This section is from the book "Safe Building", by Louis De Coppet Berg. Also available from Amazon: Code Check: An Illustrated Guide to Building a Safe House.
Another serious objection to uneven thickness in castings is the uneven cooling; that is, the thin parts cool before the thicker ones, and the consequence is the production of internal strains in the metal, the parts which have cooled rapidly, contracting and pulling away from the other parts. This means tension, and a constant tension right in the metal and before any external strains come upon it; this frequently is the cause of the sudden and otherwise inexplicable failure of the casting.
It is essential, therefore, that not only should cast-iron cool evenly, but also as slowly as possible, to allow the contracting parts to relax.1 It is well-known that if a casting- column or lintel for instance - be heated in a fire and then suddenly be cooled by a stream of water played against it, that it is liable and likely to snap in two from the sudden tension put on the metal in rapid cooling. For this reason all such castings should be surrounded with non-conducting, fireproof materials wherever placed in inflammable buildings, or surrounded by inflammable goods.
Another essential in good castings is, not to have them too big and heavy ; otherwise it is impossible to keep the entire mass of same fluidity; that is, to prevent parts from cooling before other parts have yet been filled with molten metal. The result of such castings is apt to be weak from internal strains, due to unequal casting and cooling, to be weak on account of the thickness of the metal, and to be spongy or filled with air bubbles, due to the gases escaping at one end and mixing with the molten metal running in at the other end. Sand-holes in castings are, of course, due to the use of a poor quality of sand or improper ramming or to the presence of dirt, scum or other impurities.
Thin castings strongest.
Danger of Sud-aen Cooling.
Big castings bad.
1When castings of unequal thicknesses are required,as for instance in flywheels and pulleys, the strain from unequal shrinkage are prevented by so removing the sand from the hot casting as to bave all parts cool together. The thin parts or arms being kept covered until the heavier rim and hub have been hurried in cooling by uncovering them.
Another essential in good castings is to round all angles, that is, not to go abruptly from one surface to another at a right or sharp angle to the first: this, for the same reason that uneven thickness is avoided, namely, the danger of internal tensional strains.
In casting plates it is desirable to have a large circular hole, if possible, in the centre of the plate, or else several holes, which will largely prevent the great strain, in cooling, on the centre of the mass.
The contraction of cast-iron in cooling, from its bulk when fluid depends on its thickness and its mixture. It is about 1/8" per foot for thin castings, for castings 1 1/2" thick about 1/12" per foot, and for castings 2" thick about 1/16" per foot. The moulds and patterns are, therefore, made correspondingly larger than required by the architect's drawings.
All that has been said of iron-castings applies equally to steel-castings. But as these are made at a temperature some 700° to 800° F. higher than for cast-iron, the contraction is, of course, much greater, and also the danger from unequal shrinkage. To avoid this, steel-castings are usually annealed, that is, left for some 24 hours in a furnace at 1700° F. - (the melting point being about 3500° F.) -and are then allowed to cool slowly.
In the manufacture of rolled or wrought iron the pig-iron forms the basis, but is sometimes first refined, though in many of the grades the refining is omitted. This refining consists in the conversion of the carbon contained in the pig-iron from the graphitic to the combined state. This is done by breaking the pig-iron into convenient lengths for handling and melt-ing the iron and adding to it different substances to attract and remove the earthy impurities. To reduce the carbon the most usual process is the forcing of strong currents of air through tuyeres downwards onto the melted mass, which stirs it up and uniting with the carbon, forms a carbonic oxide gas and ignites and burns part of the carbon away. The melted mass is run out into shallow metal troughs, and cooled suddenly by streams of cold water. The object of this sudden cooling, is to make white iron, that is, to prevent the remaining carbon from separating itself and resuming the graphitic form.
The refined (or pig) iron is next puddled; this consists sometimes, but rarely, in remelting the iron, being the third melting, adding flux to attract the impurities, they and the flux being run off as iron slag; but usually in puddling the iron while it is still in pasty form, immediately after the removal of the carbon in the second melting or refining process. The pure iron forming a pasty mass is collected into separate lumps called puddle balls or loups, by the puddler, who stirs the mass around with his puddling bar, thus gradually forming the balls. When the puddle ball is of sufficient size, usually weighing from one to six hundred pounds, the puddler draws it out of the fire, and throws it on a wheelbarrow or truck ; it is now run as quickly as possible to a steam hammer, where it is thoroughly pounded and thus welded into wrought-iron; or in more modern mills it is put into a rotary (Burden) squeezer which consists of a corrugated pinion and iron collar, situated one within the other, but eccentrically, the pinion revolving inside the collar, which is stationary, the puddle ball going in at the wide end, is kept constantly revolving by the corrugations, and at the same time is squeezed and drawn through and reduced between the pinion and collar, finally emerging at the other (narrow) end in cylindrical shape, and in nature partially rolled-iron, the slag and gases being squeezed out. From here it is taken while still heated, and several times run through between a pair of rollers which revolve in opposite directions under great pressure ; these rollers have grooves along their surfaces, the largest at one end and diminishing gradually towards the other, thus gradually shaping the mass as desired into "muck bars," which are pieces of partially rolled-iron of convenient size and shape for handling, usually from 3 inches to 6 inches wide and 1/2 inch to 3/4 inch thick in section. The length of each bar, of course, depends upon the weight of puddle ball.