It may be remarked as a general rule that the hardness of cutting tools is " inverse as the hardness of the material to be cut," which seems anomalous, and no doubt is so, if nothing but the cutting properties of edges is considered; but all cutting edges are subjected to transverse strain, and the amount of this strain is generally as the hardness of the material acted upon; hence the degree of temper has of necessity to be such as to guard against breaking the edges. Tools for cutting wood, for example, are harder than those usually employed for cutting iron; for if iron tools were always as carefully formed and as carefully used as those employed in cutting wood, they could be equally hard. (' Workshop Manipulation.')

Steel plunged into cold water when it is itself at a red heat becomes excessively hard. The more suddenly the heat is extracted the harder it will be. This process of "hardening," however, makes the steel very brittle, and in order to make it tough enough for most purposes it has to be "tempered." The process of tempering depends upon another characteristic of steel, which is that if (after hardening) the steel be reheated, as the heat increases, the hardness diminishes. In order then to produce steel of a certain degree of toughness (without the extreme hardness which causes brittleness), it is gradually reheated, and then cooled when it arrives at that temperature which experience has shown will produce the limited degree of hardness required. Heated steel becomes covered with a thin film of oxidation, which grows thicker and changes in colour as the temperature rises. The colour of this film is therefore an indication of the temperature of the steel upon which it appears. Advantage is taken of this change of colour in the process of tempering, which for ordinary masons' tools is conducted as follows : - The workman places the point or cutting-end of the tool in the fire till it is of a bright-red heat, then hardens it by dipping the end of the tool suddenly into cold water.

He then immediately withdraws the tool and cleans off the scale from the point by rubbing it on the stone hearth. He watches it while the heat in the body of the tool returns, by conduction, to the point. The point thus becomes gradually reheated, and at last he sees that colour appear which he knows by experience to be an indication that the steel has arrived at the temperature at which it should again be dipped. He then plunges the tool suddenly and entirely into cold water, and moves it about till the heat has all been extracted by the water. It is important that considerable motion should be given to the surface of the water while the tool is plunged in, after tempering, otherwise there will be a sharp straight line of demarcation between the hardened part and the remainder of the tool, and the metal will be liable to snap at this point.

In very small tools there is not sufficient bulk to retain the heat necessary for conduction to the point after it has been dipped. Such tools, therefore, are heated, quenched, rubbed bright, and laid upon a hot plate to bring them to the required temperature and colour before being finally quenched. In some cases, the articles so heated are allowed to cool slowly in the air, or still more gradually in sand, ashes, or powdered charcoal. The effect of cooling slowly is to produce a softer degree of temper.

The following table shows the temperature at which the steel should be suddenly cooled in order to produce the hardness required for different descriptions of tools. It also shows the colours which indicate that the required temperature has been reached : -

Colour of Film.

Temp. Fahr.

Nature of Tool.

Very pale straw yellow

430°

Lancets and tools for metal.

A shade of darker yellow

440°

Razors and do.

Darker straw colour

470°

Penknives.

Still darker straw yellow

490°

Cold chisels for cutting iron, tools for wood.

Brownish yellow

500°

Hatchets, plane irons, pocket knives, chipping chisels, saws, etc.

Yellow tinged with purple ..

520°

Do. do. and tools for working granite.

Light purple Dark purple

530° 550°

Swords, watch-springs, tools for cutting sandstone.

Dark blue

570°

Small saws.

Pale blue

600°

Large saws, pit and hand saws.

Paler blue with tinge of green

630°

Too soft for steel instruments.

The tempering colour is sometimes allowed to remain, as in watch springs, but is generally removed by the subsequent processes of grinding and polishing. A blue colour is sometimes produced on the surface of steel articles by exposing them to the air on hot sand. By this operation, a thin film of iron oxide is formed over the surface, which gives the colour required. Steel articles are often varnished in such a way as to give them an appearance of having retained the tempering colours. The exact tempering heat required to produce the same degree of hardness varies with different kinds of steel, and is arrived at by experience.

There are several ways of heating steel articles both for hardening and tempering. They may be heated in a hollow or in an open fire, exposed upon a hot plate, or in a dish with charcoal in an oven, or upon a gas stove. Small articles may be heated by being placed within a nick in a red-hot bar. If there is a large number of articles, and a uniform heat of high degree is required, they may be plunged into molten metal alloys, or oil raised to the temperature required.

In hardening steel, care must be taken not to overheat the metal before dipping. In case of doubt, it is better to heat it at too low than too high a temperature. The best kinds require only a low red heat. If cast steel be overheated, it becomes brittle, and can never be restored to its original quality. If, however, the steel has not been thoroughly hardened, it cannot be tempered. The hardness of the steel can be tested with a file. The process of hardening often causes the steel to crack. The expansion of the inner particles by the heat is suddenly arrested by the crust formed in consequence of the cooling of the outer particles, and there is a tendency to burst the outer skin thus formed.

When the whole bulk of any article has to be tempered, it may either be dipped or allowed to cool in the air. It does not matter which way they become cold, provided the heat has not been too suddenly applied; for when the articles are removed from the heat, they cannot become more heated, consequently the temper cannot become more reduced. But those tools in which a portion only is tempered, and in which the heat for tempering is supplied by conduction from other parts of the tool, must be cooled in the water directly the cutting part attains the desired colour, otherwise the body of the tool will continue to supply heat and the cutting part will become too soft.

When toughness and elasticity are required rather than extreme hardness, oil is used instead of water both for hardening and tempering, and the latter process is sometimes called "toughening." The steel plunged into the oil does not cool nearly so rapidly as it would in water. The oil takes up the heat less rapidly. The heated particles of oil cling more to the steel, and there is not so much decrease of temperature caused by vaporisation as there is in using water. Sometimes the oil for tempering is raised to the heat suited to the degree of hardness required. When a large number of articles have to be raised to the same temperature, they are treated in this way.

Saws are hardened in oil, or in a mixture of oil with suet, wax, etc. They are then heated over a fire till the grease inflames. This is called being "blazed." After blazing the saw is flattened while warm, and then ground. Springs are treated in somewhat the same manner, and small tools after being hardened in water are cooled with tallow, heated till the tallow begins to smoke, and then quenched in cold tallow.

Annealing or softening steel is effected by raising hardened steel to a red heat and allowing it to cool gradually, the result of which is that it regains its original softness.

Case-hardening is a process by which the surface of wrought iron is turned into steel, so that a hard exterior, to resist wear, is combined with the toughness of the iron in the interior. This is effected by placing the article to be case-hardened in an iron box full of bone-dust or some other animal matter, and subjecting it to a red heat for a period varying from 1/2 hour to 8 hours, according to the depth of steel required. The iron at the surface combines with a proportion of carbon, and is turned into steel to the depth of 1/16 to 3/8 in. If the surface of the article is to be hardened all over, it is quenched in cold water upon removal from the furnace. If parts are to remain malleable, it is allowed to cool down, the steeled surface of those parts is removed, and the whole is then reheated and quenched, by which the portions on which the steel remains are hardened. Gun-locks, keys, and other articles which require a hard surface, combined with toughness, are generally case-hardened. A more rapid method of case-hardening is conducted as follows : - The article to be case-hardened is polished, raised to a red heat, sprinkled with finely powdered prussiate of potash. When this has become decomposed and has disappeared, the metal is plunged into cold water and quenched.

The case-hardening in this way may be made local by a partial application of the prussiate. Malleable castings are sometimes case-hardened in order that they may take a polish.

Many further details on hardening, tempering, softening, and annealing steel will be found in Workhop Receipts, Third Series, pp. 256-295.