IRON-IRON CARBIDE PHASE DIAGRAM

The iron-iron carbide phase diagram (Fig. ) furnishes a map showing the ranges of compositions

and temperatures in which the various phases such as austenite, ferrite, and cementite are present in

slowly cooled steels. The diagram covers the temperature range from 60O0C to the melting point of

iron, and carbon contents from O to 5%. In steels and cast irons, carbon can be present either as iron

carbide (cementite) or as graphite. Under equilibrium conditions, only graphite is present because

iron carbide is unstable with respect to iron and graphite. However, in commercial steels, iron carbide

is present instead of graphite. When a steel containing carbon solidifies, the carbon in the steel usually

solidifies as iron carbide. Although the iron carbide in a steel can change to graphite and iron when

the steel is held at ~ 90O0C for several days or weeks, iron carbide in steel under normal conditions

is quite stable.

The portion of the iron-iron carbide diagram of interest here is that part extending from O to

2.01% carbon. Its application to heat treatment can be illustrated by considering the changes occurring

on heating and cooling steels of selected carbon contents.

Iron occurs in two allotropic forms, a or 8 (the latter at a very high temperature) and yThe temperatures at which these phase changes occur are known as the critical temperatures,

and the boundaries in Fig. 2.2 show how these temperatures are affected by composition. For pure

iron, these temperatures are 91O0C for the a-y phase change and 1390° for the y-8 phase change

Changes on Heating and Cooling Pure Iron

The only changes occurring on heating or cooling pure iron are the reversible changes at —9100C
from bcc a iron to fee y iron and from the fee 8 iron to bcc y iron at ~1390°C.

Changes on Heating and Cooling Eutectoid Steel

Eutectoid steels are those that contain 0.8% carbon. The diagram shows that at and below 7270C the

constituents are a-ferrite and cementite. At 60O0C, the a-ferrite may dissolve as much as 0.007%

carbon. Up to 7270C, the solubility of carbon in the ferrite increases until, at this temperature, the ferrite contains about 0.02% carbon. The phase change on heating an 0.8% carbon steel occurs at

7270C which is designated as A1, as the eutectoid or lower critical temperature. On heating just above

this temperature, all ferrite and cementite transform to austenite, and on slow cooling the reverse

change occurs.When a eutectoid steel is slowly cooled from —7380C, the ferrite and cementite form in alternate layers of microscopic thickness. Under the microscope at low magnification, this mixture of ferrite

and cementite has an appearance similar to that of a pearl and is therefore called pearlite.Changes on Heating and Cooling Hypoeutectoid Steels

Hypoeutectoid steels are those that contain less carbon than the eutectoid steels. If the steel contains

more than 0.02% carbon, the constituents present at and below 7270C are usually ferrite and pearlite;

the relative amounts depend on the carbon content. As the carbon content increases, the amount of

ferrite decreases and the amount of pearlite increases.

The first phase change on heating, if the steel contains more than 0.02% carbon, occurs at 7270C.

On heating just above this temperature, the pearlite changes to austenite. The excess ferrite, called

proeutectoid ferrite, remains unchanged. As the temperature rises further above A1, the austenite

dissolves more and more of the surrounding proeutectoid ferrite, becoming lower and lower in carbon

content until all the proeutectoid ferrite is dissolved in the austenite, which now has the same average

carbon content as the steel.On slow cooling the reverse changes occur. Ferrite precipitates, generally at the grain boundariesof the austenite, which becomes progressively richer in carbon. Just above A1, the austenite is substantiallyof eutectoid composition, 0.8% carbon.

Changes on Heating and Cooling Hypereutectoid Steels

The behavior on heating and cooling hypereutectoid steels (steels containing >0.80% carbon) is

similar to that of hypoeutectoid steels, except that the excess constituent is cementite rather than

ferrite. Thus, on heating above A1, the austentie gradually dissolves the excess cementite until at the

Acm temperature the proeutectoid cementite has been completely dissolved and austenite of the same

carbon content as the steel is formed. Similarly, on cooling below Acm, cementite precipitates and

the carbon content of the austenite approaches the eutectoid composition. On cooling below A1, this

eutectoid austenite changes to pearlite and the room-temperature composition is, therefore, pearlite

and proeutectoid cementite.

Early iron-carbon equilibrium diagrams indicated a critical temperature at ~768°C. It has since

been found that there is no true phase change at this point. However, between —768 and 79O0C there

is a gradual magnetic change, since ferrite is magnetic below this range and paramagnetic above it.

This change, occurring at what formerly was called the A2 change, is of little or no significance with

regard to the heat treatment of steel.

Effect of Alloys on the Equilibrium Diagram

The iron-carbon diagram may, of course, be profoundly altered by alloying elements, and its application
should be limited to plain carbon and low-alloy steels. The most important effects of the
alloying elements are that the number of phases that may be in equilibrium is no longer limited to
two as in the iron-carbon diagram; the temperature and composition range, with respect to carbon,
over which austenite is stable may be increased or reduced; and the eutectoid temperature and composition
may change.
Alloying elements either enlarge the austenite field or reduce it. The former include manganese,
nickel, cobalt, copper, carbon, and nitrogen and are referred to as austenite formers.
The elements that decrease the extent of the austenite field include chromium, silicon, molybdenum,
tungsten, vanadium, tin, niobium, phosphorus, aluminum, and titanium; they are known as
ferrite formers.
Manganese and nickel lower the eutectoid temperature, whereas chromium, tungsten, silicon,
molybdenum, and titanium generally raise it. All these elements seem to lower the eutectoid carbon
content.

Grain Size—Austenite

A significant aspect of the behavior of steels on heating is the grain growth that occurs when the
austenite, formed on heating above A3 or Acm, is heated even higher; A3 is the upper critical temperature
and Acm is the temperature at which cementite begins to form. The austenite, like any metal
composed of a solid solution, consists of polygonal grains. As formed at a temperature just above
A3 or Acm, the size of the individual grains is very small but, as the temperature is increased above
the critical temperature, the grain sizes increase. The final austenite grain size depends, therefore, on
the temperature above the critical temperature to which the steel is heated. The grain size of the
austenite has a marked influence on transformation behavior during cooling and on the grain size of
the constituents of the final microstructure. Grain growth may be inhibited by carbides that dissolve slowly or by dispersion of nonmetallic inclusions. Hot working refines the coarse grain formed by
reheating steel to the relatively high temperatures used in forging or rolling, and the grain size of
hot-worked steel is determined largely by the temperature at which the final stage of the hot-working
process is carried out. The general effects of austenite grain size on the properties of heat-treated
steel

Microscopic-Grain-Size Determination

The microscopic grain size of steel is customarily determined from a polished plane section prepared
in such a way as to delineate the grain boundaries. The grain size can be estimated by several methods.
The results can be expressed as diameter of average grain in millimeters (reciprocal of the square
root of the number of grains per mm2), number of grains per unit area, number of grains per unit
volume, or a micrograin-size number obtained by comparing the microstructure of the sample with
a series of standard charts.

Fine- and Coarse-Grain Steels

As mentioned previously, austenite-grain growth may be inhibited by undissolved carbides or nonmetallic
inclusions. Steels of this type are commonly referred to as fine-grained steels, whereas steels
that are free from grain-growth inhibitors are known as coarse-grained steels.
The general pattern of grain coarsening when steel is heated above the critical temperature is as
follows: Coarse-grained steel coarsens gradually and consistently as the temperature is increased,
whereas fine-grained steel coarsens only slightly, if at all, until a certain temperature known as the
coarsening temperature is reached, after which abrupt coarsening occurs. Heat treatment can make
any type of steel either fine or coarse grained; as a matter of fact, at temperatures above its coarsening
temperature, the fine-grained steel usually exhibits a coarser grain size than the coarse-grained steel
at the same temperature.
Making steels that remain fine grained above 9250C involves the judicious use of deoxidation
with aluminum. The inhibiting agent in such steels is generally conjectured to be a submicroscopic
dispersion of aluminum nitride or, perhaps at times, aluminum oxide

Phase Transformations—Austenite

At equilibrium, that is, with very slow cooling, austenite transforms to pearlite when cooled below
the A1 temperature. When austenite is cooled more rapidly, this transformation is depressed and
occurs at a lower temperature. The faster the cooling rate, the lower the temperature at which transformation
occurs. Furthermore, the nature of the ferrite-carbide aggregate formed when the austenite
transforms varies markedly with the transformation temperature, and the properites are found to vary
correspondingly. Thus, heat treatment involves a controlled supercooling of austenite, and in order
to take full advantage of the wide range of structures and properties that this treatment permits, a
knowledge of the transformation behavior of austenite and the properties of the resulting aggregates
is essential.