Weld iron is a name occasionally used to describe malleable iron, indicating that it is pure enough to be welded. Malleable iron is produced by the puddling process. It is made by melting up pig iron and scrap in an open-hearth furnace and burning out the greater part of the silicon, manganese, and carbon in the order named. As the burning continues the melting point of the iron rises, and it becomes a pasty mass permeated with slag from the stirring. This nearly pure iron is gathered into "puddle balls," and taken to the rolls at a white heat. It is rolled or hammered out into long strips, the strips are cut, reheated to white heat, and welded together between the rolls or beneath the hammer. To produce the finest wrought iron, the process of rolling and welding is repeated. This is the puddling process in brief; its object is to squeeze and work the slag out of the iron and to give the iron a fibrous structure like rolled copper.

1 Journal of the Iron and Steel Institute, 1895, Vol. I, p. 463.

The principal chemical difference between wrought iron and steel is the carbon content. In fact, except for its high carbon, basic open-hearth steel is purer than most malleable irons.

All weldable iron is not malleable, but it is weldable in proportion to its malleability. As is seen in the foregoing process, the iron is welded several times.