Unreliability of Fracture as a Test of Quality. It might perhaps be supposed from the analyses of a regular series. of samples such as those above given that it would be possible by examining the fracture of a sample of pig iron to determine its chemical composition and physical properties. It is true that in many foundries where a "rule of thumb" method is followed it is still customary to determine the suitability of pig iron for a particular purpose by the appearance of the fracture. There is no doubt that men of long experience, working with known irons, can in this way obtain uniform results in a manner which is surprising to those who have had less practical experience of the subject. But should there be any change in the source of supply of the iron, or any variation in the rate of cooling in the pig, the foundry manager is apt to find himself entirely at fault if he depends merely upon fracture; nor can even an experienced chemist determine, from the appearance of samples which are submitted to him, exactly, or, in many cases, even approximately, what the composition is likely to be. It must be remembered that there are so many variables in connection with the chemical composition of pig iron, some of which produce one effect, and others almost exactly the opposite, that however useful a guide fracture may be in some cases, it is entirely misleading in the remainder. The intelligent and successful iron-founder of the present day, while carefully examining the fracture of all the iron he uses, also adopts physical tests to control the quality of his mixture, and checks these from time to time by chemical analyses. It is only by such a combination of science and practice that satisfactory results can be continuously obtained. But before the iron-founder can make his mixtures with intelligence it is necessary for him to understand the influence of the various elements which are constantly present in the pig iron which he uses, and this information will, as far as possible, be given in the next lecture. LECTURE III. CONSTITUENTS OF CAST IRON. CARBON IN CAST IRON. FORMS OF OCCURRENCE OF CARBON. COMBINED CARBON. SEPARATION OF GRAPHITE. SILICON IN CAST IRON. SILICON IN THE FOUNDRY. SULPHUR, PHOSPHORUS, MANGANESE, ALUMINIUM, ARSENIC, COPPER, AND TITANIUM IN CAST IRON. COMPOSITION OF STRONG CAST IRON, AND OF TYPICAL AMERICAN IRONS. LECTURE III. Constituents of Cast Iron. CAST IRON is not an element, nor is it a compound, nor an alloy. It is a complex aggregation which includes elements and compounds, and supplies examples both of chemical combination and of mechanical admixture. In the previous lecture it was shown that the substances which are usually present in cast iron include carbon, silicon, sulphur, phosphorus, and manganese. In addition to these, other elements are met with in greater or less proportion. Titanium, for example, is not an uncommon constituent, especially when the iron has been produced from Scandinavian ores. Arsenic, too, is usually present in small quantities; in other cases, copper, chromium, and tungsten can also be detected. With sufficient care traces of many other elements can be recognised by the analyst, but these are of no importance to the practical iron-founder. In old analyses it was not uncommon to find calcium or aluminium returned as being present in cast iron, but it may now be regarded as proved that these elements are not met with in ordinary practice, though they do occur in alloys of iron produced in the electric furnace. The elements which are regularly present in cast iron may be divided into three classes, acccording as to whether they are essential, fairly constant, or variable in amount. In the first class carbon stands alone. It is an essential constituent of all cast iron, since with less than some 2 per cent. of carbon the characteristic properties of cast iron are not obtained. In the second class of elements are included those which are fairly constant in all grades of pig iron made from a particular ore mixture. These elements are phosphorus and manganese, and also the less common constituents, arsenic, copper and titanium. It has already been seen that the proportion of manganese and of phosphorus is very similar in all grades of iron made from Cleveland ironstone, and the same is true with irons made from, say, Cumberland hematite, or Lake Superior magnetite, or any other ore of tolerably uniform composition. It may be mentioned, however, incidentally, that sometimes the proportion of manganese is less in a white iron than in a grey iron from the same ore mixture. This is probably due to the effect of the sulphur in the pig, but the difference is not so great as to materially affect the product from the point of view of the iron-founder. The elements of the third class are those which vary with the temperature and rate of working of the blast furnace; with small variations in the composition of the charge; and other similar circumstances, and which therefore may vary from day to day, or even from hour to hour. These elements are silicon and sulphur, and to them the iron-founder must pay special attention if a uniform product is desired. It will now be convenient to consider in detail the influence and state of combination of each of the elements which are ordinarily present in cast iron. Carbon in Cast Iron. When pure iron is heated with pure carbon, the iron readily takes up more or less of the carbon, the amount which is absorbed depending upon circumstances of temperature and time. If the iron is not pure a third condition is introduced, namely, the proportions of other elements which may be present. The effect of time and temperature in the absorption of carbon is well illustrated in the ordinary cementation process for the production of steel. In this process bars of iron are heated in contact with carbon for periods which vary according to the quality of steel which it is desired to produce. The time taken is usually about seven days, but this may be longer or shorter if specially high or low carbon steel is desired. By heating iron with excess of carbon in a crucible to a somewhat higher temperature than is employed in the steel converting furnace, it is easy to introduce, in two hours or less, some 2.5 per cent. of carbon, which is more than is taken up during cementation in a week. But by raising the iron to an even higher temperature, while still keeping it in contact with solid carbon, the proportion of carbon |