INFORMATION FROM TEMPERATURE CHANGES Allotropic Changes in Pure Iron.—Much information about metal can be gained by studying the rate at which its temperature changes during uniform cooling. As long as the temperature falls uniformly, the loss by radiation is accounted for by loss in sensi ble heat. But suppose the surroundings are unchanged yet the rate of cooling is arrested, or even reverses—evidently some chemical or physical change is taking place inside the metal which generates a portion of the heat which is being constantly radiated. Such spontaneous heat evolutions or absorptions, arrests in the time-temperature curves, in purest iron occur at 77o, 91o, 1,390 and 1,535° C. Pure iron or ferrite at room temperature consists of an aggregate of tiny crystals of so-called alpha iron, crystallized in body centred arrangement. On heating, this ferrite gradually loses its magnetism, becoming practically non-magnetic at 77o° C, but without changing its atomic arrangements. This non-magnetic alpha iron has been called beta iron, and was erroneously thought to be the cause of the hardening phenomenon—it being given a fictitious hardness. At 91o° C alpha iron recrystallizes on heating with absorption of considerable heat and a contraction of in volume. The elementary cubes in the space lattice now have atoms at each corner and at the centre of each face, a face-cen tred cubic form known as gamma iron. At 1,390° the reverse change takes place: so-called delta iron existing between 1,390° and the melting point appears to be a form of non-magnetic alpha iron, stable at high temperature. At 1,535° the metal melts, with large absorption of heat and profound change in all physical properties. Precisely the reverse changes occur on slow cooling, as noted in the following thermometric chart.
A, and A3 lag somewhat on reversing the direction, i.e., occur at slightly lower temperature on cooling than the reverse change during heating.
the left of AEB the alloy is solid. The region between the liquidus and the solidus denotes that at. those temperatures and composi tions the alloy is in a mushy state, part solid, part liquid.
The way a 1% carbon steel solidifies is worthy of study. Its condition at any temperature may be investigated by drawing a vertical ordinate at the composition representing r % carbon. The diagram shows immediately that the first solid material appears on cooling to 1,450° C (point m). This solid is not pure iron, but a solid solution of carbon in iron whose chemical composition is found at point n on the solidus (about 0.2%). This solid is much richer in iron than the melt, so its appearance leaves the melt richer in carbon, and it must be cooled further before more solid appears. Thus, by continuous solidification of iron poor in car bon and drop in temperature, the sample reaches let us say r.400° C. At 1,40o° C the melted portion has acquired a composition represented by point p (about 1.6% carbon), and the solid ap pearing from it is a solid solution containing 0.4% carbon (point q).
And so it goes: at 1,300° the remaining mother liquor has 2.7% carbon; the solidifying material has o.8% carbon.
The first-formed solid material will act as a nucleus for the later which solidifies as the melt chills down. This causes the early-formed solid, low in carbon, to be in contact with later formed solids higher in carbon. Yet at this lower temperature the material in the cores of the crystals has a greater capacity for carbon than it had before, since the solubility of carbon in creases with decreasing temperature down to 1,130° C, and there is a pronounced tendency for carbon to pass from the high carbon melt, inward through the outer crystalline layers, until the carbon content of the solid solution becomes more nearly uniform. It requires a very slow cooling or a long annealing to cause entire equalization of carbon ; the so-called solid solution alloys cast and cooled at normal rates, usually show a cored structure under the microscope, caused by systematic variations in chemical com position between the material first solidified and the last freezing mother liquid. Assuming that the cooling has been very slow, and the equalization of carbon in the solid portions perfect, the last of the mother liquor in the i% steel disappears at 1,23o° C (point r, fig. 6) the metal is entirely solid.