Until these 19th-century developments, steel was an expensive commodity and only used for a limited number of purposes where a particularly hard or flexible metal was needed, as in the cutting edges of tools and springs. The widespread availability of inexpensive steel powered the Second Industrial Revolution and modern society as we know it. Mild steel ultimately replaced wrought iron for almost all purposes, and wrought iron is no longer commercially produced. With minor exceptions, alloy steels only began to be made in the late 19th century. Stainless steel was developed on the eve of World War I and was not widely used until the 1920s.