The strongest joints of all are produced by fusion welding, as will be duly understood from later chapters.
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.
_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 Tin 0 10 20 30 40 50 60 Hardness 3·9 10·1 12·16 14·5 15·8 15·0 14·6
Lead 34 33 32 30 20 10 0 Tin 66 67 68 70 80 90 100 Hardness 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.