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Testing of Structural Steel 23

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TESTING OF STRUCTURAL STEEL 23. Chemical Tests. These are made in order to determine the percentage of the metal loids present in the steel—the chief of which are carbon, silicon, manganese, sulphur, and phosphorus. In making a melt of steel in the Bessemer or open-hearth furnaces, chemical tests are first made of the various parts of the charge and of the recarburizing material. A careful computation is then made as to how much of each of these is required to make steel of a given carbon and other metalloid content. During the progress of the open-hearth heat, small test bars are made at frequent intervals from the metal, and quick determinations of the carbon content are made. By this means it is ascertained when the carbon has burned down to the desired percentage; then the "recar burizer" is added and the material poured.

The making of chemical tests is of great im portance; but the heat treatment of steel—that is, the working of it when it is cooling—has such an effect on this material that steel of various physical properties can be made from a bath of the same chemical composition.

Testing of Structural Steel 23

However, as most structural steel receives the same heat treatment, chemical analysis is a good guide to physical properties. Chemical tests are certainly invaluable in indicating the presence of sulphur and phosphorus.

24. Physical Tests. After the steel is manufactured, various physical tests are often insisted upon by the purchaser, and are con ducted in order to make sure that the material is of the required strength and ductility. For it is not always true that the stronger the steel the less ductile it is, although that is the case as a general rule.

Since the compression test is difficult to make, and since the tensile stress indicates closely what the result of the compressive test would be, the tensile test is the one usually em ployed. A piece of iron is put into a machine of which Fig. 18 is a diagrammatical representa tion. The vertical screws drive the head H in a vertical direction. The bar to be tested is put in at t; and the machine, which runs by hand or power, is started. The head H moves down ward; and at last the test piece, which is held together tightly by the clamps, is pulled apart. The scale beam B is kept balanced by the opera tor moving the weight w backward or forward, and the amount of pull on the piece is registered at all times during the pull.

The highest value registered is called the total strength of the bar; and if this amount is divided by the area of the cross-section of the bar, there is obtained the ultimate unit-stress often called simply the ultimate strength. As the area of the bar is measured in square inches, and the total strength is in pounds, it follows that the ultimate strength is in pounds per square inch. For example, if the bar was inch in diameter, and the highest value registered on the scale was 18,715 pounds, the ultimate strength would be 18,715-:-0.3068=61,000 pounds per square inch. The value 0.3068 is found in column 5 (C 261), and is the area of a bar. If the bar were inches in diameter, and the beam registered 86,273 pounds, the ulti mate strength would be 86,2734-1.4849=58,100 pounds per square inch, the 1.4849 being the area in square inches of a bar, which is found in column 5 (C 261).

The ultimate strength may be defined as the force required to pull apart a bar one square inch in cross-section.

At first, as the pull on the bar in the machine increases, the bar stretches at a uniform rate; that is, for every pound increase in weight, there is a corresponding amount of stretch, and this amount is constant for each pound increase up to a certain point. When a certain pull on the bar is reached, the amount of stretch per pound of tension changes quickly, and is considerably greater than that previously shown, being 1,200 to 1,300 times as great. Not only is the amount greater, but the rate of increase is variable, be coming greater and greater as the test pro gresses, until, when the bar breaks, the stretch for 1,000 pounds of pull of the machine is about 1,500 times what it was at first. The amount of stretch—or elongation, as it is called— is ex pressed in percentage of the length over which it is measured, and it is an indication of the ductility of the metal or its ability to bend con siderably without cracking. The percentage of elongation for any certain class of steel is prac tically constant, being almost independent of the size of the section, provided the measured length is the same.

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