Welding - theory, practice, apparatus and tests

In spite of the numerous data on the theory, practice, apparatus, and tests of welding contained in the trade journals and metallurgical books, no previous attempt has been made to present this data in sequence under one cover. But in the last fifteen years the subject has begun to be of interest and importance. The electric, thermit, and hot-flame processes are welding all of the metals and are doing repeat and repair work that has never before been attempted. New brazing methods have also been successfully tried out and the range of good solders greatly increased.

I have given separate chapters to the commercial metals. Few of the metallurgies give much space to the working properties of the metals, especially the welding property, which is often merely mentioned.

Test and cost data must be taken with a grain of salt. The test data have been compiled from various sources. Those tests given for iron are standard and cannot be questioned. Those tests given for the special processes are more recent and in most cases have been made by interested parties. They are no doubt accurate, but at present the special processes cannot be so well represented by test data, as by the actual work they turn out. The same may be said of cost data. The prospective purchaser of welding machinery must figure the cost of his apparatus plus the cost of labor and the depreciation. But above all, he must satisfy himself that the apparatus he chooses is the best for his kind of welding.

According to the Standard Dictionary, to weld is to “unite, as heated metal, in one piece or mass under the hammer or by pressure.”

The Century Dictionary says, "To unite or consolidate, as pieces of metal or metallic powder, by hammering or compression, with or without previous softening by heat." "term is more generally used when the junction of the pieces is effected without the actual fusing point of the metal having been reached." While the Standard adds, "Metals are weldable in proportion to the length of time they will stay under heat in a plastic condition without melting."

Welding is distinguished from soldering, which is, according to the Standard, "To unite, as two metallic substances, by solder." The Century, "To unite by a metallic cement." "Every kind must be used as its own melting point, which must be always lower than that of the metals to be united."

I give these definitions because there is some confusion of the terms; and naturally so, as the two processes often are undistinguishable. Thus two unlike metals, as iron and platinum, may be welded; while a fractured steel bar may be united by placing platinum foil between the pieces, pressing strongly together and heating moderately. This is strictly welding, yet the platinum foil is solder. In the recent processes of welding by fusion, the molten metal becomes a solder. Brazing is classed as soldering, but when brass is brazed the process is as nearly welding as the so-called autogenous weld.

The word autogenous is misapplied to welding. It means self produced. The melted weld of the oxy-hydrogen or acetylene flame is a soldering process in which the metal produces its own solder. However, it makes a catchy trade-name.

Welding, under different names, is a property possessed by many substances, both elemental and compound. According to Roberts-Austen, who devotes considerable space to the flowing property of metals, "welding is the property possessed by metals, which on cooling from the molten state pass through a plastic stage before becoming perfectly solid, of being joined together by the cohesion of the molecules that is induced by the application of an extraneous force, such as hammering."

In general, welding occurs if cohesion between the molecules of the two pieces can be induced. This cohesion may amount to diffusion when the two pieces are of unlike substance, and the metal at the weld will be found to be an alloy of the two, Thus gold and lead, pressed together for several weeks, will weld at ordinary temperature. At 100 deg. C. they will weld in less time, and the weld will be an alloy of gold and lead.

Welding and diffusion are not inseparable, however. For the welding by diffusion of lead and gold is weaker than the first- named cold weld. While the diffusion of mercury through an other metal invariably produces weakness of that metal, sometimes disintegration.

Regelation is the name given to the welding of two pieces of ice. Faraday is credited with this discovery. He found that two pieces of ice slightly below freezing point, if pressed together will weld. Wrightson states that both iron and ice suffer a drop in temperature when pressed together. He heated two irons to the plastic state in an electric welder and pressed them together. The recording pyrometer showed a sudden fall of from 19 deg. to 57 deg. C. He further states that iron in- creased almost 7 per cent, in volume on becoming plastic, and tries to trace an analogy between the behavior of iron and ice. It has since been found that revelation is a property possessed in some degree by most crystalline substances. Pure crystalline salts will regelate under pressure at moderate temperature. Even such a substance as bismuth will regelate.

Evidently welding depends upon two things:

1. The flow.
Most of the so-called solids are fluid to some extent. Highly crystalline, refractory rocks will flow under great pressure. The walls of some deep mines have flowed together in the course of time. A rod of glass or of sealing-wax will bend or flow if it supports a weight for several days. Lead, sodium, etc., flow readily under pressure. Flow is almost synonymous with malleability, the difference being a matter of time. Many substances which flow slowly will not withstand the shock of the hammer.

Most metals flow at all temperatures from normal to melting point,, but they are the most easily weldable within the range of greatest plasticity. But their welding also depends upon

2. The wetting or cohesion of the two substances.
Two pieces of the same or different substance will not weld if their surfaces do not cohere, no matter how malleable or fluid they may be. Aluminum is a notable instance. The metal is quite malleable at most temperatures, but a microscopic film of oxid prevents the two surfaces from wetting one another. Iron in a lesser degree is troubled with a coating of oxid at welding temperature. Any flux which will clean off both surfaces will allow a weld to be made. In proportion to the ease with which one can have and hold a clean surface of the metals in the range of plasticity, in that proportion will welding be feasible. The welding of malleable metals is dependent on the behavior of the oxids which form on their surface. In proof of this is the remarkable experiment of Chernoff 1 in 1877. He showed that a partial weld of two pieces of iron could be made at the low temperature of 650 deg. C., which is at least 700 deg. below common welding heat. The two surfaces were planed and highly polished. Pressure was applied for several days, when it was found that there was a partial weld. This is similar to the well-known experiment in physics where two plane and highly polished surfaces of glass are pressed together. The surfaces will cohere to some extent.

These two experiments seem to show that whatever assists cohesion, assists the welding. There are numerous instances of welding among non-metallic substances which do not oxidize at the welding heat. Glass is too well-known to need explanation. Pieces of horn can be joined under pressure of hot plates if the horn be kept moist.

Metals newly nascent, in a fine powder, can be welded into a solid piece by a stroke of the hammer. Apparently for the reason that the grains of powder have bright, clean faces. Most of the malleable metals are so weldable.


THEORIES OF WELDING

In 1877, Holley 1 advanced the theory that irons weld in pro- portion to their mobility or flowing, and inversely as oxidation of the welding surfaces occurs. He thought that the more plastic or more nearly melting point the irons were, the more readily they would weld. But with every increase in heat was a corresponding readiness to oxidize especially on the part of carbon and iron. This oxid interposed a mechanical difficulty to perfect welding.

This theory does not satisfy Campbell 2 who insists that impurities tend to crystallization in the body of the iron. Carbon, which is the principal offender, and sulphur, phosphorus, and other ingredients, all form alloys or compounds with the pure ferrite. Ferrite itself is exceedingly malleable and mobile. But a mixture of ferrite and several of the carbon compounds, as cementite, martensite, etc., is stiff above red heat in proportion to the carbon present. Campbell thinks that such a steel, which is really a mineral with a granitic structure, will not weld, because it refuses to flow. He claims that oxidation troubles are actually less, because the chemical combination of the iron oxid with the impurities and their oxids would give a self-fluxing surface.

According to Campbell, then, those impurities which caused decided crystallization with accompanying brittleness, interfered with the flow at high heat and prevented welding. Manganese, it is true, makes a more brittle iron, up to i . 20 per cent; but it prevents crystallization of sulphur, etc., and is an aid in welding.

For ordinary and commercial purposes the welding must be done in a few seconds' time, and the previous cleaning and heating must not take long. This at once limits to a very few the number of metals which can be welded; were it not for the recent remarkable advance due to the electric, oxy-hydrogen and acetylene processes of melting, welding would be confined to iron, platinum, nickel, and gold. Other metals would be joined by soldering and brazing, and even then the metal worker would have great difficulties with aluminum and many alloys.

I will first take up iron and steel welding. As much research work has been done on the metallurgy of iron as on all of the other metals combined. It is extremely probable that many of the difficulties and problems arising from proportions of impurities and methods of producing will apply equally to other metals. For this reason, and because of its overwhelming importance, I will treat of iron more thoroughly.


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Welding - theory, practice, apparatus and tests