Critical Points

: The Working Of Steel

One of the most important means of investigating the properties of

pure metals and their alloys is by an examination of their heating

and cooling curves. Such curves are constructed by taking a small

piece and observing and recording the temperature of the mass at

uniform intervals of time during a uniform heating or cooling.

These observations, when plotted in the form of a curve will show

whether the temperature of t
e mass rises or falls uniformly.

The heat which a body absorbs serves either to raise the temperature

of the mass or change its physical condition. That portion of the

heat which results in an increase in temperature of the body is

called sensible heat, inasmuch as such a gain in heat is apparent

to the physical senses of the observer. If heat were supplied to the

body at a uniform rate, the temperature would rise continuously,

and if the temperature were plotted against time, a smooth rising

curve would result. Or, if sensible heat were abstracted from the

body at a uniform rate, a time-temperature curve would again be a

smooth falling curve. Such a curve is called a cooling curve.

However, we find that when a body is melting, vaporizing, or otherwise

suffering an abrupt change in physical properties, a quantity of

heat is absorbed which disappears without changing the temperature

of the body. This heat absorbed during a change of state is called

latent heat, because it is transformed into the work necessary to

change the configuration and disposition of the molecules in the

body; but it is again liberated in equal amount when the reverse

change takes place.

From these considerations it would seem that should the cooling

curve be continuous and smooth, following closely a regular course,

all the heat abstracted during cooling is furnished at the expense

of a fall in temperature of the body; that is to say, it disappears

as sensible heat. These curves, however, frequently show horizontal

portions or arrests which denote that at that temperature all

of the heat constantly radiating is being supplied by internal

changes in the alloy itself; that is, it is being supplied by the

evolution of a certain amount of latent heat.

In addition to the large amount of heat liberated when a metal

solidifies, there are other changes indicated by the thermal analysis

of many alloys which occur after the body has become entirely

solidified. These so-called transformation points or ranges may

be caused by chemical reactions taking place within the solid,

substances being precipitated from a solid solution, or a sudden

change in some physical property of the components, such as in

magnetism, hardness, or specific gravity.

It may be difficult to comprehend that such changes can occur in

a body after it has become entirely solidified, owing to the usual

conception that the particles are then rigidly fixed. However, this

rigidity is only comparative. The molecules in the solid state

have not the large mobility they possess as a liquid, but even so,

they are still moving in circumscribed orbits, and have the power,

under proper conditions, to rearrange their position or internal

configuration. In general, such rearrangement is accompanied by a

sudden change in some physical property and in the total energy

of the molecule, which is evidenced by a spontaneous evolution or

absorption of latent heat.

Cooling curves of the purest iron show at least two well-defined

discontinuities at temperatures more than 1,000 deg.F., below its

freezing-point. It seems that the soft, magnetic metal so familiar

as wrought iron, and called alpha iron or ferrite by the

metallurgist, becomes unstable at about 1,400 deg.F. and changes into

the so-called beta modification, becoming suddenly harder, and

losing its magnetism. This state in turn persists no higher than

1,706 deg.C., when a softer, non-magnetic gamma iron is the stable

modification up to the actual melting-point of the metal. These

various changes occur in electrolytic iron, and therefore cannot be

attributed to any chemical reaction or solution; they are entirely

due to the existence of allotropic modifications of the iron in

its solid state.

Steels, or iron containing a certain amount of carbon, develop

somewhat different cooling curves from those produced by pure iron.

Figure 45 shows, for instance, some data observed on a cooling

piece of 0.38 per cent carbon steel, and the curve constructed

therefrom. It will be noted that the time was noted when the needle

on the pyrometer passed each dial marking. If the metal were not

changing in its physical condition, the time between each reading

would be nearly constant; in fact for a time it required about 50

sec. to cool each unit. When the dial read about 32.5 (corresponding

in this instrument to a temperature of 775 deg.C. or 1,427 deg.F.) the

cooling rate shortened materially, 55 sec. then 65, then 100, then

100; showing that some change inside the metal was furnishing some

of the steadily radiating heat. This temperature is the so-called

upper critical for this steel. Further down, the lower critical

is shown by a large heat evolution at 695 deg.C. or 1,283 deg.F.

Just the reverse effects take place upon heating, except that the

temperatures shown are somewhat higher--there seems to be a lag

in the reactions taking place in the steel. This is an important

point to remember, because if it was desired to anneal a piece of

0.38 carbon steel, it is necessary to heat it up to and beyond

1,476 deg. F. (1,427 deg.F. plus this lag, which may be as much as 50 deg.).

It may be said immediately that above the upper critical the carbon

exists in the iron as a solid solution, called austenite by

metallographers. That is to say, it is uniformly distributed as atoms

throughout the iron; the atoms of carbon are not present in any fixed

combination, in fact any amount of carbon from zero to 1.7 per cent

can enter into solid solution above the upper critical. However,

upon cooling this steel, the carbon again enters into combination

with a definite proportion of iron (the carbide cementite, Fe3C),

and accumulates into small crystals which can be seen under a good

microscope. Formation of all the cementite has been completed by

the time the temperature has fallen to the lower critical, and

below that temperature the steel exists as a complex substance

of pure iron and the iron carbide.

It is important to note that the critical points or critical range

of a plain steel varies with its carbon content. The following

table gives some average figures:

Carbon Content. Upper Critical. Lower Critical.

0.00 1,706 deg.F. 1,330 deg.F.

0.20 1,600 deg.F. 1,330 deg.F.

0.40 1,480 deg.F. 1,330 deg.F.

0.60 1,400 deg.F. 1,330 deg.F.

0.80 1,350 deg.F. 1,330 deg.F.

0.90 1,330 deg.F. 1,330 deg.F.

1.00 1,470 deg.F. 1,330 deg.F.

1.20 1,650 deg.F. 1,330 deg.F.

1.40 1,830 deg.F. 1,330 deg.F.

1.60 2,000 deg.F. 1,330 deg.F.

It is immediately noted that the critical range narrows with increasing

carbon content until all the heat seems to be liberated at one

temperature in a steel of 0.90 per cent carbon. Beyond that composition

the critical range widens rapidly. Note also that the lower critical

is constant in plain carbon steels containing no alloying elements.

This steel of 0.90 carbon content is an important one. It is called

eutectoid steel. Under the microscope a properly polished and

etched sample shows the structure to consist of thin sheets of

two different substances (Fig. 46). One of these is pure iron,

and the other is pure cementite. This structure of thin sheets

has received the name pearlite, because of its pearly appearance

under sunlight. Pearlite is a constituent found in all annealed

carbon steels. Pure iron, having no carbon, naturally would show no

pearlite when examined under a microscope; only abutting granules

of iron are delicately traced. The metallographist calls this pure

iron ferrite. As soon as a little carbon enters the alloy and a

soft steel is formed, small angular areas of pearlite appear at the

boundaries of the ferrite crystals (Fig. 47). With increasing carbon

in the steel the volume of iron crystals becomes less and less, and

the relative amount of pearlite increases, until arriving at 0.90

per cent carbon, the large ferrite crystals have been suppressed and

the structure is all pearlite. Higher carbon steels show films of

cementite outlining grains of pearlite (Fig. 48).

This represents the structure of annealed, slowly cooled steels.

It is possible to change the relative sizes of the ferrite and

cementite crystals by heat treatment. Large grains are associated

with brittleness. Consequently one must avoid heat treatments which

produce coarse grains.

In general it may be said that the previous crystalline structure

of a steel is entirely obliterated when it passes just through the

critical range. At that moment, in fact, the ferrite, cementite or

pearlite which previously existed has lost its identity by everything

going into the solid solution called austenite. If sufficient time

is given, the chemical elements comprising a good steel distribute

themselves uniformly through the mass. If the steel be then cooled,

the austenite breaks up into new crystals of ferrite, cementite

and pearlite; and in general if the temperature has not gone far

above the critical, and cooling is not excessively slow, a very

fine texture will result. This is called refining the grain;

or in shop parlance closing the grain. However, if the heating

has gone above the critical very far, the austenite crystals start

to grow; a very short time at an extreme temperature will cause

a large grain growth. Subsequent cooling gives a coarse texture,

or an arrangement of ferrite, cementite and pearlite grains which

is greatly coarsened, reflecting the condition of the austenite

crystals from which they were born.

It maybe noted in passing that the coarse crystals of cast metal

cannot generally be refined by heat treatment unless some forging

or rolling has been done in the meantime. Heat treatment alone does

not seem to be able to break up the crystals of an ingot structure.