- 0,02) = 88%Wt% Cementite = 100 (0,8- 0,02) /6,67- 0,02) = 12%

SteelsIf the carbon content of the cooled solid is less than Eutectoid (about 0,8% C) the solid is identified as a hypoeutectoid steel: most steels are this form. If the carbon content is more then 0,8% then the solid is a hypereutectoid steel. Hypereutectoid steels with carbon content over 1,2% C are very brittle. Few steels are made with carbon contents over 1,2%.

Generally in order to increase the strength of steel other alloying elements are added which increase the strength while retaining toughness and ductility

A eutectoid solid raised to a temperature just above 723oC and held at this temperature for some time will become a homogeneous austenite phase. If this solid is then slowly cooled the entire structure will change to a lamellar structure of alternate plates of ferrite and cementite. This structure is called pearlite because of its mother-of-pearl appearance. As calculated above pearlite consists of 88% (mass) ferrite and 12% (mass) cementite

Transformation of a hypereutectoid steel (> 0,8% C) will result in pearlite in a matrix of cementite. Transformation of a hypoeutectoid steel (< 0,8% C ) will result in pearlite in a matrix of α Ferrite.

Transformation on high cooling rates Below is an Isothermal Transformation (IT) diagram, also called a TTT (Time, Temperature, Transformation) curve for a eutectoid steel test piece

which has been rapidly cooled in a bath at a set temperature, held for a time and then water quenched.

It can be seen that if the transformation is allowed to take place at a higher temperature then, as above coarse pearlite is formed.

If the test piece is cooled to a lower temperature in a bath a finer pearlitic structure results.

If the test piece is rapidly cooled to a temperature below a value Ms (which varies with the carbon content then a new metastable phase is produced called Martensite. Martensite is a supersaturated solid solution of carbon in ferrite.

If the test piece is cooled rapidly at a temperature between 220oC and 525oC a phase structure between pearlite and martensite is formed. This is called Bainite

BainiteBainite is a constituent which forms from austenite in a tempertures range below 530 oC and above Ms . Bainite forms together with pearlite in steel which are cooled somewhat too fast to form a complete pearlite structure. Bainite is like pearlite a mixture of ferrite and iron carbide but in a different form. The bainite structure varies from a featherlike pattern to pattern of lens shaped particles depending on the temperature range of formation. (Featherlike constituent in upper temperature range and lens like in the lower temperature range). Bainite is harder, stronger and tougher than ferrite-pearlite structures at lower temperatures.

MartensiteMartensite is the hardest structure formed from austenite. It is a distorted BCC (tetrogonal) it is a body centred tetrogonal structure. The distortion is caused by trapped carbon atoms which have not been able to nucleate into cementite

Some features of martensite are listed belowThe crystal structure is a stressed structure which is resistance to dislocation movement it is therefore strong and relatively brittle. There are various types of martensite depending on the carbon content: for C <0,2% the martensite is in the form of well defined thin strips (laths),for C <= 0,6% plates of martensite are formed , for C < = 1,2% the martensite is in the form of arrays of well defined plates The martensite phase initiates at a temperature Ms and is complete at a temperature Mf. Ms varies from 500oC for C <= 0,1% to 200oC for C <= 1,2%

The Iron-Carbon Equilibrium Diagram

Printable Version

Abstract: A study of the constitution and structure of all steels and irons must first start with the iron-carbon equilibrium diagram. Many of the basic features of this system influence the behavior of even the most complex alloy steels. For example, the phases found in the simple binary Fe-C system persist in complex steels, but it is necessary to examine the effects alloying elements have on the formation and properties of these phases. The iron-carbon diagram provides a valuable foundation on which to build

knowledge of both plain carbon and alloy steels in their immense variety.

A study of the constitution and structure of all steels and irons must first start with the iron-carbon equilibrium diagram. Many of the basic features of this system (Fig. 1) influence the behavior of even the most complex alloy steels. For example, the phases found in the simple binary Fe-C system persist in complex steels, but it is necessary to examine the effects alloying elements have on the formation and properties of these phases. The iron-carbon diagram provides a valuable foundation on which to build knowledge of both plain carbon and alloy steels in their immense variety.

Fig. 1. The iron-carbon diagram.

It should first be pointed out that the normal equilibrium diagram really represents the metastable equilibrium between iron and iron carbide (cementite). Cementite is metastable, and the true equilibrium should be between iron and graphite. Although graphite occurs extensively in cast irons (2-4 wt % C), it is usually difficult to obtain this equilibrium phase in steels (0.03-1.5 wt %C). Therefore, the metastable equilibrium between iron and iron carbide should be considered, because it is relevant to the behavior of most steels in practice.

The much larger phase field of γ-iron (austenite) compared with that of α-iron (ferrite) reflects the much greater solubility of carbon in γ-iron, with a maximum value of just over 2 wt % at 1147°C (E, Fig.1). This high solubility of carbon in γ-iron is of extreme importance in heat treatment, when solution treatment in the γ-region followed by rapid quenching to room temperature allows a supersaturated solid solution of carbon in iron to be formed.

The α-iron phase field is severely restricted, with a maximum carbon solubility of 0.02 wt% at 723°C (P), so over the carbon range encountered in steels from 0.05 to 1.5 wt%, α-iron is normally associated with iron carbide in one form or another. Similarly, the δ-phase field is very restricted between 1390 and 1534°C and disappears completely when the carbon content reaches 0.5 wt% (B).

There are several temperatures or critical points in the diagram, which are important, both from the basic and from the practical point of view.

• Firstly, there is the A1, temperature at which the eutectoid reaction occurs (P-S-K), which is 723°C in the binary diagram.

• Secondly, there is the A3, temperature when α-iron transforms to γ-iron. For pure iron this occurs at 910°C, but the transformation temperature is progressively lowered along the line GS by the addition of carbon.

• The third point is A4 at which γ-iron transforms to δ-iron, 1390°C in pure iron, hut this is raised as carbon is added. The A2, point is the Curie point when iron changes from the ferro- to the paramagnetic condition. This temperature is 769°C for pure iron, but no change in crystal structure is involved. The A1, A3 and A4

points are easily detected by thermal analysis or dilatometry during cooling or heating cycles, and some hysteresis is observed. Consequently, three values for each point can be obtained. Ac for heating, Ar for cooling and Ae (equilibrium}, but it should be emphasized that the Ac and Ar values will be sensitive to the rates of heating and cooling, as well as to the presence of alloying elements.

The great difference in carbon solubility between γ- and α-iron leads normally to the rejection of carbon as iron carbide at the boundaries of the γ phase field. The transformation of γ to α - iron occurs via a eutectoid reaction, which plays a dominant role in heat treatment.

The eutectoid temperature is 723°C while the eutectoid composition is 0.80% C(s). On cooling alloys containing less than 0,80% C slowly, hypo-eutectoid ferrite is formed from austenite in the range 910-723°C with enrichment of the residual austenite in carbon, until at 723°C the remaining austenite, now containing 0.8% carbon transforms to pearlite, a lamellar mixture of ferrite and iron carbide (cementite). In austenite with 0,80 to 2,06% carbon, on cooling slowly in the temperature interval 1147°C to 723°C, cementite first forms progressively depleting the austenite in carbon, until at 723°C, the austenite contains 0.8% carbon and transforms to pearlite.

Steels with less than about 0.8% carbon are thus hypo-eutectoid alloys with ferrite and pearlite as the prime constituents, the relative volume fractions being determined by the lever rule which states that as the carbon content is increased, the volume percentage of pearlite increases, until it is 100% at the eutectoid composition. Above 0.8% C, cementite becomes the hyper-eutectoid phase, and a similar variation in volume fraction of cementite and pearlite occurs on this side of the eutectoid composition.

The three phases, ferrite, cementite and pearlite are thus the principle constituents of the infrastructure of plain carbon steels, provided they have been subjected to relatively slow cooling rates to avoid the formation of metastable phases.

The austenite- ferrite transformationUnder equilibrium conditions, pro-eutectoid ferrite will form in iron-carbon alloys containing up to 0.8 % carbon. The reaction occurs at 910°C in pure iron, but takes place between 910°C and 723°C in iron-carbon alloys.

However, by quenching from the austenitic state to temperatures below the eutectoid temperature Ae1, ferrite can be formed down to temperatures as low as 600°C. There are pronounced morphological changes as the transformation temperature is lowered, which it should be emphasized apply in general to hypo-and hyper-eutectoid phases, although in each case there will be variations due to the precise crystallography of the phases involved. For example, the same principles apply to the formation of cementite from austenite, but it is not difficult to distinguish ferrite from cementite morphologically.

The austenite-cementite transformationThe Dube classification applies equally well to the various morphologies of cementite formed at progressively lower transformation temperatures. The initial development of grain boundary allotriomorphs is very similar to that of ferrite, and the growth of side plates or Widmanstaten cementite follows the same pattern. The cementite plates are more rigorously crystallographic in form, despite the fact that the orientation relationship with austenite is a more complex one.

As in the case of ferrite, most of the side plates originate from grain boundary allotriomorphs, but in the cementite reaction more side plates nucleate at twin boundaries in austenite.

The austenite-pearlite reactionPearlite is probably the most familiar micro structural feature in the whole science of metallography. It was discovered by Sorby over 100 years ago, who correctly assumed it to be a lamellar mixture of iron and iron carbide.

Pearlite is a very common constituent of a wide variety of steels, where it provides a substantial contribution to strength. Lamellar eutectoid structures of this type are widespread in metallurgy, and frequently pearlite is used as a generic term to describe them.

These structures have much in common with the cellular precipitation reactions. Both types of reaction occur by nucleation and growth, and are, therefore, diffusion controlled. Pearlite nuclei occur on austenite grain boundaries, but it is clear that they can also be associated with both pro-eutectoid ferrite and cementite. In commercial steels, pearlite nodules can nucleate on inclusions.

The Metallurgy Of Carbon Steel The best way to understand the metallurgy of carbon steel is to study the ‘Iron Carbon Diagram’. The diagram shown below is based on the transformation that occurs as a result of slow heating. Slow cooling will reduce the transformation temperatures; for example: the A1 point would be reduced from 723°C to 690 °C. However the fast heating and cooling rates encountered in welding will have a significant influence on these temperatures, making the accurate prediction of weld metallurgy using this diagram difficult.

• Austenite This phase is only possible in carbon steel at high temperature. It has a Face Centre Cubic (F.C.C) atomic structure which can contain up to 2% carbon in solution.

• Ferrite This phase has a Body Centre Cubic structure (B.C.C) which can hold very little carbon; typically 0.0001% at room temperature. It can exist as either: alpha or delta ferrite.

• Carbon A very small interstitial atom that tends to fit into clusters of iron atoms. It strengthens steel and gives it the ability to harden by heat treatment. It also causes major problems for welding , particularly if it exceeds 0.25% as it creates a hard microstructure that is susceptible to hydrogen cracking. Carbon forms compounds with other elements called carbides. Iron Carbide, Chrome Carbide etc.

• Cementite Unlike ferrite and austenite, cementite is a very hard intermetallic compound consisting of 6.7% carbon and the remainder iron, its chemical symbol is Fe3C. Cementite is very hard, but when mixed with soft ferrite layers its average

hardness is reduced considerably. Slow cooling gives course perlite; soft easy to machine but poor toughness. Faster cooling gives very fine layers of ferrite and cementite; harder and tougher

• Pearlite A mixture of alternate strips of ferrite and cementite in a single grain. The distance between the plates and their thickness is dependant on the cooling rate of the material; fast cooling creates thin plates that are close together and slow cooling creates a much coarser structure possessing less toughness. The name for this structure is derived from its mother of pearl appearance under a microscope. A fully pearlitic structure occurs at 0.8% Carbon. Further increases in carbon will create cementite at the grain boundaries, which will start to weaken the steel.

• Cooling of a steel below 0.8% carbon When a steel solidifies it forms austenite. When the temperature falls below the A3 point, grains of ferrite start to form. As more grains of ferrite start to form the remaining austenite becomes richer in carbon. At about 723°C the remaining austenite, which now contains 0.8% carbon, changes to pearlite. The resulting structure is a mixture consisting of white grains of ferrite mixed with darker grains of pearlite. Heating is basically the same thing in reverse.

• Martensite If steel is cooled rapidly from austenite, the F.C.C structure rapidly

changes to B.C.C leaving insufficient time for the carbon to form pearlite. This results in a distorted structure that has the appearance of fine needles. There is no partial transformation associated with martensite, it either forms or it doesn’t. However, only the parts of a section that cool fast enough will form martensite; in a

thick section it will only form to a certain depth, and if the shape is complex it may only form in small pockets. The hardness of martensite is solely dependant on carbon content, it is normally very high, unless the carbon content is exceptionally low.

• Tempering The carbon trapped in the martensite transformation can be released by heating the steel below the A1 transformation temperature. This release of carbon from nucleated areas allows the structure to deform plastically and relive some of its internal stresses. This reduces hardness and increases toughness, but it also tends to reduce tensile strength. The degree of tempering is dependant on temperature and time; temperature having the greatest influence.

• Annealing This term is often used to define a heat treatment process that produces some softening of the structure. True annealing involves heating the steel to austenite and holding for some time to create a stable structure. The steel is then cooled very slowly to room temperature. This produces a very soft structure, but also creates very large grains, which are seldom desirable because of poor toughness.

• Normalising Returns the structure back to normal. The steel is heated until it just starts to form austenite; it is then cooled in air. This moderately rapid transformation creates relatively fine grains with uniform pearlite.

• Welding If the temperature profile for a typical weld is plotted against the carbon equilibrium diagram, a wide variety of transformation and heat treatments will be observed.

Note, the carbon equilibrium diagram shown above is only for illustration, in reality it will be heavily distorted because of the rapid heating and cooling rates involved in the welding process.

a)

b)

c)

d)

Mixture of ferrite and pearlite grains; temperature below A1, therefore microstructure not significantly affected.

Pearlite transformed to Austenite, but not sufficient temperature available to exceed the A3 line, therefore not all ferrite grains transform to Austenite. On cooling, only the transformed grains will be normalised.

Temperature just exceeds A3 line, full Austenite transformation. On cooling all grains will be normalised

Temperature significantly exceeds A3 line permitting grains to grow. On cooling, ferrite will form at the grain boundaries, and a course pearlite will form inside the grains. A course grain structure is more readily hardened than a finer one, therefore if the cooling rate between 800°C to 500°C is rapid, a hard microstructure will be formed. This is why a brittle fracture is most likely to propagate in this region.

• Welds The metallurgy of a weld is very different from the parent material. Welding filler metals are designed to create strong and tough welds, they contain fine oxide particles that permit the nucleation of fine grains. When a weld solidifies, its grains grow from the course HAZ grain structure, further refinement takes place within these course grains creating the typical acicular ferrite formation shown opposite.