Blast furnaces make pig iron, which is high in carbon and impurities. Hardness and brittleness increases as carbon content goes up. Steels in the lower reaches of the carbon league are weldable on the farm. So are those in the middle, though they need greater care over rod selection, joint preparation and subsequent cooling. High carbon steels are unweldable by normal methods. Adding dashes of other elements to carbon steels gives a wide range of tougher alloy steels.
Chart Courtesy Fox Chapel Publishing