It cannot, therefore, be welded over the smith's forge, and can be welded only by one of the recent special processes.

There are an infinite number of kinds of iron, due to the proportions of the combined impurities and the method of treatment during the milling. Each of these so-called alloys will act differently at the welding temperature, while many of them cannot be welded by the smith at any temperature. There are a few general rules which will help the worker in the selection of his material. But there is so much room for failure in welding the best irons that what cannot be blamed on the smith and his material must be laid to the devil.

Carbon in the combined state should be kept below o. 30 per cent, because it is a hardener and makes brittle in the combined state. Carbon has been found to amalgamate with iron to form a number of alloys, which are unworkable in proportion to their carbon content. W. C. Unwin1 gives 0.90 carbon as the limit for welding iron with silica flux; 1.10 carbon as the limit with any flux. These alloys,2 pearile, martensite, cementite, etc., mineralize in the iron and form a granitic structure on cooling. It is likely that all of the mineral impurities of iron behave in this manner.

In the uncombined form, as graphite, carbon does not affect the welding property, except that it becomes an enemy to the homogeneous structure of the metal, just as does slag.

Slow cooling, drawing the temper, keeps the combined carbon at a minimum. Silicon in a quantity approximately over 1 per cent and aluminum even more potently, also reduce combined carbon to graphite. However, the objection to silicon is that it makes a brittle alloy, and aluminum oxidizes to the disadvantage of the metal. Slow cooling is valueless for the reason that welding is done at a high heat, when iron is capable of absorbing carbon in greater quantity.

Hence high carbon in the stock cannot be toned down and should be avoided. It should never be above 0.50, and ought to be below o. 25 for the best results.

Silicon causes brittleness, which does not yield to welding heat. It must be kept below 0.20 per cent. Silicon is the element which differentiates cast iron from mild irons and steel. An iron with gray-colored fracture contains too much silicon to use in welding.

1 "Testing of Materials of Construction," W. C. Unwin.

2 Wm. Campbell, Electrochemical and Metallurgical Industry, Feb., 1904.

Welding Property Of Silicon Steels 1

It will be noted that the second series of steels gave perfect welds with a content of 0.504 silicon, by above analysis; probably accounted for by the high manganese, which will prevent crystallization.

Phosphorus makes the "rotten iron" for thin and sharp castings when present in quantity approximately over o. 10 per cent. Malleable iron for welds should contain less than o. 03 per cent of it, not so much to assist welding, as to insure a strong joint on cooling.

Sulphur is one of the deadly enemies of the smith. Its effect on iron is especially disastrous at welding temperature, even when not otherwise apparent. In quantity approximating over 0.10, it causes "red shortness," brittleness. It should be kept as low as possible. Manganese is the remedy for sulphur, used in the production of iron, but the smith cannot correct it with manganese himself. Very low sulphur content is the secret of the success of Swedish iron in the arts.

Manganese may be present up to about 1.50 per cent, in iron or steel to be welded. Up to this limit its effect is generally beneficial. Campbell states that between 2 and 6 per cent it forms a brittle unworkable alloy. Hadfeld's steel, with about 7 per cent, manganese, again becomes malleable.

Manganese is the cheapest tonic of the iron producer. It reduces both sulphur and oxygen in the iron and rises to the top as slag. The remainder, if any, forms a tough workable alloy with the iron.

1 Campbell, "Metallurgy of Iron and Steel."