This section is from the book "An Elementary Outline Of Mechanical Processes", by G. W. Danforth. Also available from Amazon: An elementary outline of mechanical processes.
There are now in use many important means of uniting metals solidly together. These have many applications in manufacturing and one or more of them may frequently be availed of in making permanent repairs to broken machinery or equipment under emergency conditions.
Methods of joining metals include the following, some of which have been described:
(4) Electric welding.
(5) Thermit welding.
(6) Oxy-aeetylene and oxy-hydrogen welding.
(7) Burning on.
Closely akin to these methods is the use of metal cements, which stop cracks in castings, seams, and joints of patches held in place by aid of bolts and iron straps. A leaky pipe or boiler seam may often be stopped by the use of sal ammoniac and iron filings, or by a mixture of Portland cement and coal tar. A patented compound known as Smooth On is very effective for mending broken machinery parts and repairing leaks in boilers, tanks and castings.
Joining metals implies an actual metal to metal contact. There is always an oxide or other coating over a metal surface exposed to air. This must be removed before two metals can be joined, and it is done principally by scraping, filing, pickling, etc. The last thin coating of oxide which forms during heating is removed by action of a flux. Union between the two metals is then accomplished (1) by bringing the contact surfaces of both metals up to the melting point of at least one of them, as in soldering and brazing, which necessitates the melting of the solder or brazing metal, or (2) by bringing both metals to a state of incipient fusion at the points where they are to be welded and pressing them together firmlv.
There seems no doubt that the process of welding iron or steel detracts from their strength. The weld may not break under a tensile test, but the metal on one side of the weld may break instead. The high heating necessary for welding changes the grain size of iron and steel, making it coarse and thus weaker than before. This condition can be remedied by reheating welded bars to the critical point and in this way restoring the fine grain size, or in a measure by a thorough hammering while the weld is yet very hot.