METALLOGRAPHY AND HEAT TREATMENT The fact that the hardness, toughness and general desirability of a piece of steel could be profoundly modified by a simple heating and cooling has been known since time immemorial—in fact what we now know as steel was perhaps not recognized as having great merit until the fortunate discovery of heat treat ment was made. This is a quite mysterious happening, this heat treatment, and it is not surprising that down through the long centuries when the art was guarded as a closed secret, many super stitions arose regarding the efficacy of certain heating fires and quenching fluids, and much quackery had free scope to practice its hocus-pocus. Within the past century the chemist and physicist have done much to clear up the mystery by analyzing the various operations; we now know the essential differences between the various steels, irons, fuels, atmospheres, and quenching fluids. Only since the beginning of the century, however, have intensive studies with the microscope, pyrometer, hardness and tension testing machines, brought forth a knowledge of the changes which go forward in steel or iron during a heating and cooling cycle, and working hypotheses of the causes of hardening (see METAL LOGRAPHY). Logically, an ingot in a soaking pit prior to rolling, or a billet in a heating furnace prior to forging, may be said to be undergoing heat treatment, but from a workshop viewpoint the term is restricted to a heating and cooling to restore original duc tility and workability in a partly finished piece, or a heating and cooling—sometimes repeated—to induce special properties in the finished article.
(q.v.), in regard to size, depend upon the pieces to be treated and the temperature required. A well designed furnace is one which easily attains the necessary temperature, and maintains that heat uniformly in all parts, with an atmosphere which does not deteriorate the 'furnace, and yet has the desired effect or no effect, upon the metal being treated. Fuels may be coke or coal, gas, oil or electricity. No source of heat, pure heat, disassociated from the products of combustion, has any virtue over another source—it is merely a matter of which is the most convenient and economical. Gas and electricity are now favoured because of their ease of manipulation to give uniform tempera ture—choice depends upon the relative cost per thermal unit, and the atmosphere required. For instance, it is handiest to use elec tric resistors for heating elements in a furnace for annealing transformer sheets in a hydrogen atmosphere. Gas or oil fuel would probably be chosen for treatments on products such as roller bearings running into tons per day. Recuperators or re generators are frequently installed to save heat. Mechanical de vices for putting the metal in and taking it out, such as charging machines, furnace bottoms built on wheels or moved on rollers, revolving heating muffles and conveyors, are installed to save labour wherever the weight of material receiving uniform treat ment is very great. Temperature control is most important. Well conducted shops issue definite instructions on all important work giving the rate of increase in heat, duration at heat and speed of cooling. Continuous furnaces are devised to maintain this cycle automatically. Batch furnaces are controlled by hand, or by automatic control of the fuel burners through pyrometers (q.v.), which are installed in all furnaces, sometimes several in a single furnace, and indicate or make a permanent record of the tem perature at all times. An important method of localized heating is by high frequency currents ; pieces such as crankshafts are rapidly heated one at a time at only those bearing surfaces which are to be hardened.
within the furnace is important. In an open flame furnace scaling (or the production of oxide on the metal's surface) may be minimized by feeding rather less air than the gas or oil requires for complete combustion. This deficiency of oxygen means that little or none will be available to react with the hot metal. In long heating of thin sheet, or long annealing of tool steel, where the surface must be preserved, the work is protected from the furnace atmosphere by closed boxes or tubes. Protective atmospheres have been highly developed in the 193o 40 decade. Steel and gas are heated together in a muffle, or heated without a muffle by electric resistors or by flames contained in alloy tubes. Atmospheres for bright annealing are relatively easy to prepare of flue gas, partially burned fuel gas, or even gas en gine exhaust ; due to the comparatively low annealing tempera ture-65o° C—any mixture of an excess of carbon monoxide over carbon dioxide will protect the steel. It must be fairly dry to prevent tarnishing by steam on the cooling cycle. Protection during higher heating-85o° C—for quenching is far more diffi cult because the reactions are much more rapid and the steel must neither oxidize nor must it gain or lose carbon. Such at mospheres are bone-dry and oxygen-free mixtures of hydrogen, nitrogen, and carbon monoxide ; they may be prepared from burned natural gas (methane, CH4) by removing excess oxygen by hot copper, washing out CO2 with caustic, and absorbing mois ture in activated alumina. On the other hand, certain reactions
may be induced during heat treatment—for this purpose the ma terial is packed in boxes with substances forming the necessary gaseous atmosphere, or the furnace is a closed muffle through which a stream of the proper gas slowly travels. Case hardening (q.v.), for instance, is done by packing the metal pieces in car bonaceous compounds and heating, or by piping in a gas which is principally carbon monoxide.
to cool heated metal, may be done in various liquids, depending upon the speed required. All kinds of solutions of outlandish materials have been used, but there is no virtue in a quenching fluid except the ability to abstract heat quickly. In practice, the most drastic quench is given by a properly dis posed nest of nozzles, spraying cold water at high pressure on the hot steel. Next comes a bath of iced brine; then cold water, then high flash point oil, then boiling water, then molten lead, then air blast, then quiet air, and even slower cooling may be done in 8 hours by burial in lime ; a dying furnace may require from 24 hours up to seven days. Quenching baths require methods for circulating the medium, maintaining its correct temperature either by heating or refrigerating coils, and dashing it against the metal so no air or steam bubbles remain attached to the hot metal, else soft spots will occur on an otherwise hardened sur face. Conveyors or other mechanical methods of getting the hot metal rapidly into the quench and removing it when cold, are installed to save labour. In order to understand what goes on during these heating and cooling cycles, an introduction to the science of metallography and the so-called iron-carbon equilibrium diagram is necessary.
(q.v.) is the study of the inner structure of metals and how this varies with the chemical composition of the alloy and its past heat treatment or the work that has been done on it. It also includes the study of the relationship between the inner structure of metal and the physical properties, such as strength, toughness, and hardness. In this study the use of a microscope is a long step in advance of the ancient practice of breaking the metal and looking at the fracture. It enables the student to magnify the apparent size of the grains a thousand fold. A sample of the piece under study is given a high polish, etched slightly in a suitable acid, and placed on a microscope stage with a special illuminating device for opaque objects (see MICROSCOPE) . The resulting structure may by appropriate means be photographed for permanent record, and for study in connec tion with information on its past history, and its physical strength, hardness or electrical characteristics. Important physical proper ties affected by heat treatments include strength, toughness, hard ness, electrical and magnetic characteristics, corrosion resistance. Of these, the first three are most relied upon. Machines and methods for making tension tests and determining the ultimate strength, elastic limit and ductility are described under STRENGTH OF MATERIALS. Hardness is measured by the size of impression left by a steel ball or a diamond cone weighted down by a given load. Toughness or impact strength is measured by the amount of energy absorbed from a swinging hammer by the instantaneous fracture of a standardized notched test piece.
Viewed through a microscope, an etched metal surface presents a mosaic patchwork of interlocking and nice-fitting grains (Plate III., figs. 12 and i7). Much evidence is at hand to support the idea that these granules, despite their irregular boundaries, are crystalline; i.e., the ulti mate atoms are in an orderly geometric arrangement of rows and ranks. The fact was finally proven by X-rays in a manner easi est explained by this analogy: If a street lamp is viewed through a window screen the brilliant centre appears to have a series of bright rays extending outward, caused by reflections of light from every wire so placed as to throw light back into the eye. The geometric pattern of rays is caused by the geometric arrange ment of the wires; arguing by analogy, if the atoms of metal are arranged in definite geometric patterns, then an X-ray beam penetrating the crystal would be reflected in definite ways and emerge in a geometric pattern, in fact, a mathematician can pre dict the atomic spacing and arrangement from the diffraction pat tern so produced. Thus it is found that wrought iron at room temperature is in the body centred cubic arrangement. That means that if the space occupied is subdivided by imaginary planes into a space-lattice of tiny cubes regularly placed on top and to both sides of each other, the iron atoms are located at the corner of each of these imaginary cubes, and each cube will have one atom in its centre.