Newcombe calls attention to the ambiguity of language in which the conditions for an effective weld are often stated, and introduces extracts from some of the latest writers on the subject. Edward Williams, at the conclusion of a paper "On the Manufacture of Rails/' read before the Iron and Steel Institute in September 1869, said welding was the one thing needful, and we should never lose sight of it. The chance of obtaining thorough welding would be much increased by not insisting on more toughness and fibre than is absolutely necessary to guard against so much brittleness as would bring about breakages of the rail in work. In the discussion on that paper, Sir William Armstrong said that in the manufacture of guns on the coil system, a perfect welding is just of as much importance as it is in the manufacture of rails The conclusion arrived at, both at Els - wick and Woolwich, was this, that in proportion as the iron has a steely character, so in proportion is it unfavourable for welding. The indication of its steely character was obtained by taking a specimen of iron heated to a certain point, and then plunging it into water, f its tensile strength was found to be increased beyond a certain limit, it was rejected as unfavourable for welding.
The iron welds most perfectly which undergoes no increase of strength in the process of hardening. Williams, in reply to questions, defined good welding to be a combination of effects, an actual amalgamation of the surfaces, and sol-dering together by means of the cinder In proportion as there is more of the absolute contact and less of the soldering, so is welding good, and vice versa. Where there is no contact of the actual metallic surfaces, and nothing but the soldering of the layers together by means of the cinder, it is poor welding; and it is the poorer the thicker the cinder. Where you have a large proportion of surface actually brought in contact with the layer next to it, then you have good welding. But perfect welding - that is, complete contact of surfaces, or anything at all approaching it - is impossible.
Mattieu Williams, in treating on lamination and blistering, says that when a blacksmith makes a weld in a common open fire, he throws sand on the surface to be joined, the object being to flux the scale - that is, to convert the oxide into fusible silicate. This being done, he brings the fluxed surfaces together, and by hammering forces out the liquid silicate, and thus brings clean surfaces of pure iron together, which at a proper heat unite perfectly. If he had a film of oxide between the surfaces it would prevent welding. Following up this principle, Mattieu Williams obtained from the potteries some "slip," or finely - ground flint used in glazing earthenware, mixed this with sufficient water to form a sort of paint or whitewash, and with a whitewashed brush painted it over the surface of the piles on both sides of each layer. He treated several piles of the finest quality of iron in this manner. They were rolled into boiler - plates, none of which showed any signs of lamination. He believes that by this means lamination may be effectually prevented.
At a meeting of the Iron and Steel Institute, Richard Howson, in a paper read before that society, "On Welding Iron," said that in order to obtain complete metallic contact the skill of the workman had to be exercised - 1st, in heating the iron sufficiently; 2nd, in protecting the surface from oxidation by means of a flux; 3rd, in forming the surfaces in such a way that the flux has a means of escape when the ends are closed up under the hammer.
Having thus given a resume of the latest theories on the subject, New - combe examines them from a practical standpoint, to ascertain how far they are supported or contradicted by the most advanced practice of the day; and as Howson's views are of the most recent date, and may fairly be supposed to include much that had previously been said on the subject, he takes up the consideration of his conditions first, on the necessity for proper heats to secure good welds.
There is no operation connected with smithing which requires more careful handling, or gives more anxiety to the smith, than the process of welding, for on the successful issue of one weld in the manufacture of a single article may depend either the success or loss of much labour and money. It may therefore repay us to examine with care the conditions to be observed in obtaining a good heat. In the first place, the fuel must be as free from metallic impurities as possible, especially sulphur, as it readily combines with iron, and with it forms sulphide of iron, which is naturally detrimental to the formation of a good weld; 2nd, by a proper construction of the hearth, and arrangement of twyer, to obtain the requisite chemical combination necessary for a proper combustion of the fuel for heating purposes. This is effected by placing the twyer about 4 in. below the level of the hearth for lighter kinds of work, and 6 to 8 in. for heavy kinds. But even with good fuel and the arrangement of twyer just spoken of, we may obtain two kinds of heat - viz., a carbonaceous (reducing) heat, or au oxidizing (destroying) heat; the carbonaceous is that which is required by the smith to reduce his iron to a welding condition, to obtain which he must maintain a constant supply of heated fuel between his twyer and the iron to be heated, and covering it likewise if the whole mass is to be heated.
The chemical action which here takes place may be explained thus:- The oxygen of the air, after passing the twyer, comes in contact with the heated carbon in the fuel; chemical union then takes place; 1 part of the carbon combines with 2 of oxygen, forming carbonic acid; this, in passing through the heated fuel above it, takes up another part of carbon and forms carbonic oxide, which is composed of 2 parts carbon and 1 oxygen; and so long as this action can be maintained, we have a reducing heat suitable for bringing iron into the welding state with the formation of the least amount of oxide on the surfaces of the iron; and if we fail to obtain those conditions, and allow the fuel to become deficient in quantity between the twyer and the iron when it is in a semi - welding state, then we have a chemical action of a different kind, for the oxygen then being in excess, through a deficiency of carbon, readily combines with the iron, and forms a cinder or oxide of iron. This combination results in great loss of iron.