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Description Of Welding

Welding is the intimate union produced between the surfaces of two pieces of metal when heated to proper temperature and hammered together. The union is so close that when two bars of metal are properly welded the place of junction is as strong relatively to its thickness as any part of the bar. To weld bar iron to another piece of iron requires an intense heat. Wrought iron at the welding temperature possesses the property of expanding when cooled and contracting when heated, and the welding property is intimately connected with the critical condition in which this abnormal behaviour is exhibited.

The condition known as "the welding state " of iron or steel is one which exists only within a very limited range of temperature. If the smith takes his iron bars out of the fire at too low a temperature, welding cannot be effected. If, on the other hand, the iron is too hot, a failure is also certain.

The range of temperature, during which impact or pressure causes the union, known as welding, of two masses of iron or steel, is therefore comprised within narrow limits, and the familiar operation is really a critical one.

In welding by the forging process the parts to be united are heated within the critical range of temperature, considerably less than that of fusion. The smith in striking with his hammer is assisted by the molecules which approach, though never arrive at, liquidity. This condition is favourable to the interpenetration of the molecules and consequent adhesion of the surfaces on hammering.

It has been shown by Sir Thomas Wrightson that the phenomenon of welding is akin to that of regelation, or the sticking together of ice under pressure. To prove this an experiment was made which showed a distinct decrease in temperature, amounting to 106° F., at 2,550° F. during welding, a similar result being known to take place during regelation. This abstraction of heat is caused by the melting of the iron or ice in either case, and the consequent need of latent heat for the liquefaction.

If the welding of two metals is caused by fusion by the aid of combustible gases in conjunction with oxygen, this latter acting as the gas of combustion, or by electricity, or even by the Alumino-Thermic process, the metals become subject to considerable alteration in their properties chemically as well as physically, offering thereby difficulties of great extent in the production of a perfect weld.

The welding by aid of combustible gases is very similar to casting; and if it may be termed a process of casting, then also must the conditions attributed to that process be applicable to welding more or less. The difference would then appear to be the composition of the moulds, those of sand being substituted by the metal to be welded, which, however, is of superior and more finished make as compared with the metal to be used for the completion of the weld. The molten or liquefied metal is poured at a considerable temperature into the grooved metal pieces to be welded, which have the temperature of the surrounding atmosphere only, and is then subjected to a mechanical treatment, in order to increase the strength and ductility of the weld so that it may resemble that of wrought iron more or less.

Although the cast-iron expands at the moment of solidification and thus seemingly produces a weld, the subsequent cooling from a red heat to the ordinary temperature leads to a still greater contraction, and the net result is an imperfect weld. To overcome this difficulty, mechanical means must be employed to facilitate the union between the molten mass and the metals, converting the former, as far as possible, into a similar condition to that of the superior metal. The cou-traction that takes place during cooling after solidification is not uniform, but is interrupted by certain arrests or expansions which occur at particular temperatures. The shrinkage in welds is, however, by no means a constant quantity, but varies with the proportions of the weld and with the character of the metal used.

The strength and solidity of the weld are affected by the bulk of metal employed to complete the weld, and also by the form of the article to be welded. Thus, if a sample of cast-iron which would be suitable for a weld of small size be employed for making a very heavy weld, it will be found that, owing to the slower cooling in the latter case, the grain of the metal becomes much more open, and the strength is proportionately diminished. On the other hand, if the same metal were used for very small welds, the chilling in the seam or groove of the weld would tend to make the product close and hard, and in many cases this would be so marked as to make the weld quite brittle. The grade of the iron used must therefore depend upon the size of the weld to be made, and probably a closer grained iron must be used for large than for small welds. At the same time, it is generally found that the strength of a large weld per unit of area is somewhat less than that of a smaller one, since the closeness of grain is usually though not always associated with increased tenacity.

It is also very important that in large welds, where strength is required, no sharp or re-entering angles should occur, as these in all cases lead to the formation of planes of weakness in the weld. When the metal cools in the weld a crystalline structure is developed, the crystals forming at right angles to the cooling surface. If this cooling surface be curved, the crystals interlace so as to yield a strong weld of uniform structure, while on the other hand, whenever a sharp change of curvature takes place a plane of weakness is the result.

It is much to be desired that some plan could be adopted by which a test piece weld would indicate exactly and directly the physical qualities of a weld of the same metal, but no method of doing this has been devised or seems likely to be.

Until it has been fully made clear what takes place during the operation of autogenous welding, the changes that take place in the chemical as well as the physical compositions not only of the metal parts to be welded together, but also of the liquefied metal which, from the welding bar is poured into the seam of the weld, and more so the intermediary and gradual interlacing action upon the two metals, it seems useless as well as fruitless to discuss what kind of repairs may or may not be made especially upon marine boilers, so as to satisfy not merely the insurance inspectors but to a greater extent to secure that safety in public conveyance which is of vital importance.

Discussion and professional examination in this direction would tend more quickly to solve the difficulties so vividly apparent in the great and important industry of autogenous welding.