If the melting points of 2 metals sensibly differ, then the welding point of the one may be near the melting point of the other, and the difference in the degree of plasticity, so to speak, between the 2 pieces may be so considerable that when they are brought under the hammer at the welding point of the least fusible, the blow will produce a greater effect upon the latter, and create an inequality of fibre. This constitutes the difficulty in welding steel to wrought iron. A difference at the rate of expansion of the 2 pieces to be welded produces unequal contraction, which is a manifest disadvantage. (Percy.) Hard cast steel and wrought iron differ so much in their melting points that they can hardly be welded together. Blister and shear steel, or any of the milder steels, can, however, be welded to wrought iron with ease, care being taken to raise the iron to a higher temperature than the steel, as the welding point of the latter is lower is consequence of its greater fusibility.