NATURE AND STRENGTH OF IRON.
214. The table below shows the properties of the several descriptions of iron used in engineering.
| Wrought Iron. | Cast-Iron. | Iron Wire. | Boiler Plate. | Designation of the quality. |
|---|---|---|---|---|
| 480 | 450 | 480 | Weight per cubic foot in lbs. | |
| 15000 | 4500 | 25000 | 12740 | Resistance to extension in lbs. per sq. inch. |
| 11000 | 25000 | 7500 | Resistance to compression in lbs. per sq. in. | |
| .0000066 | .00000608 | .00000685 | .0000066 | Expansion per degree Fahrenheit in lengths. |
| .0000000424 | .000000106 | .0000000446 | .0000000524 | Extension per lb. per square inch. |
| .000000149 | .000000083 | .000000189 | Compression per lb. per square inch. | |
| 90 to 66 | 20 to 111 | 127 to 75 | Ratio of extensive to compressive strength. | |
| 12500 | 17500 | Resistance to detrusion, or shearing. | ||
| 55 | 31 | Relative transverse strength. | ||
Column four refers to boiler plate when built into tubes.
After wrought iron has become a little compressed, its power to resist a crushing force is very much increased.
215. The tenacity of wrought iron is increased by heating. Experiments upon thirty varieties gave the following mean result, the temperature ranging from 500° to 700° Fahrenheit.
Strength when
| Cold. | Hot. | Cooled. |
|---|---|---|
| 60,000 | 64,000 | 70,000 |
216. Stirling’s process of toughening cast-iron, by the addition of malleable scrap, increases the strength in the following ratio:—
| The mean tensile strength of cast-iron being | 18,000 lbs. |
| And the compressive strength being | 105,000 lbs. |
| When Stirling-toughened the tensile strength is | 23,000 lbs. |
| And the compressive strength | 130,000 lbs. |
The strength of cast-iron increases rapidly up to the twelfth or fifteenth recasting, when it is nearly doubled; after the fifteenth melting the strength decreases.
217. Wrought iron exposed for some time to vibration, as in the case of railroad axles, or iron which has been wrought with light hammers, loses its toughness and becomes “short,” (crystalline). The fibre may be restored in such cases by reheating and cooling slowly.
218. GENERAL RATIOS OF THE STRENGTH OF IRON.
| Tension. | Compression. | Cross Strain. | |
|---|---|---|---|
| Cast, | 300 | 1,666 | 31.68 |
| Wrought, | 1,000 | 733 | 55.40 |
OF THE STRENGTH OF BOILER PLATES.
219. The strength of rolled boiler plates is no greater in the direction of the fibres than crosswise, but is more regular; whence the length of the fibre must be placed as nearly as possible with the direction of the force.
A mean of twelve experiments, by Mr. Fairbairn, gives the tensile strength of wrought iron plates as 50,960 lbs. per square inch; and the compressive strength of plates, when built into tubes, as 30,464 lbs., or for safe use in practice, for extension, 12,740 lbs., and for compression, 7,500 lbs. In the remarks upon girder bridges the matter of riveting will be considered.