When at a white heat it is so pasty that if two pieces at this temperature be firmly pressed together and freed from oxide or other impurity they unite intimately and firmly.
"The facility with which steel may be welded to steel diminishes as the metal approximates to cast iron with respect to the proportion of carbon; or, what amounts to the same thing, it increases as the metal approximates to wrought iron with respect to absence of carbon.
"Hence in welding together two pieces of steel - cceteris paribus - the more nearly their melting points coincide - and these are determined by the amount of carbon they contain - the less should be the difficulty."
Puddled steel welds very indifferently, and so does cast steel containing a large percentage of carbon. The mild cast steels, also shear and blister steel, can be welded with ease.
In forging and welding and tempering steel tools, more than the requisite heat is detrimental, as it opens the grain of the steel and makes it coarse. The heat should be applied regularly, irregular heat causes fracture and irregular grain.
In welding cast steel borax or sal-ammoniac, or mixtures of them, are used as fluxes.1
If the melting points of two 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 two 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 produce an inequality of fibre."
"This constitutes the difficulty in welding steel to wrought iron.
"A difference in the rate of expansion of the two pieces to be welded produces unequal contraction, which is a manifest disadvantage." 2
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 in consequence of its greater fusibility.
It is not certain that other metals do not become pasty before fusion, but the range of temperature through which it occurs is so small that it would be scarcely possible to hit upon it with any certainty in practice.
1 16 parts borax, 1 part sal-arnmoniac, boiled over a slow fire, and when cold ground to powder, may be used.
2 Percy's Metallurgy.