perature, and plunge one into each of the above liquids, and it will be found that the hardness will be in the order given, mercury producing the greatest degree of hardness, and coal-tar the least. The hardening power of water is influenced by other substances which may be present in it, as well as by its temperature, thus: sulphuric acid and dissolved salts increase the conducting power and hasten the cooling, rendering the steel harder. Hot water on the other hand will induce less hardness. Further, the rate of cooling is also dependent on the way the steel is held in the liquid. The rapidity with which the steel is cooled down to 300° C. has a greater effect on the hardening than the subsequent cooling to the ordinary temperature. The hardening power of steel increases with the amount of carbon, as does also its brittleness, both of which properties are often further increased by the presence of other elements.

The cause of hardening is generally attributed to the sudden cooling fixing the condition of the particles in the same way as when hot, that is, the carbon is retained in the combined form; but much of the brittleness of hardened steel results from the unequal contraction, caused by the different rates of cooling of the interior and the exterior, which inequality induces so strong a tension between the various parts, that a slight force is often sufficient to overcome their cohesion.

Tempering. By cautiously re-heating hardened steel to a certain point, the tension is greatly released, some carbon or carbide is set free, and the metal loses its brittleness, and becomes softer in proportion to the temperature to which it is raised; so that by this means various degrees of hardness may be produced in the same steel to suit the requirements of different articles; this operation is termed "tempering." The surface of the steel is made bright before heating, and when the temperature reaches 220° C. a faint yellow colour appears,

succeeded by other colours as the process proceeds, so that the colour indicates the temperature of the steel, and therefore the degree of softness or "temper." These colours are due to a thin coating of oxide, which forms on the surface and produces the colour as an optical effect. The following table shows the tempers used for various articles, the lowest temperature indicating the hardest temper :

When the steel is heated much beyond this point it becomes too soft for any kind of tools. The term "temper" is used to express the hardness, whether due to treatment or composition.

All kinds of steel expand on hardening, varying with the amount of carbon present. The temperature to which steel should be raised for purposes of hardening, must be regulated according to the quantity of carbon it contains, since overheating is highly injurious. The harder the brand of steel, that is, the more highly carbonised it is, the lower will be the temperature required; moreover a small article will require a lower temperature than a large one made of the same steel, because it is more suddenly cooled. Bulky articles, such as the face of an anvil, cannot be hardened by plunging into water face downwards, but must be placed upright in water and deluged with a large stream of cold water falling from above. The same remarks apply also to smaller articles having steel faces.

In hardening files it is important to protect the teeth from oxidation during heating, which is done by covering them with a fusible paste, made of common salt and flour or ale-grounds. The files are then heated in a cokefire, then placed on a block with lead rests, and straightened with a leaden hammer. The workman, holding the tang by means of tongs, dips the file in a saturated solution of salt and water, and removes it when the hissing noise ceases. If the file has warped, it may be sprung a little by means of pressure exerted by the workman. The hardening of edge-tools is effected by heating in coke-fires and plunging them into water. They are then rubbed bright and tempered by placing on a thick cast-iron plate which is below a red-heat. Those tools, such as plane-irons, which are made of a combination of steel and iron, invariably become convex on the steel side, and require to be set by placing them iron side downwards. on an anvil, and hammering the steel side with a particular shaped hammer. The setting of cast steel chisels, knives, etc., is done by placing the convex side on the anvil, and striking the other side with a setting hammer.

Saws are hardened by heating them in an air-furnace, and dipping into whale-oil, or a mixture of whale-oil and tallow. Care is required in removing them from the furnace so as not to bend them with the tongs. They are next tempered by being passed over a clear charcoalfire, until the oily matter is burnt off. The saws are now flattened while warm, and any buckling removed by careful hammering. A method adopted by some hardeners of steel, is to heat the article in a bath of molten lead kept at a uniform temperature, by which all parts are equally heated. This plan is said to be well adapted for all kinds of cutlery as well as files. In hardening irregular shaped masses of steel the more bulky part should be immersed in the water first if possible, and the more pointed parts last. To prevent buckling in pieces of irregular shape and thickness, encasing the thinner parts

in iron, and hardening as a whole, is sometimes resorted to.

Welding steel. It is much more difficult to weld steel than iron, and the greater the amount of carbon, the greater is the divergence. Various fluxes have been recommended to prevent the carbon burning away, and to render the welding surfaces clean, but have not come into common use; sand, and a mixture of borax and salammoniac being the best. It is important that the temperature of the metal should not be raised higher than absolutely necessary; the higher the amount of carbon the lower should be the temperature used, hence the necessity of heavier hammers. The pieces should be tapped gently at first, and when they have begun to adhere they may be struck more heavily. The temperature at the finish must not be too low. It is best to work the steel rapidly, reheating as often as necessary (in order to prevent finishing too cold), and to anneal the whole piece when finished, so as to correct the effects of local hardening. When steel is to be welded to iron, the latter may be at a higher temperature with advantage.

1. What is steel?

QUESTIONS.

2. What is the influence of phosphorus and silicon respectively on steel?

3. Describe the physical properties of an alloy of iron containing but little carbon and 15 per cent. of manganese. 4. How would you distinguish steel from iron?

5. Why is the presence of sulphur so injurious in steel? 6. How is steel hardened and tempered, and what is the object of the latter operation?

7. In what states does carbon exist in steel hardened in the usual manner, and in the same steel after annealing? 8. In hardening a circular saw of large dimensions, what steps would you take to prevent buckling?

9. Describe the process to be gone through in tempering au ordinary cast steel chisel.

10. Describe the process of hardening files.

11. How are edge-tools hardened and tempered?

12. Describe the comparative results of tempering mild steel in water and in oil, and so far as you are able, give a reason for the difference.

13. What class of ores is best adapted for the manufacture of steel?

14. What are the peculiar advantages of the Swedish wrought-iron employed for making the best steel?

15. Describe the process of welding steel. Why is it more difficult to weld steel to iron than iron to iron?

CHAPTER XII.

PRODUCTION OF STEEL BY PUDDLING, CEMENTATION, ETC.

THE different methods of making steel may be classified thus:-(1) By direct methods as in the Catalan forge, etc.; (2) From pig-iron in the finery; (3) By puddling pig-iron; (4) By the cementation process; (5) By the treatment of blister-steel, or iron and carbon in crucibles; (6) By pneumatic processes, such as that of Bessemer; (7) In open-hearths, such as that of Siemens.

1. The direct methods in the Catalan-forge, Siemens' rotator, Blair's furnace, etc., have been described when treating of iron.

2. Finery method.--This differs but little from that employed in making iron, the operation being conducted so as to leave sufficient carbon in the bloom to constitute steel. This is done by prolonging the operation, thus leaving the iron a longer time in contact with incandescent carbon, and by using a less oxidising blast. The best pig-irons for this purpose are the strongly mottledand spiegel-eisen varieties. This method is now almost obsolete.