A Treatise On The Principles And Practice Of Dock Engineering

DEFECTS IN CASTINGS. T37 the iron either at the exact composition required, or comparatively pure, so that the requisite carbon may be added to it. Siemens-Martin steel results from the reduction of a mass of crude iron, often with the admixture of an ore rich in oxide, the whole being melted in an open hearth exposed to the intense heat of a regenerative furnace. It is a much slower process than the Bessemer, but it produces a steel of a more generally trustworthy character, and it is frequently specified for bridgework.. Of the above processes, two modifications exist—viz., (a) the Acid, and (6) the Basic— according to whether the lining of the converters, or ot the furnaces is siliceous or calcareous. In the basic process, additions of calcined lime are made to the bath of molten metal in order that it may combine with the excess of phosphorus, and remove it in the form of slag. In the acid process this step is not taken, and hence the necessity for purer ores. Practical Observations on Manufactured Iron. Defects in Castings. —The engineer should have sufficient acquaintance with foundry methods to enable him to appreciate the difficulties ot success- ful casting, to understand the proper distribution of the metal for the purpose intended, and to distinguish between defects which are trilling and those which are of vital importance. Founders incur considerable risks and many failures in their endeavours to reproduce large and intricate patterns, and such work should not lightly be rejected on account of some insignificant surface blemish, when otherwise sound and serviceable. On the other hand, there are surface indications, apparently slight, which reveal serious internal defects. The necessity for having the metal thoroughly fluid, in order that it may penetrate to all parts of a large mould, sometimes causes it to be heated to such an extent that it burns into the sand of the mould, and instead of producing the clear blue skin of the ideal casting, a rough white surface is the result. This affects green sand moulds rather than those of loam or dry sand. The most common defects of castings are the presence of blow or air holes due to the generation of steam and gases by the damp sand, the want of sufficient venting, and an imperfect supply of metal. A certain amount of dampness in the sand of moulds and cores is necessary to secure adhesion of the particles, but an excess of moisture produces steam. An insufficient number of vents causes particles of air to be imprisoned in the varions parts, and an imperfect supply of molten metal lacks the head to secure homogeneity. Very often these blowholes are not manifest until the casting is machined, and occasionally they escape notice altogether. It is obvions that they are a source of weakness wherever they occur. The author has noticed a hydraulic pressure pipe develop an almost imperceptibly fine jet through the thickest part of the flange, while the