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HARDENING AND TEMPERING

Hardening means making harder. Steel which contains more than 0.75 per cent carbon becomes very hard and very brittle when hardened. High carbon steel is hardened by carrying out the following operations. The steel is first slowly heated to a proper temperature. Having been heated to red color, it is then rapidly cooled in oil, water, brine or some other liquid. The process of cooling is called quenching the steel, thus the hardening operation consists of heating and quenching. Alloy steels are usually hardened by special ways. The hardness produced by heat treatment depends upon the: 1) amount of carbon in steel; 2) temperature of heated steel; 3) speed of cooling.

The critical temperature, or critical point, is the temperature at which a piece of steel is properly hardened. When steel is heated to the critical temperature, the grain becomes very fine, that is the crystals get smaller, some other changes in the physical properties of steel take place, too, in this condition. The critical temperature is different for different kinds of steel. It depends upon the amount of carbon in steel. The more carbon the steel contains, the less it should be heated for hardening. In other words, the more carbon the steel contains, the lower is the critical point. The critical temperature may be tested with a magnet. Steel is magnetic until it reaches the critical temperature, then it is non-magnetic. Thus, a piece of steel to be hardened may be heated until it becomes non-magnetic; it should be then rapidly cooled (quenched).

Temper is the hardness, toughness and brittleness of a metal. Tempering, also called drawing the temper, means taking some of the hardness and brittleness out of hardened steel so that it could do good work. Having been tempered, the steel becomes stronger because its grain gets finer; as we already know, steel with a coarse grain is weaker than steel with a fine grain. Hardened steel is too hard and too brittle for many tools. A hardened cutting tool will break easily while cutting with it; it is therefore better to have a cutting tool tough and not too hard. Tempering brings about this condition.

Having hardened the steel, it is polished and heated. The different temperatures will show in the form of different colors on the polished surface. Each temperature or color is a different hardness or temper. The temperatures for tempering are from 400 to 600 degrees of Fahrenheit. When the right color appears, the steel is cooled (the speed of cooling is not important here). Thus the steel is a little softened and will be tougher and less brittle.

Structure of metals

Strength together with plasticity is the combination of properties that makes metals so valuable in industry. In addition to strength and plasticity, metals have many favourable characteristics, such as resistance to corrosion, electrical and heat conductivity, etc.

The characteristics of metals are due to two structural factors: first, the atoms of which the metallic state is composed; and second, the way in which these atoms are arranged.

When a metal becomes solid, it crystallizes. The crystallization or solidification is accompanied by a complete change in the atomic arrangement of the metal. The atoms of liquid metal become arranged into a definite pattern, forming small solid bodies of regular geometric shape such as cubes, etc., when crystallization occurs.

If liquid is cooled slowly in a crucible, nuclei form at the temperature of freezing, and these nuclei continue to grow until the liquid has changed to solid.

Knowing that metals are composed of many crystals or grains, and that each grain in turn is composed of a great many atoms all arranged in some pattern, how can we understand the plastic flow that must take place when metals are deformed during a bending operation or during the drawing out of a piece of a metal? This deformation may be understood as shearing; that is, when a metal is subjected to stresses exceeding its elastic limit, the crystals of the metal elongate by an action of slipping or shearing which takes place within the crystals and between the crystals. If deformation of the metal continuous, the crystals become remarkably elongated. This plastic flow of the metal, resulting in permanent deformation of the crystals is accompanied by marked changes in the physical properties of the metal. The tensile strength, yield point and particularly hardness are increased, but not the scratch hardness or ductility of cutting, as in machine operations in a lathe. The stiffness remains about the same, though in some cases it may be increased as much as 3 per cent. With the increase in hardness and strength, the plasticity or formability is reduced. Ultimately, if deformation of the crystals is continued, the metal becomes brittle. This process changing the physical properties of a metal is called work-hardening.

If the temperature of work-hardened metal is raised above normal, the deformation begins to disappear and the metal returns to the normal condition of structure and properties.