As the metal when ready to pour may not always be of the desired temperature (in fact the temperature changes during tapping, going up steadily), it is best to pour the thin castings first, provided the iron be hot enough for this, then the medium weight castings, and finally the thick ones. By this time the metal will be very hot and the danger from excessive mottling avoided. Great care should be exercised to see that patterns are so proportioned that no shrinkage occurs in the interior of the metal. This is certain to take place at abrupt changes of section, at sharp angles and in heavy parts. Hence all sharp junctions on patterns should have fillets, and where great changes of section cannot be avoided, chills should be placed against the work. This will send the shrinkage into the interior of the casting where it will not matter so much.
the carbon, without attempting to burn it out by prolonged annealing, and hence we have a black heart in the casting. This is especially notice able as we make very heavy work, compar atively speaking. Sections of one inch are common, and even heavier work is done, but with the use of chills in casting them, so that the carbon be surely all combined, and the cast ing dead white before the annealing is begun. Otherwise, if more than a slight mottling is present in the fracture, the casting is sure to come out "rotten" in strength (as it is called) when leaving the anneal. The temperature of the bath of molten metal has an important bear ing on this, for with a very hot metal, heavy sections can be cast and still have their fracture white, while the same castings would be gray were the bath of melted metal colder.
Recent developments have brought about a division of the American "black-head" malle able process into the making of very low car bon, soft, but weak castings on the one hand, and medium carbon, strong, but stiff castings on the other. The former is for general use, and the latter for specification work of very high grade.
The contraction of a white casting, as made for malleable purposes is 3/16 inch to !/16 inch In general three things affect the state of a hard casting which will allow it to anneal prop erly or not. First the chemical composition of the metal itself. Second the thickness of the sections of the casting, and third the pouring temperature. The last two items have been gone over above. It remains to give specifica tions for the first. The most powerful agent affecting the state of the carbon present in a casting is the silicon. As it is necessary to have a casting white in fracture as it leaves the sand, the silicon must be very low. Then with
the proper pouring temperature, and when poured into sections suitable for the composi tion employed, the results will be good. Natu rally this will principally depend upon the thick ness of the work made, as this cannot be changed, and then upon the heat of the iron, which can be regulated by careful melting. The thinner the castings, the higher the silicon the mixture can stand. Thus for pipe fittings, the silicon may run up to 1 per cent in the casting. For exceptionally heavy sectioned castings the silicon may have to run down as low as 0.45 per cent in order to get the best results. When charcoal irons were used exclu sively (these standing more punishment in melting than the coke irons of the present day), the silicon oftentimes ran as low as 0.28 per cent in a casting and still this was first-rate. The general average, however, for all around medium and fairly heavy work is 0.65 per cent silicon in the casting, which means about 0.85 per cent to 0.90 per cent in the mixture; 0.45 per cent may be considered the lowest range for heavy work, and 1.25 per cent the highest for the lightest of castings.
The phosphorus should not exceed 0.225 per cent, the manganese not over 0.30, the sulphur as low as possible, preferably not over 0.05 in the casting, though in Europe, where the long anneal counteracts this evil, the sulphur goes very high, sometimes even up to 0.40 per cent.
The lower the total carbon, down to 2.75 be low when trouble arises, the stronger will be the casting. (The total carbon may be run as low as 2.25, but only with the best of melting and annealing practice). Hence steel scrap is added to make the metal low in its carbon con tent. This is a much better plan than to refine the iron in the process to get the carbon low. In general it is best to simply melt a mixture, and then get it out of the furnace as quickly as possible, in order to get it away from oxidizing influences as quickly as may be. Five to 10 per cent of steel may be added, also malleable scrap, if necessary. In a 10-ton heat the best propor tion of the mixture is five tons of pig iron, one ton malleable scrap, 500 pounds steel scrap and the balance the sprees of the previous work. The practical effect of these steel and other additions is about as follows: 100 pounds, wrought iron scrap equal 250 pounds steel, equal 2,000 pounds malleable scrap. Mixtures thus arranged come out about the same in strength, all other things being equal.