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.
A simple method of determining the quality is by the cold bend test, and is readily understood. It consists in bending by blows of a sledge-hammer, until the two sides approach each other to within a distance equal to the thickness of the bar.
If the iron endures this treatment without showing signs of fracture on the back of the bend at A, it can be rated as of the very best quality. The test is more severe on a square than a round bar, the fibres becoming very much strained at the corners.
The basis of strength is made on small bars of about one inch or less in diameter, while large bars will not show the same ultimate strength per square inch.
Modulus Of Elasticity is a term used to designate such a weight as would extend a bar through a space equal to its original length, supposing the elasticity of the bar to be perfect.
Then if one ton extends an inch bar of wrought iron one ten-thousandth of its length, it is evident that, upon the supposition that the bar is perfectly elastic, 10,000 tons would produce an extension equal to double the length of the bar. Hence on this assumption 10,000 tons, or 20,000,000 pounds, will be the modulus of elasticity of the wrought iron stated in weight.
The modulus of elasticity is found to vary from 25,000,000 to 29,000,000 pounds, and will of course vary according to the character of the material tested, being greater in higher than in lower grades of iron. Careful and repeated rolling improves the quality of iron, the best being that which has been reworked.
To resist compression, wrought iron is usually taken equal to its tensile strength.
The average weight of wrought iron is 480 lbs. per cubic foot. A bar one inch square, three feet long, weighs therefore exactly ten pounds.
Steel is a compound principally of iron, in the proportion of one tenth (0.1) of one per cent to two (2.0) per cent of carbon, with a very low percentage of phosphorus, silica, manganese, etc. Steel above three tenths (0.3) of one per cent of carbon is used for "Tool Steel"; below fifteen hundredths (0.15) of one per cent it is called "Mild Steel".
The greater the percentage of carbon the higher will be the tensile strength.
Steel is distinguished from iron by its fine grain ; it can be made more uniform, varies more in quality and has greater power of hardening.
Of the many processes by which mild or soft steel is manufactured, one of the latest is that in which the air is forced at high pressure through the metallic iron, the silicon first burned out, and then the carbon contained in the pig iron consumed; the length of time required being about 20 minutes to each charge. The color of the flame determines the instant at which the pressure of air shall cease. This is when the carbon has been entirely consumed, at which time the metallic iron is ready for conversion to steel.
Then recarbonizing material, ferro-manganese, is put in, either molten or in lumps, the exact quantity being carefully weighed, in order that the steel may be of such quality as the purpose for which it is to be used requires. The molten metal is then formed into ingot moulds, and transferred to the blooming mill. The result has been to change cast iron into a material as soft as the best quality of wrought iron, capable of being forged to shapes, bent cold, punched, drilled, or welded.
What has been said in regard to the rolling of iron also applies to steel; being rolled at a lower heat, it is denser, and therefore stiffer and stronger.
Steel beams are rolled from a billet.
To make a 6-inch beam 35 pounds per yard, a billet 7 by 5 1/2 inches is used, and it requires eleven passes in the rolls to finish it. For 7, 8, and 9 inch beams billets 9 by 7 1/2 inches are taken, and fifteen passes are necessary to finish them. A 10-inch beam requires a 9 by 9 1/2 inch billet and needs seventeen passes; a 12-inch beam, a billet 10 by 10 inches, and the beam is finished in seventeen passes.
For steel beams above these sizes the billets are shaped in a blooming mill in the following form:
The remarks concerning the superiority of steel I beams over wrought iron can also be applied to channels and other shapes.
Steel has a weight two (2.0) per cent greater than that of wrought iron; and an ultimate tensile strength in the "mild steels" of 70,000 lbs. per square inch with an elastic limit of 36,000 pounds.