In composition, wrought iron differs from cast iron and steel in two important features, viz.: (1) In having had removed, as an essential of its manufacture, the greater part of the five elements usually contained in iron. In this respect it is near the composition of mild steel. (2) In containing, as a result of the process of manufacture, a quantity of slag (usually called cinder) which surrounds each iron crystal in a thin sheath, and preserves the identity of the crystal as a fiber when a bar of wrought iron is elongated by rolling or hammering. In this respect it differs from steel, which is crystalline and without much slag.
The chief properties of wrought iron are as follows, viz.:
(a) It is very malleable and ductile, and can be readily forged, particularly when heated.
(b) It cannot be cast, as it is fusible only at a very high temperature (about 2800° F.), and merely becomes pasty at the usual furnace temperatures, though because of this quality it is readily united by welding.
(c) It cannot be hardened, due to lack of carbon.
(d) If pulled apart, the fracture shows a fibrous break. 6
Wrought iron gets its name from the fact that it may be wrought into various shapes readily under the hammer; also it is called malleable iron in England, because of its great malleability, but it must not be confused with malleable castings, also called malleable cast iron or merely malleable iron in America.
While wrought iron and mild steel resemble each other, there are certain distinct advantages of wrought iron which cause it to be retained for some uses. Among its advantages are (1) it welds better than does steel, (2) lasts longer when exposed to weather or to water, (3) is better to resist shock and vibration (fatigue), in use, and (4) its fibrous structure arrests fracture, as its breaking is in the nature of a gradual tearing, which often gives warning of a dangerous stress, while steel breaks suddenly.
Among the disadvantages of wrought iron are, (1) its elastic and tensile strength are lower than those of steel, (2) and its production is more costly.