methods this is falling into disuse, because its estimation is not sufficiently accurate, and temperature is therefore regulated by thermometers in closed furnaces, when hardening is done in quantities.

The heating of steel preparatory to hardening must be done gradually. If done in the fire, the fire should not be too hot when the steel is inserted, but the temperature be increased gradually as that of the steel increases.

Difficulties arise in work of irregular shapes, having thick and thin portions adjacent. If the temperature is not uniform, warping will inevitably result due to different rates of expansion. Attention therefore should be given to the heating of thick portions. When work is of uniform dimensions the heating must be regular, and therefore slow, and the article turned about. The threads of taps, and teeth of milling cutters are frequently protected with

soap.

Hardening is effected in cold water, preferably rain water, as it contains less lime than well water. Water which has been in long use is reputably better than fresh water. Water of icy coldness is best for hardening. Rock salt, or lime are often added.

Articles when being quenched, if of fair dimensions, should be moved about in the water to bring them into contact with fresh cool water, and remove them from the spheroidal globules produced by the heated work. The reason of this is illustrated by the necessity for pouring a stream of water over anvil faces, and the faces of large dies for smiths' forgings. Mercury, which does not volatilise, is for this reason an ideal hardening agent. Articles of considerable length must be plunged perpendicularly to prevent distortion, as well as be moved about vertically.

The more rapidly heat is abstracted, the better the hardening. This explains why objects must be moved about in the water, and why mercury is an effective agent, being a good conductor of heat. A large volume of liquid is necessary to prevent rise of temperature of the water during quenching.

By raising steel to the temperature for hardening, the iron carbide Fe,C, or cement carbon, is dissolved in the iron. The effect of

sudden cooling is to prevent the carbide from returning to its soft condition.

Successful hardening, therefore, depends, besides the percentage of carbon in the steel, on the temperature at which it has been cooled, and on the rapidity of the cooling. If too highly heated the steel will be brittle, if too low it will not be thoroughly hardened. Brinnel, experimenting on steel containing 5 per cent. carbon, denoted the temperature at which the carbon passes from the cement to the hardening state by the letter W, and that at which it passed from the hardening to the cement form as V, this being suitable for the annealing of steel. If the temperature W is exceeded previous to quenching, the steel becomes more crystalline and brittle; if it is not reached, the steel is not fully hardened. Mr Brayshaw has stated that a difference of 5° Cent. is sufficient to make the difference between good and bad hardening. Some experiments made with a view to determine the best temperatures for hardening proved that the variation between moderate and maximum hardness occurs between 977° Cent. and 1078° Cent., a range say of 100°. A bar of fine Jessop steel was taken and notched at distances of about 60 mm., and each section numbered, and heated in a lead bath, hardened, broken off, and tested. No injury was suffered at 1078° Cent., but over that temperature deterioration set in, while below 977° Cent. the hardening was imperfect.

Though rapid cooling is essential to hardening, yet when there is considerable mass the cooling must be delayed, otherwise cracks will form, due to the exterior hardening and shrinking before the interior. This makes the hardening of large drills and milling cutters so difficult. The subsequent letting down for temper helps to release the internal stresses set up.

Hardening in air is adopted in some cases, the object being waved about in a current of air, either natural, or blast. It is said that Damascus blades were tempered in this way in winter time, when a cold strong north wind was utilised. This air was allowed to come through a slit in the forge wall on the red-hot blade held in front of the slit. The high-speed food steels are hardened in air, being either ni

Thin articles to be hardened must have the scale removed by grinding, otherwise the heating will not be uniform.

The difficulties and uncertainties of hardening are due largely to the variations in the qualities of different steels. Hence different temperatures are necessary for heating and quenching the various brands. And as shades of colours vary with temperatures, the precise estimations of these become difficult. As overheating is always to be avoided, it becomes necessary to lay down the rule, that the lowest heat which is suitable should be ascertained in all cases. See also Annealing, Armour Plate, Case Hardening, Tempering, &c.

Hardening Compounds. Physics added to water when steel is deficient in carbon, or to expedite the cooling action of the water. They include common salt, rock salt, alum, sal-ammoniac, corrosive sublimate, ammonia, parings of hoofs, yellow prussiate of potash, borax.

Recipes for making steel very hard are the following:-8 oz. of powdered prussiate of potash, 8 oz. of powdered borax, 6 oz. of salt dissolved in 8 gallons of water. Or, one half teaspoonful of wheat flour, one teaspoonful of salt to two teaspoonfuls of water, made into a paste to cover the steel to be hardened, which requires to be warmed for the purpose. Then heat to cherry red, and quench in cold water. Another is, 2 oz. of saltpetre, 2 oz. of sal-ammoniac, 2 oz. of crushed alum, 1 lb. of salt, to 3 gallons of soft water. Another, 1 oz. of corrosive sublimate, 8 oz. of salt, 6 quarts of soft water.

Hardening Furnaces.-The development of the cycle industries and of other special industries on the interchangeable system of manufacture has been favourable to the growth of special furnaces for heating articles uniformly for hardening purposes. Gas is the fuel used, the temperature is regulated, and also the time required for heating. A few descriptions of typical furnaces are here given.

In a machine by the American Gas Furnace Co. for balls for bearings, and similar objects, a cylindrical body, with its axis horizontal and lined with fire-brick contains a cast-iron

formed. Its shaft is a wrought-iron pipe carrying a spiral, all revolving together, driven by worm gear. There is a hopper at one end in which the work is placed. A scoop revolving with the cylinder picks up portions and drops them into a feeding funnel, by which they are discharged into the inner spiral, the revolution of which feeds them along to the farther end, whence they drop into the spiral way in the outer cylinder, and so work back towards the hopper end, and drop from the cylinder into the hardening bath.

Supply valves for gas and air permit of the regulation of the temperature of the drum, and a friction cone on the countershaft, driving to pulleys on the worm shaft, allows the time required for heating and delivery to be regulated with precision. Preliminary heating is required to raise the temperature of the spiral ways. The articles need not be of the same dimensions. If the temperature is regulated to suit the heaviest articles, the thinnest will not be overheated. The machine is suitable for hardening cycle balls, nuts, screws, bolts, &c., at the rate of from 1,500 to 2,000 lb. per day.

As quenching the work raises the temperature of the oil used, the output cannot exceed the capacity of the bath, which should be large. The bath has therefore to be maintained cool by water jacketing, the water being circulated at a rate to keep the bath at a proper temperature, as regulated by a thermometer. Or the oil is run off from the top, passed through pipes immersed in cold water, and pumped back cool to the bottom of the tank. Or if the water is not sufficiently cool, ice may have to be used.

In another design by the same firm, a link belt is used for conveying the articles to be hardened through the furnace. The latter encloses the belt, excepting for an opening through which the articles are dropped on the belt. The same methods of regulating temperature and speed are employed as in the previous example. In another an endless chain carries pins standing up vertically, while the chain is moving through the heating furnace, and which, on leaving, throw off the articles stuck on them

In

into the cooling bath; the sag of the chain lying below the furnace, which is carried on legs. This is used for pinions, cones, shells, and other articles provided with holes. another design the articles are carried in trays that are rotated two or three times in a minute in the furnace, being suspended from rods connected by spokes to a central driving shaft, operated by worm gear.

Fig. 138, Plate VIII., illustrates the gas hardening shop of the Birmingham Small Arms Company, from which it will be seen that the centre of the shop is occupied with benches, provided with a few vices, and rows of furnaces run down against the walls, the chimneys leading into a common horizontal flue.

Hardenite. Martensite which contains the maximum amount of carbon. The term martensite is given to the microscopic appearance of steel that has been quenched suddenly from a high temperature. Under the microscope it appears like a system of interlacing crystalline fibres.

used on cranes. These must always be run metal to metal without lubricant. Under hard service these, if of soft metal, will wear themselves out in a few months. Worm gears, and spiral gears, the friction on which is excessive. Spiral gears under hard duty will wear out in a few months. If expense is no consideration, phosphor bronze is better than iron, or phosphor bronze may run with iron or with steel. The same remark applies to worm gears, but the trouble with these is got over by running them in oil. Nevertheless the metal should be hard. The slides and strips of machine tools. The temptation to use soft metal is great, because the surfaces to be tooled are large, and because soft metal is less liable to produce curving and distortion, and uncertain shrinkage in cooling. This is a case in which a mean must be struck. Toothed wheels should be made of close-grained metal. They soon wear if soft, especially high-speeded wheels.

The limit to the use of hard tough metal is when it becomes so highly contractile that unsafe shrinkage strains are set up. These produce internal stresses, and sometimes "draws' in vital sections. But a metal must be very

A turner or planer is complacent when he sees the chips falling down in great chunks "like cutting cheese," and the graphite dust much in evidence. But this is not the proper metal for wearing parts, though it is suitable enough for frames and solid masses that simply afford support to mechanism, or form connections. This is well understood in making engine cylinders, and liners, slide bars, crosshead guides, &c., in regard to which specifications are generally strict. Such castings cost more for tooling, but the friction of their parts is so excessive that it is necessary to insist on close-grained metal, moderately hard, and slippery. But there are thousands of other details which are subject to just as much wear, and which should properly be of cylinder metal, or a mixture running very close to it. The following may be instanced out of many. The reversing cone clutches, such as

Occurs. These evils are seen at their worst in steel castings, the excessive shrinkage of which occasioned much trouble in the early days of steel making. Many steel castings have to be stiffened up with brackets, and very large radii in weak sections, solely with the view of affording sufficient strength to prevent the shrinkage strains from causing fracture at those sections.

A foundry man who is experienced in mixing is able to obtain metal of any grade by observing the fracture of the ingredients before they are put into the cupola. To a large extent he is helped by using proportions of pig of known brands, but in regard to scrap the appearance of a fractured surface is a sound guide. Grey open pig, and scrap iron yield soft open castings. In the mottled irons, and their judicious proportioning lies the principal secret of getting castings of varying degrees of mottle, toughness, and hardness. Good coke, of course, is essential, and thorough melting. In time the laboratory will largely supersede this kind of experience,