= in no the ola, cial , in as of ons ron The now His will Is or that mes ball Fully tion has Kage Dove his that ing, ntly i nedy adry phur rged pose. has d be of may iron. tions are made in the length or thickness of the patterns, so as to produce a different size, or to give additional strength. If in this way what may be called the "balance" of the pattern is interfered with, the casting is very apt to crack on cooling, even though the metal be tolerably satisfactory in composition. In order to understand this effect it is necessary to consider the condition of a specimen of cast iron during cooling. If, for example, we consider the case of a casting in the shape of a cube of one inch square, the outside will naturally cool and become rigid more rapidly than the inside; but all the sides of the cube will be equally affected, and the result will be equal shrinkage in all directions. If, however, twelve such cubes are placed side by side so as to make a bar of twelve inches long by one inch square section, it will now be found that as the outside and ends cool more quickly than the inside and B A, fracture; B, increased thickness. FIG. 30.-Diagram illustrating Cause B B indicates probable position of hollow place or sponginess due to irregular thickness. FIG. 31.--Diagram illustrating Cause of Spongy Part of Casting. middle, contraction will take place chiefly in the direction of length. As a general rule, therefore, it will be found that the thinner the casting the greater will be the contraction of length as compared with that which takes place in other directions. A simple illustration of the way in which fracture can be produced by altering the pattern is furnished by a rectangular casting shown in plan in fig. 30. If the original pattern, as indicated by the black and dotted lines, were satisfactory, but if for some reason the one side be increased in thickness as shown by the line B, it is probable that during cooling a fracture would take place on the thin side, or at A, because the thin side would now contract more than the thick side, and cast iron is not ductile, and does not extend under stress. In following out the same idea, and starting from a simple pattern as shown in fig. 31, if the original uniform rectangular design be altered from the shape as indicated by the dotted line, so as to give a large mass of metal in one part, it is probable that during cooling a hollow part or spongy portion may develop somewhere towards the centre of the larger mass of the casting (as at B), since this portion remains longer fluid. Such places are frequently met with when there is a considerable change in the thickness of the pattern. There is yet another form of faulty design which is responsible for FIG. 32. Fractures of White Cast Iron, showing crystalline structure- weakness in iron castings, and which is connected with the crystallization of the metal during cooling. If, for example, some sulphur is melted and is poured into a mould and then allowed to cool slowly, it will be found on fracture to have developed a crystalline structure, and that the crystals have been formed at right angles to the cooling surface. Cast iron crystallizes in the octahedral system, and though its power to crystallize is not nearly so marked as in the case of sulphur, still, to some extent no doubt, the same influences are at work, and there is evidence that, as a general rule, a crystalline structure at right angles to the cooling surface is set up in cast iron when it passes from the fluid to the solid state. This effect is most readily observed in cast iron which has been chilled, in which case a distinct crystalline character is often to be plainly seen, as illustrated in fig. 32. If now we consider a corner seen in the illustration given in fig. 33, it will be observed that the two sets of crystals cross on a line which bisects the angle, and where these crystals meet a plane of weakness is developed. For this reason cylinders should ing effect of sharp angles in a casting. never be cast with flat bottoms and sharp FIG. 33.-Diagram showcorners, as a comparatively moderate internal pressure will often force out the bottom of such a cylinder in a single piece. In every case where strength is required, sharp corners, or sharp re-entering angles, should be avoided, and wherever possible a smooth contour adopted. There should also be no abrupt or unnecessary changes in the thickness. Note.-In connection with this portion of the subject, two recent articles on the Foundry may be read with advantage, viz., "Modern Foundry Appliances," by Dr R. Moldenke, Cassier's Magazine, vol. xxiii. p. 251, and “The Equipment of the Foundry,” by J. Horner, ibid, vol. xxiv. p. 486. Among larger works, "The Metallurgy of Cast Iron," by T. D. West, and "Modern Foundry Practice," by J. Sharp, may be consulted. |