For a given percentage of phosphorus in the finished product, it follows therefore that the original stock must contain no more phosphorus than that allowed at the finish.

This implies the use of practically non-phosphoric ores for the acid process.

The acid-lined open hearth in this respect stands on the same footing as the acid-lined converter, and the original stock must be of known composition so far as sulphur and phosphorus are concerned, for there is no appreciable elimination of these elements, and the finished product will show a percentage equal to the average of the material charged.

In the basic Bessemer process the distinctive feature of the basic vessel is a lining which resists the action of basic slags. This is usually made of dolomite, or limestone containing a small proportion of magnesia.

During the earlier stages of the process of combustion the chemical reactions in the metal of the basic converter are practically identical with the reactions in the acid vessel up to the point when the combustion of the carbon has been carried to its limit. From this point onwards to the end of the blow the comparison with the acid process ceases, and the distinctive feature of the basic system, viz. the combustion of the phosphorus and sulphur, begins. The initial content of phosphorus can be burnt out and reduced to a desirable limit.

This dephosphorization is in a similar manner the characteristic feature of the basic open hearth as compared with the acid open hearth.

In both these basic processes it is, then, possible to use an initial stock of pig or scrap having a higher percentage of the undesirable elements than is possible in the acid processes; or, in other words, a less pure ore can be utilized.

An important distinction between the converter and open-hearth system lies in the fact that whereas the initial charge of pig-iron or scrap can be converted into steel by the former process in from fifteen to twenty minutes, the same transformation by the latter process occupies some nine to twelve hours. In the opinion of many authorities, this difference of time exercises an important influence on the quality of the resulting material by reason of the fact that greater opportunities are afforded in the longer process of testing the quality at frequent stages of the process.1 , The important question, by which of the processes can the best and most reliable mild steel be produced for structural purposes, is one which would probably be answered by British, American, or German steel makers from points of view not wholly unconnected with the great commercial interests involved in the supply and use, in their respective countries, of phosphoric or non-phosphoric ores.

It may, however, be generally admitted that for uniformity of quality, and general excellence of material for all purposes where great reliability is essential, the product of the open hearth, either acid or basic, stands pre-eminent.

In support of this view a series of tests is appended, representative of present-day open-hearth practice in this country, and similar tests might be multiplied almost indefinitely.

The tests cover, it will be seen, a large range of sections of structural material, such as are commonly employed in every-day use, and they have been exhibited at some length in order that the student may be assured of the practical application of the series to the work he may have under consideration.

1 Various modifications of the open-hearth process (involving the consideration of various points of steel works practice into which it is not necessary here to enter) are known as the Bertrand-Thiel process, the Talbot process, the Twynam process, and the Monell process.

The tests are the samples of a large quantity of mild steel employed in ordinary structural work as represented and described in Chapters III. to VI., and are representatives of the material from which the majority of the girder-work, columns, roofing, etc., represented by the illustrations in this volume, have been manufactured.1

The material was supplied under ordinary commercial conditions by some seven or eight firms in both England, Wales, and Scotland, and are therefore fairly indicative of present practice in open-hearth work in Great Britain. The material was supplied under the following specification -

All mild steel required for structural purposes is to be of British manufacture, made by the open-hearth process, either acid or basic.

To be cleanly rolled and true to the thicknesses and sections specified, free from scale, laminations, cracked edges, and every other defect.

The edges of all plates to be cleanly sheared, except where otherwise specified, and truly square. The surfaces of finished plates to be quite fair and flat, except where otherwise directed.

All steel to be of such strength and quality that it shall not fracture under tensile stresses or with elongations less than those shown in the following table: -

1 These tests, together with the chemical analyses, were carried out by Mr. R. H. Harry Stanger, Assoc. M. Inst. C. E., A. M. I. Mech. E., Broadway Testing Works, Westminster.

Samples selected for testing as specified above are to be planed parallel for a length of 8 inches. The sectional area to be fractured is, whenever possible, to be not less than ½ square inch. The steel must also be capable of bearing the following tests: -