Suppose we have an iron or steel that will weld. There are still many good reasons for discrediting the safety of the joint until it has been tested. They are:

1. Imperfect Contact.

a. The two surfaces may give the appearance of perfect union, but a considerable percentage of the central portion may be faulty. For if the metal in course of puddling is not relieved of its enclosed slag, this will be finally pressed out into thin laminae. These cleaving planes will be parallel with the bar until it is welded, when they will be upset in all directions at the joint. A network of such planes now running across the bar instead of parallel to it will greatly diminish the tensile strength.

b. Then, again, the smith may carelessly allow some of his flux to stay within the weld. All the hammering in the world will not remedy it. In most cases one of the pieces can be scarfed or pointed, so that the first contact of the hot pieces will be at a central point. As the hammering proceeds, the slag will be forced out of the joint as fast as the pieces weld (see Fig. 2). This caution applies equally to the smithed weld or the Lafitte joint (see page 158).

Fig. 8. - Correct shapes for jump and lap welds.

Fig. 9. - Incorrect shapes for jump and lap welds.

c. Or the smith may not flux his bars correctly, thereby allowing unreduced and uncombined oxid of iron to get in the weld. It is not a difficult thing to get this rust incorporated in the weld, because it is soluble in iron. Dissolved rust makes a "burnt" or brittle joint. Clean silica sand is the common flux. The smith dips his hot bars into a box of the sand kept near by, and often sprinkles a handful over the pieces when hammering if the pieces are large. The reaction between the sand and the rust is rapid. Iron silicates, easily fusible, are formed. They cover the fresh iron surface with a thin glass, which prevents further oxidation.

2. Insufficient hammering or "working" may account for the poor weld. The iron may be perfectly joined, yet the structure is weak and uncertain. Wrought iron has a fibrous structure similar to wood. The smith must do his best to continue this structure through the weld. With a lap weld he can start with a heavy upset which can be hammered down. Hammering serves the double purpose of consolidating the metal and of laying the cleavage planes perpendicular to the blows. Much hammering is impossible with jump and butt welds. The lapweld-ing of wrought-iron pipe and the scarf-welding of chain links-must depend on pressure for their perfect finishing. Welds of steel cannot often be much benefited by working. Steel is an alloy, without structure. To be strong it must be solid and homogeneous. Campbell's advice is to keep the critical temperature high and keep down the silicon and phosphorus when choosing your steel.

3. Too high a heat is responsible for some bad welds. Very great heat will unsettle the structure of the iron in the stock a considerable distance from the joint. Because the bar breaks near, but not on, the joint, is no proof of the soundness of the weld. The structure of the iron is impaired wherever the heat is beyond the critical point of crystallization of any important impurity. By this is meant that if there be an appreciable amount of carbon, for example, it will tend to mineralize the iron when the latter is cooled down past the crystallization point of the first carbon-iron mineral. A number of other impurities, silicon, aluminum, arsenic, etc., are presumed to act in like manner. So the smith must work his weld and also all of the surrounding metal that has been raised above this critical temperature. Just what is the critical temperature is unknown to him, because it varies with every iron, according to the amount and proportion of its impurities, links and small-bore pipe, where the entire piece must be heated, are especially subject to this disadvantage.

Oftentimes welds are made which cannot be worked to advantage. Such welds are apt to be weak. The only present remedy is to select for such purpose iron as free as possible from objectionable impurities. Machine welds, such as small chain

4. Large welds are unsafe.

Just what is the maximum cross-section of practicable welding cannot be safely laid down. Welding is not advisable where it is difficult to get a good welding heat, to flux properly, to joint the pieces well and accurately, or to work the weld when made. To weld a ship's sternpost or skeg or rudder-post was a matter of weeks of expensive and often hopeless labor before the advent of thermit (see page 147). To weld a large driving-rod, an embossing die, crushing roll, or propellor shaft was impossible. The largest iron pipe used, however, could be welded; also chain links a foot or more long, and 2- or 3-inch stock could be safely welded.

Campbell says,1 "Welds of large rods of forging steel are entirely unreliable. Electric methods do not offer a solution of the problem, for during the process the metal is heated far beyond the critical temperature." Thermit, since discovered, does offer a partial solution, because a thermit weld can be reinforced with as much metal as needed, and the metal itself can be varied to meet the requirements. Its cost is to be considered. The oxy-acetylene melt-weld should recommend itself for like reasons. More than one torch can be used for large work (see page 75).

Effect Of Impurities, Etc

In order to make a good weld of iron, the metal must possess plasticity, which is directly affected by the contained impurities. The welding temperature seems to be the point at which the iron becomes semiplastic, but is still not sufficiently hot to burn or oxidize rapidly. For the reason that this temperature varies considerably for the different alloys of iron, it is impracticable to make any definite rules. A number of impurities or alloy formers affect the welding property to a marked degree even when present in fractional parts of 1 per cent. Substances which cause "red shortness," such as sulphur, or which oxidize to form an impervious skin, such as aluminum, must be avoided in iron.

In the case of cast iron, which is iron high in carbon and silicon, the metal passes suddenly from a crystalline to a liquid condition.

1 "Metallurgy of Iron and Steel " p. 588.