The grade of iron for a particular class depends upon the intended use of the casting. The natural grades of pig iron are varied by mixing with each other and by the addition of scrap. The pig irons are placed in one of three grades known as No. 1, No. 2 and No. 3. There is no absolute distinction between them for they merge into each other by imperceptible gradations. The metal, when cast, contains from three to five per cent, of cent, besides such elements as silicon, sulphur, phosphorus and manganese. In some cases it also contains arsenic, copper, etc. Carbon is the distinguishing element of pig iron. It occurs in two forms; the combined, and graphitic or uncombined. The difference in the grades of iron depends upon the relative propitious of these two forms of carbon. A large percentage of the carbon in the soft, gray irons is in the uncombined form. In the hard, white irons this carbon is almost wholly combined. The carbon contained in cast iron never exceeds five per cent.
No. 1 iron contains the largest amount of uncombined carbon. It is of a dark bluish gray when broken. The fracture also shows numerous graphitic scales. This iron is weak and has a low tensile strength. It is used for light castings intended for ornamental work. Sometimes this iron contains so much uncombined carbon that, when melted, kinh appears on the surface. Kish is a carburet of iron, which, when cold, appears in bright shining scales. In the ladle, as a liquid, it floats upon the surface. It possesses most of the properties of graphite but contains less carbon. When kish appeal's the iron is unlit for use.
No. 2 iron contains a smaller amount of uncombined and more combined carbon. The fracture is more regular than that of No. 1. The crystals are smaller and the color a lighter gray. It is harder and stronger than No. 1, and is used for the general run of foundry work.
No. 3 iron contains less uncombined and more combined carbon than cither No. 1 or No. 2. The fracture is regular. The crystals arc still smaller and the color still lighter than No 2. It is also more dense and stronger than No. 2. It is suitable for use in heavy work. When melted it is less fluid than either of the other two. It is too hard to be readily worked in a machine.
There are still higher numbers. These are known as forge irons and are suited only for puddling.
The effect of the other impurities, such as silicon, sulphur, phosphorus and manganese is also very marked. These always exist in much smaller quantities than carbon. In Lake Superior charcoal iron there will usually be found about 2.25 per cent, of silicon ; .03 per cent, of sulphur; .10 per cent, of phosphorus and from .15 to .18 per cent, of manganese. When silicon occur's above a certain per cent, the iron is weakened. If this proportion is large the iron is made hard and brittle. Small quantities of sulphur strengthen ; phosphorus weakens iron, and makes it more fluid when melted. Manganese gives iron a white appearance and renders it more brittle. Where iron, is to be cast in a chilly as explained later, the addition of powdered manganese tends to increase the depth of the chill.
Any degree of hardness and strength may l>e obtained by mixing the above-described irons in suitable proportions. Where iron is to be subjected to blows as in heads and anvils of steam hammers, there should be a preponderance of No. 3. For steam engine cylinders and all castings to be machine finished, where the surface must be hard and smooth there should be a preponderance of No. 2. The iron selected should be compact and free from an excess of uncombined carbon. It may be hardened by mixing a little scrap with it.
Remelting iron tends to harden it, to drive out the uncombined carbon and to give it the characteristics of No. 3. When iron is said to be a good scrap carrier, it is meant that it contains a considerable percentage of uncombined carbon. This excess of uncombined carbon makes up for the deficiency of the same in the scrap. The casting produced by the mixture has the characteristics of castings made from No. 2 iron of good quality, except that it is not as strong.
The tensile strength of cast iron varies from 20,000 pounds to 40,000 pounds per square inch of section.