“The next inference would be that by increasing temperature we chiefly improve the quality of welding. If temperature is increased to fusion, welding is practically perfect; if to plasticity and mobility of surfaces, welding should be nearly perfect. Then how does it sometimes occur that the more irons are heated the worse they weld?
“1. Not by reason of mere temperature, for a heat almost to dissociation will fuse wrought iron into a homogeneous mass.
“2. Probably by reason of oxidation, which, in a smith’s fire especially, necessarily increases as the temperature increases. Even in a gas furnace a very hot flame is usually an oxidizing flame. The oxide of iron forms a dividing film between the surfaces to be joined, while the slight interposition of the same oxide, when diffused throughout the mass by fusion or partial fusion, hardly affects welding. It is true that the contained slag, or the artificial flux, becomes more fluid as the temperature rises, and thus tends to wash away the oxide from the surfaces; but inasmuch as any iron with any welding flux can be oxidized till it scintillates, the value of a high heat in liquefying the slag is more than balanced by its damage in burning the iron.
“But it still remains to be explained why some irons weld at a higher temperature than others; notably, white irons high in carbon, or in some other impurities, can only be welded soundly by ordinary processes at low heats. It can only be said that these impurities, as far as we are aware, increase the fusibility of iron, and that in an oxidizing flame oxidation becomes more excessive as the point of fusion approaches. Welding demands a certain condition of plasticity of surface; if this condition is not reached, welding fails for want of contact due to mobility; if it is exceeded, welding fails for want of contact due to excessive oxidation. The temperature of this certain condition of plasticity varies with all the different compositions of irons. Hence, while it may be true that heterogeneous irons, which have different welding points, cannot be soundly welded to one another in an oxidizing flame, it is not yet proved, nor is it probable, that homogeneous irons cannot be welded together, whatever their composition, even in an oxidizing flame. A collateral proof of this is, that one smith can weld irons and steels which another smith cannot weld at all, by means of a skilful selection of fluxes and a nice variation of temperatures.
“To recapitulate. It is certain that perfect welds are made by means of perfect contact due to fusion, and that nearly perfect welds are made by means of such contact as may be got by partial fusion in a non-oxidizing atmosphere or by the mechanical fitting of surfaces, whatever the composition of the iron may be within all known limits. While high temperature is thus the first cause of that mobility which promotes welding, it is also the cause, in an oxidizing atmosphere, of that ‘burning’ which injures both the weld and the iron. Hence, welding in an oxidizing atmosphere must be done at a heat which gives a compromise between imperfect contact due to want of mobility on the one hand, and imperfect contact due to oxidation on the other hand. This heat varies with each different composition of irons. It varies because these compositions change the fusing points of irons, and hence their points of excessive oxidation. Hence, while ingredients such as carbon, phosphorus, copper, &c., positively do not prevent welding under fusion, or in a non-oxidizing atmosphere, it is probable that they impair it in an oxidizing atmosphere, not directly, but only by changing the susceptibility of the iron to oxidation.”
In welding steel to iron both are heated to as high a temperature as possible without burning, and a welding compound or flux of some kind is used.
In welding steel to steel the greatest care is necessary to obtain as great a heat as possible without burning, and to keep the surfaces clean.
An excellent welding compound is composed as follows: Copperas 2 ozs., salt 4 ozs., white sand 4 lbs., the whole to be mixed and thrown upon the heat, as is done when using white sand as described for welding iron. An equally good compound is made up of equal quantities of borax and pulverized glass, well wetted with alcohol, and heated to a red heat in a crucible. Pulverize when cool, and apply as in the case of sand only.
A welding compound for cast steel given by Mr. Rust in the Revue Industrielle is made up as follows: 61 parts of borax, 20 parts of sal-ammoniac, 163⁄4 parts of ferrocyanide, and 5 parts of colophonium. He states that with the acid of this compound cast steel may be welded at a yellow red heat, or at a temperature between the yellow, red, and white heats. The borax and sal-ammoniac are powdered, mixed, and slowly heated until they melt. The heating is continued until the strong odor of ammonia ceases almost entirely, a small quantity of water being added to make up for that lost by evaporation. The powdered ferrocyanide is then added, together with the colophonium, and the heating is continued until a slight smell of cyanogen is noticed. The mixture is allowed to cool by spreading it out in a thin layer.