3. How is the lining of a Bessemer-converter effected during the blow? Give the composition of the ordinary lining. 4. What kind of pig-iron is considered essential for the acid Bessemer-process?

5. What is meant by the basic Bessemer-process, and how does it differ from the acid process?

6. What is the nature of the gases given off from the Bessemer-vessel during the blow?

7. How do you account for the removal of phosphorus from the iron by the basic process?

8. What is the nature of basic slag, and in what state of combination does phosphorus exist in it?

9. Name any applications of basic slag to useful purposes. 10. Show by means of a sketch in vertical section the arrangement of a Bessemer-converter during the blow and also during the period of pouring.

11. Describe the ladle employed for receiving the charge of a Bessemer-converter.

12. Describe Clapp and Griffiths' modification of the Bessemer-process, and state what advantages are claimed

for it.

CHAPTER XIV.

OPEN-HEARTH PROCESS.

THIS process is conducted in a Siemens' regenerative furnace and comprises three modifications:-1. Pig and scrap process; 2°. Pig and ore process; 3°. A combination of the two former is now generally adopted and termed the Siemens-Martin process.

The first or Martin-process consists of melting malleable iron with pig-iron, preferably one low in silicon and containing manganese. When the charge is melted, it may be kept in fusion, because the intensity of the oxidising action may be easily maintained. In order to hasten the operation, the pig-iron is charged into the

furnace in the liquid state and speedily raised to a whiteheat; the malleable iron, previously made red-hot, is then added in lumps. With a neutral flame, No. 1 grey pig will dissolve 9 times its weight of Bessemer-scrap, while No. 3 pig will not dissolve more than 4 times its weight, and when the flame is oxidising, considerably less. The oxide of iron Fe3O4, formed by oxidation, reacts on the carbon of the pig-iron, producing carbonic oxide, which, on escaping, agitates the bath of metal, and thus tends to make it uniform in composition. When the whole is melted a test is taken, and when the metal shows the proper fracture and toughness as well as the right degree of decarburisation by Eggertz' test, it is run into a ladle, and cast into ingotmoulds as in the Bessemer-process. This method of working is only possible with the best pig-iron, so that the usual plan is to completely decarburise, and then add spiegel-eisen or ferro-manganese. The latter, containing less carbon, produces a milder steel than the former.

The second or Siemens-process is worked with pigiron and pure rich oxides, the latter replacing the scrap-iron of the Martin process. In the latter the iron takes up carbon from the pig-iron, but in the Siemens method the oxide of iron assists the atmosphere in its oxidising action. It partly removes the silicon, manganese, and carbon, so that the process is hastened and the material used is less costly. The difficulty has been to make the light ore thoroughly penetrate the denser metal. This dif ficulty has been partly avoided by adding the ore in small portions at intervals. The weak point of this method is the corrosive action of the basic oxide on the lining of the furnace, forming an acid silicate of iron, which increases with the amount of ore used, especially when a large portion is added at one charge. At Landore the charge of ore and scrap is added cold, and 8 to 12 per cent. of spiegel or ferro-manganese is added at the conclusion as usual.

The third modification or Siemens-Martin process is similar to the Siemens method in the mode of working. Pig-iron is first charged into the hot furnace, and then iron and steel scrap are added in small quantities at a time, and a certain quantity of rich oxide of iron. Spiegel or ferro-manganese is added at the conclusion.

In the three modifications as above described, it has been assumed that the pig-iron contains but little phosphorus and sulphur. These elements are only to a small extent removed by the process, in consequence of the siliceous nature of the slag, which is but feebly oxidising, so that phosphate of iron could have but a temporary existence.

The Siemens open-hearth furnace is of the reverberatory type, having a slightly concave bed lined with sand, which has been well rammed in and strongly heated (see Fig. 40). The bed also slightly inclines towards the taphole, which is situated beneath the middle working door on the front side of the furnace. In the newer forms of this furnace the roof has a strong slope from each side to the centre, giving a very plunging flame; the air-space beneath the bed is enlarged, the gas-ports are longer and narrower, and the regenerators are larger and roofed with semi-circular arches, so as to give a larger exhaust flue for the flame, and thus modify its temperature before it reaches the regenerators.

Pernot's furnace with moveable bed, already described as used in puddling iron, is also used for making steel. (See Fig. 39, p. 93.) The inclination of the bed and the rotation of the furnace cause a more energetic oxidising action than occurs in the Siemens furnace, so that less scrap and ore are necessary for the charge. At St. Chamond the pig-iron is charged after being heated to redness, and the steel- or iron-scrap is added cold at the same time. The bed is made to rotate three to four times per minute during charging and working. The pig-iron melts first and

forms a liquid bath, and the scrap is alternately exposed to the action of the air and molten metal by the motion of the bed. The pig-iron is thus uniformly oxidised by the oxide of iron Fe,04 formed at each revolution, when the unmelted iron is exposed to the air, the iron thus acts as a vehicle for oxygen.

It is claimed for this method that with a 5 ton charge, 5 operations may be completed in 24 hours, each charge only requiring 7 cwts. of coal, and 14 cwts. for the preliminary heating of the pig-iron. The charge consists of th pig-iron and ths crop ends of rails, etc. At the conclusion of the refining, spiegel-eisen or ferro-manganese is added as in the Siemens process.

Ponsard has modified the Pernot furnace by the addition of two twyers inserted in the side lining. These twyers work alternately, with the object of hastening the first part of the process. The bed only moves half a revolution, since in a complete revolution the twyers would be alternately in and out of the metal. The central spindle upon which the hearth rotates is made hollow, and through this passage the blast passes to the twyers. This method then is a combination of the Bessemer and Siemens systems of making steel.

Open-hearth basic process.-The object of a basic lining, as in the Bessemer converter, is to enable common pig-iron containing phosphorus and sulphur to be used. The fixed lining of calcined dolomite and tar, and the addition of lime to the charge, causes a highly basic slag to be formed, which takes up a portion of the phosphorus and sulphur as well as the silicon. In the basic Bessemerprocess, the phosphorus largely passes into the slag during the "after-blow," when the carbon is reduced to a minimum, so that this after-blow, with the consequent loss of iron, seems essential. In the Siemens furnace, this refining influence is absent, and Gruner states that unless a similar contrivance be adopted, such Ponsard's, it is doubtful whether mild steel, free from

as

phosphorus, can be produced from common phosphoric pig-iron.

Dick's open-hearth furnace.-This furnace Fig. 67, is the same in principle as the ordinary Siemens furnace,

but differs in construction and arrangement of the parts. The furnace or melting chamber (A) consists of a circular or oval body with an iron or steel casing. It is placed on a platform, supported by girders, and left entirely clear underneath, so that the bottom is kept cool and the lining better preserved. The four regenerators (BB) form four circular towers, and instead of being situated below the bed of the furnace, are placed in pairs at opposite sides of the furnace; each regenerator forms a separate structure, which is out of harm's way in case of the metal breaking out, and as it has only its own weight to carry it cannot get out of shape. It is very desirable to regulate the relative amounts of the heated gases passing through the regenerators, in order to control the relative amounts of heat stored up in these chambers. The tendency is for the gas-chamber to receive the largest share of the waste heat, whereas the air-chamber should be the more highly heated of the two. This is effected by the adoption of a new kind of disc-valves.

This class of furnace is adopted by the Patent Shaft and Axle Company, Wednesbury, where Mr. Wailes has made several improvements in the parts, with the result that