CRITICAL POINTS

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 the 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°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°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°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.

FIG. 45.—Inverse Rate Cooling Curve of 0.38 C Steel.

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°C. or 1,427°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°C. or 1,283°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° F. (1,427°F. plus this lag, which may be as much as 50°).

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.
0.001,706°F.1,330°F.
0.201,600°F.1,330°F.
0.401,480°F.1,330°F.
0.601,400°F.1,330°F.
0.801,350°F.1,330°F.
0.901,330°F.1,330°F.
1.001,470°F.1,330°F.
1.201,650°F.1,330°F.
1.401,830°F.1,330°F.
1.602,000°F.1,330°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.

FIG. 46.—Microphotograph of steel used in S. K. F. bearings, polished and etched with nitric acid and magnified 1,000 times. Made by H. O. Walp.

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.

FIG. 47.—Structure of low carbon steel, polished, etched and viewed under 100 magnifications. Tiny white granules of pure iron (ferrite) have small accumulations of dark-etching pearlite interspersed between them. Photograph by H. S. Rawdon.

FIG. 48.—Slowly cooled high-carbon steel, polished, etched and viewed at 100 magnifications. The dark grains are pearlite, separated by white films of iron carbide (cementite). Photograph by H. S. Rawdon.

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.