This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
The service part of a casting that is wanted to retain a certain shape, size, and smoothness, and to withstand constant wear and tear, can in most cases be chilled, when cast, by forming the shape of iron instead of sand. The iron mould or chill, when made of cast-iron, should be of the best strong iron, having very little contraction, as the sudden heating of the surfaces by the melted iron is liable to crack it, so that in a short time the face will be full of small cracks or raised blisters. When melted grey iron is poured around or against the surface of solid iron, it is chilled 1/8 in. to 1 in. in depth, depending on the hardness and closeness of the iron the mould is poured with. In order to chill this iron as deep as 1 1/2 in. and upward, there must be some cast steel melted in the cupola. The proportion will depend on the quality of the iron and steel used. Steel borings can be put into the ladles, and the hot iron let mix with them; but the best plan is to have some old steel castings or pieces of steel rails, and melt them in the cupola, and when the iron is in the ladle, mix or stir the metal with a large rod. With strong, close iron, about 1 part steel to 5 of iron will cause a chill of 1 1/2 in.
Iron for making chilled castings should be strong, as chilling iron impairs its strength. An iron that contracts very little in cooling is of the greatest importance in keeping chilled castings from checking or cracking. The following may explain the cause of chilled casting being bad. Melted iron, when poured inside a chill, similar to a roll or car-wheel chill, cools and forms a shell in a very short time, the thickness of which will depend on the hardness and temperature of the iron. It is during the course of the first 2 or 3 minutes that the checking or cracking takes place; for as soon as melted iron commences to cool or freeze, it starts to contract more or less, and as the shell thus formed becomes cool, or half-molten, it contracts and leaves the surface of the chill, so that the contracting shell stands, or holds in the pressure of the liquid iron inside. Should the mould not be dead level, the inside liquid metal will have the most pressure at the lowest point of the shell, and will cause this part to burst open. A check or crack never starts at the top part of a mould, but always at the bottom, and if you look closely at one of these cracks you will see it is the largest at the bottom, and running up to nothing.
In some cases you can see where the inside liquid iron has flowed out, and partly filled up the crack.
So far as mixing the iron is concerned, it will stand a deal of variation, and it is a poor excuse for a moulder to put the blame on the melter for 3 or 4 bad wheels out of a heat of 16. If he would make a straight edge that would reach across the top and come down on to the turned level face of the chill, and then level his flasks instead of dumping them in any shape, the melter would not get blamed so much as he does for cracked wheels.
In making chilled rolls, the temperature of the iron is as important a point as it is in the manufacture of car-wheels. The iron should be poured as dull as possible, for the duller the iron the quicker and thicker is the outside shell formed, thereby offering a stronger resistance to the pressure of the inside liquid iron. Of course, the moulder must use his judgment in cooling off the iron, for if too dull, the face of the chilled part will be cold shut, and look dirty. The rolls should be poured quickly at the neck, and the gates cut, so as to whirl the iron and keep all dirt in the centre and away from the face of the chill. When the mould is full, do not put in the feeding-rod until the neck is about to freeze up. When you do put it in, do not ram it down suddenly so as to cause a pressure on the contracting shell, which would be liable to crack it. When feeding, work the rod slowly. It is better to make the chills as hot as possible by heating them in the oven, as the iron will lie closer and make a smoother casting against a hot chill than when poured against a cold one. By having the mould dead level, the pressure will be equal all around.
Whenever there is a check or crack, you may depend that it is caused by unequal pressure of the confined liquid metal against the contracting shell.