This section is from the "Architectural Iron And Steel, And Its Application In The Construction Of Buildings" book, by WM. H. Birkmire.. Also see Amazon: Architectural Iron And Steel, And Its Application In The Construction Of Buildings.
Iron is obtained from its ores, in which it generally exists in the state of an oxide, combined with earthy or rocky matters, and frequently with carbon, sulphur, silica, manganese, etc.
The foreign substances which iron is found to contain modify in a marked manner its essential properties. Carbon adds to its hardness, but destroys some of its characteristic qualities, and produces cast iron or steel according to the proportion of carbon it contains.
Smelting is the process by which iron is separated from foreign substances with which it is combined in the ore.
It consists in raising the ore to a high heat, in contact with carbon and a suitable flux, in the blast or smelting furnace. The flux unites with the earthy matter of the ore, forming a glassy substance called slag or cinder, and the carbon unites with the oxygen of the ore, setting the iron free, which in turn unites with a portion of the carbon and forms a fusible compound, carburet of iron or cast iron. The furnace is tapped from time to time. The metal is run out and formed into bars called pigs.
Pig Iron according to the proportion of carbon which it contains, is divided into different grades, No. 1, 2/3, 4, etc. The lower numbers are the more expensive to produce; they are used for foundry purposes, and are called "foundry pig"; the high numbers are converted into wrought iron by the puddling furnace, and are termed "forge pig".
After the proper melting of the pig iron in the puddling furnace, and stirring the mass until it loses its fluidity, it is formed into balls weighing from 100 to 200 pounds, and the liquid cinder being pressed out in the squeezer, it is then passed through the puddle rolls, making a rough bar several feet in length.
To prepare rough bars for this operation, they are cut, hot or cold, by a pair of shears, into such lengths as may be best adapted to the finished bar, then placed in the heating furnace, heated to a welding heat, and passed back and forth through the finishing rolls, from which their commercial shape is derived.
The piles used in making beams are numerous, the following sketches giving a few:
In Fig. I the large plates are placed on top of each other for the flange of the I beam, with the smaller plates between for the web; the dotted lines showing the form of beam when in its finished state.
In Fig. 2 the pile is made entirely of one size plates.
Fig. 3 shows a common arrangement of piles for 7 to 10 inch beams.
The piles for 10 to 20 inch beams are built up as in Fig. 4. It will be noticed that in this form channels are used, the flanges of which are worked into the flanges of the beam. .
Some piles for 12 to 20 inch beams are made with three pieces only, one for each flange, with a groove to receive the web piece, as in Fig. 5.
Iron channel piles are formed similarly to the above.
Iron beams from 6 to 12 inches can be rolled 60 feet long as easily as 30 feet, providing the entire weight of beam is not over 2500 pounds. For deeper beams than 12 inches the length is limited by the length of pile and size of furnace; 15 feet being an extra-long furnace.
The quality of a beam is often governed by the manner of rolling. The webs being strained and worked more than the flanges, will cause wavy or buckled webs; or an unequal working on the web and flange will cause broken flanges; or the flanges do not always fill the passes of the rolls, causing wavy flanges, which are then called wire-drawn.
Channels (C's), Angles (L's), Tees (T's), and various shapes are rolled similar to I beams.
The ultimate tensile strength of prepared test bars having a sectional area of about one square inch for a length of 10 inches should not be less than 50,000 pounds. The elastic limit should be regarded as the measure of quality, and should not be less than 25,000 pounds per square inch of section, the working loads proportioned with reference to the elastic limit.
The shape of the bar has much to do with determining the breaking strain.
A round bar one inch in diameter should bend double, cold, without sign of fracture, while a square bar of the same quality-may show cracks on the edges under such a test.
By taking a number of bars of best commercial iron and fracturing them off short, some specimens will present coarse crystals whitish in color, others very fine ones of a dark gray appearance, and in others the fracture will be lustrous like satin. The coarse crystals indicate an iron poor in quality, being hard and brittle ; the fine ones an iron the reverse of this, but one on which dependence can be placed for all purposes where strength is required. The fracture exposing a soft and silky fibre indicates a high grade of iron, - the finer the fibre the better the quality.
All iron is built up, as it were, of crystals having different degrees of fineness, depending upon impurities and the manipulation.
Rolling develops fibre by elongating these crystals.
When the bar is broken off short, the ends of the elongated crystals are seen to be formed like threads.