In executing the weld, care must be taken to avoid contact of the white jet of the blowpipe flame with the metal just about to be melted, because the high temperature of this part tends to produce holes which are difficult to fill in. The distance of the white jet should vary according to the power of the blowpipe, say from 1/4 in. to 3/4 in. The flame should be so adjusted as to furnish an excess of acetylene. There need be but little fear of carbonising the metal, for the reason that the temperature of the work is comparatively low. For thin welds, up to 1/8 in. thick, it is preferable to hold the welding rod in front of the blowpipe in the direction of the edges to be welded. As soon as the latter begins to melt it is heated rapidly, and should be lowered to form one molten bath with the metal of the piece. The welding is thus done very rapidly. For great thicknesses it is preferable to obtain fusion of the welding rod, previously heated in the molten bath of the bevel. Directly after welding, the weld should be thoroughly washed in clean warm water in order to remove all remaining traces of the flux, which would otherwise continue to have a chemical action on the metal, thereby setting up corrosion.
=Welding Cast-Iron.=--The edges of the weld should be bevelled when the thickness exceeds 1/8 in.; this enables the welding to penetrate the entire thickness of the metal. Both edges must be bevelled to an angle of 45°, so as to form a right angle at the weld. The bevelling should be regular, especially at the bottom, so as not to produce holes or excess thickness at the bottom of the bevel. Workers who attempt to effect welds on cast-iron above, say, 1/4 in. in thickness, without bevelling, invariably obtain poor results, as it is impossible to get regular and thorough penetration. The bevelling of the edges may be done by chipping or grinding, etc. Grinding wheels made from a carbide of silicon abrasive are very effective for cast-iron. The edges to be welded and their immediate neighbourhood must be free from sand, dirt, and rust.
It is known that internal strains are always set up in every process of welding, due to the expansion and contraction when a metal body is heated and cooled. These strains are not unavoidable, but their effect may be minimised or nullified. In the case of cast-iron, the tendency to crack will be greatly increased if the cooling of the metal after fusion is rapid or irregular. Consequently, the article to be welded should be pre-heated slowly to about 700° F. to 1,000° F. Generally speaking, the higher the temperature of pre-heating, the less the danger of cracking. Preferably, pre-heating and subsequent slow cooling should be carried out in a muffle, particularly where light and intricate castings have to be dealt with.