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.