1911 Encyclopædia Britannica/Brazing and Soldering

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BRAZING AND SOLDERING, in metal work, termed respectively hard and soft soldering, are processes which correspond with soldering done at high and at low temperatures. The first embraces jointing effected with soldering mixtures into which copper, brass, or silver largely enter, the second those in which lead and tin are the only, or the principal, constituents. Some metals, as aluminium and cast iron, are less easily soldered than others. Aluminium, owing to its high conductivity, removes the heat from the solder rapidly. Aluminium enters into the composition of most of the solders for these metals, and the “soldering bit” is of pure nickel.

The hard solders are the spelter and the silver solders. Soft spelter solder is composed of equal parts of copper and zinc, melted and granulated and passed through a sieve. As some of the zinc volatilizes the ultimate proportions are not quite equal. The proportion of zinc is increased if the solder is required to be softer or more fusible. A valuable property of the zinc is that its volatilization indicates the fusing of the solder. Silver solder is used for jewelry and other fine metal work, arid has the advantage of high fusing points. The hardest contains from 4 parts of silver to 1 of copper; the softest 2 of silver to 1 of brass wire. Borax is the flux used, with silver solder as with spelter.

The soft solders are composed mainly of tin and lead. They occur in a large range. Common tinner’s solder is composed of equal parts of tin and lead, and melts at 370° Fah. Plumber’s solder has 2 of lead to 1 of tin. Excess of lead in plumber’s solder renders the solder difficult to work, excess of tin allows it to melt too easily. Pewterers add bismuth to render the solder more fusible, e.g. lead 4, tin 3, bismuth 2; or lead 1, tin 2, bismuth 1. Unless these are cooled quickly the bismuth separates out.

The essentials of a soldered joint are the contact of absolutely clean surfaces, free from oxide and dirt. The surfaces are therefore scraped, filed and otherwise treated, and then, in order to cleanse and preserve them from any trace of oxide which might form during subsequent manipulation, a fluxing material is used. The soldering material is compelled to follow the areas prepared for it by the flux, and it will not adhere anywhere else. There is much similarity between soldering and welding in this respect. A weld joint must as a rule be fluxed, or metal will not adhere to metal. There is not, however, the absolute need for fluxing that there is in soldered joints, and many welds in good fibrous iron are made without a flux. But the explanation here is that the metal is brought to a temperature of semifusion, and the shapes of joints are generally such that particles of scale are squeezed out from between the joint in the act of closing the weld. But in brazing and soldering the parts to be united are generally nearly cold, and only the soldering material is fused, so that the conditions are less favourable to the removal of oxide than in welding processes.

Fluxes are either liquid or solid, but the latter are not efficient until they fuse and cover the surfaces to be united. Hydrochloric acid (spirits of salts) is the one used chiefly for soft soldering. It is “killed” by the addition of a little zinc, the resulting chloride of zinc rendering its action quiet. Common fluxes are powdered resin, and tallow (used chiefly by plumbers for wiped joints). These, with others, are employed for soft solder joints, the temperature of which rarely exceeds about 600° Fah. The best flux for zinc is chloride of zinc. For brazed joints, spelter or powdered brass is employed, and the flux is usually borax. The borax will not cover the joint until it has been deprived of its water of crystallization, and this is effected by raising it to a full red heat, when it swells in bulk, “boils,” and afterwards sinks quietly and spreads over, or into the joint. There are differences in details of working. The borax is generally powdered and mixed with the spelter, and both with water. But sometimes they are applied separately, the borax first and over this the particles of spelter. Another flux used for copper is sal ammoniac, either alone or mixed with powdered resin.

As brazed joints often have to be very strong, other precautions are frequently taken beyond that of the mere overlapping of the joint edges. In pipes subjected to high steam pressures, and articles subjected to severe stresses, the joints are “cramped” before the solder is applied. That is, the edges are notched in a manner having somewhat the appearance of the dovetails of the carpenter; the notched portions overlap the opposite edges, and on alternate sides. Such joints when brazed are stronger than plain overlapping joints would be. Steam dome coverings are jointed thus longitudinally as cylinders, and the crown is jointed thereto, also by cramping. Another common method of union is that of flanges to copper pipes. In these the pipe passes freely within a hole bored right through the flange, and the solder is run between. The pipe is suspended vertically, flange downwards, and the spelter run in from the back of the flange. The fused borax works its way in by capillary action, and the spelter follows.

The “copper bit” is used in soft soldering. Its end is a prismatic pyramid of copper, riveted to an iron shank in a wooden handle. It is made hot, and the contained heat is sufficient to melt the solder. It has to be “tinned,” by being heated to a dull red, filed, rubbed with sal ammoniac, and then rubbed upon the solder. It is wiped with tow before use. For small brazed work the blow-pipe is commonly employed; large works are done on the brazier’s hearth, or in any clear coke fire. If coal is used it must be kept away from the joint.

In “sweating on,” a variation in soldering, the surfaces to be united are cleaned, and solder melted and spread over them. They are then brought together, and the temperature raised sufficiently to melt the solder.

A detail of first importance is the essential difference between the melting points of the objects to be brazed or soldered, and that of the solder used. The latter must always be lower than the former. This explains why soldering materials are used in a large range of temperatures. A few will melt at the temperature of boiling water. At the other extreme 2000° Fah. is required to melt a solder for brazing. If this point is neglected, it will often happen that the object to be soldered will fuse before the solder melts. This accident may occur in the soft Britannia and white metals at the one extreme, and in the softer brasses at the other. It would not do, for example, to use flanges of common brass, or even ordinary gun-metal, to be brazed to copper pipe, for they would begin to fuse before the joint was made. Such flanges must be made of nearly pure copper, to withstand the temperature, usually 98 of copper to 2 of tin (brazing metal). A most valuable feature in solder is that by varying the proportions of the metals used a great range in hardness and fusibility is obtainable. The useful solders therefore number many scores. This is also a source of danger, unless regard be had to the relative fusing points of solders, and of the parts they unite.  (J. G. H.)