is brought to bear upon the lead, the joint being fed with a strip of the same metal.
Soft-soldering is an operation that the beginner will not find nearly so difficult as hard-soldering or brazing, and although the strength of joints made by it is not nearly equal to that produced by the methods named, it fills a useful place within its scope. It is purely a surface union—that is, the solder adheres to the faces in contact in much the same manner as an adhesive sticks to metal; but with the assistance of fluxes, the contact is made so intimate that some force is necessary to break the joint. Soft-soldering is also of use where brazing would simply mean the ruin or destruction of the metals, as in the cases of lead, poor-quality brass, pewter, tin, zinc, and in tinplate and galvanised iron.
In silver-soldering and brazing, the silver or spelter that fuses to form the joint alloys itself so intimately with the copper or brass that it actually becomes part of the piece itself, and for all practical purposes cannot be distinguished from it. But soft-soldering is not always inferior to hard-soldering. Indeed, the surface nature of the soldering often constitutes its value.
The strongest joints of all are produced by fusion welding, as will be duly understood from later chapters.
CHAPTER II
Soft Solders
A solder should melt at a slightly lower temperature than the metals which it unites, and should possess the quality of alloying with the two surfaces, thus effecting a sound and true metallic joint. Ordinary soft solders are lead-tin alloys, and the larger the proportion of lead the commoner is the solder said to be. At an extreme is plumber’s solder, consisting of 2 parts of lead to 1 part of tin, and, at the other, the best blowpipe soft solder, which contains 2 parts of tin to only 1 part of lead. In the ordinary way, a “coarse” or “common” solder is 2 parts of lead to 1 part of tin; a “fine” or “medium” solder, 1 part of lead to 1 part of tin; and a “very fine” or “best” solder, 1 part of lead to 2 parts of tin.
Eutectic Alloys.
—Lead-tin solders are eutectic alloys—that is, they are examples of the phenomenon of a combination of two metals melting at a temperature lower than one of them would if melted separately. Thus, lead melts at about 328° C., and tin at about 232° C., yet reference to the following table, given by Mr. A. H. Hiorns, will show that the “commonest” solder mentioned fuses at 303° C., and the “best” at 175° C.
Melting points of lead-tin alloys
| Tin % | Lead % | Melting point (C.) |
|---|---|---|
| 10 | 90 | 303° |
| 20 | 80 | 278° |
| 30 | 70 | 255° |
| 40 | 60 | 230° |
| 50 | 50 | 205° |
| 60 | 40 | 187° |
| 63 | 37 | 175° |
| 70 | 30 | 185° |
| 80 | 20 | 198° |
| 90 | 10 | 215° |
Hardness of Solders.
—According to the before-mentioned authority, Saposhniko, in 1908, determined the hardness of various lead-tin alloys by Brineli’s method, by which a steel cone is forced into the metal. The results he obtained are as follow:
| Lead | 100 | 90 | 80 | 70 | 60 | 50 | 40 | 34 | 33 | 32 | 30 | 20 | 10 | 0 |
| Tin | 0 | 10 | 20 | 30 | 40 | 50 | 60 | 66 | 67 | 68 | 70 | 80 | 90 | 100 |
| Hardness | 3·9 | 10·1 | 12·16 | 14·5 | 15·8 | 15·0 | 14·6 | 16·7 | 15·4 | 14·6 | 15·8 | 15·2 | 13·3 | 4·1 |
These results, says Mr. Hiorns, show that the hardest alloy is the one with 66% (about 2 parts) of tin and 34% (about 1 part) of lead, which also is the one having the lowest melting point of all the lead-tin alloys. The results also show that tin is slightly harder than lead.
Compositions of Soft Solders.
—As already shown, solders vary in fusibility according to their composition, and the choice should be determined by the nature of the work and the properties of the metal to be soldered. Should a solder be used of too high a melting-point, the metal will itself be fused before the solder begins to flow.
A point to be particularly observed is that the introduction of a foreign substance into the solder—for example, the addition of a little zinc to a pot of “very fine” solder—will utterly spoil it and render it unworkable. To remove zinc from solder, melt the solder in a pot, take it off the fire and stir in powdered sulphur or brimstone until the whole is of the consistency of wet sand. Replace the pot on the fire and melt, but do not stir the contents. The sulphur and zinc will rise to the surface and form into a cake. Now take the pot off the fire and carefully remove the cake