The similitude of chemical constitution between steel, which usually contains about one per cent, of carbon, and cast-iron that has from three to six or seven per cent., naturally leads to the expectation of some correspondence in their characters, and which is found to exist. Thus some kinds of cast-iron will harden almost like steel, but they generally require a higher temperature; and the majority of cast-iron, also like steel, assumes different degrees of hardness, according to the rapidity with which the pieces are allowed to cool.
The casting left undisturbed in the mould, is softer than a similar one exposed to the air soon after it has been poured. Large castings cannot cool very hastily, and are seldom so hard as the small pieces, some of which are hardened like steel by the moisture combined with the moulding sand, and cannot be filed until they have been annealed after the manner of steel, which renders them soft and easy to be worked.
Chilled iron-castings, present as difficult a problem as the hardening and tempering of steel; the fact is simply this, that iron castings, made in iron moulds under particular circumstances, become on their outer surfaces perfectly hard, and resist the file almost like hardened steel; the effect is however superficial, as the chilled exterior shows a distinct line of demarcation when the objects are broken.
The late Mr. Ransome, of Ipswich, took out a patent in 1803, for ploughshares cast on this principle: the under sides and points are hard from the chilling process, and these from resisting abrasion more than the softer parts, maintain a comparatively thin edge; - an imitation of the beautiful provision in nature for preserving the acute edges of the front teeth of the rat, squirrel, and other rodentiae, the external enamel of which is always left in advance from the wearing away of the softer bone or ivory beneath, and acts almost in the manner of a carpenter's chisel.
The production of chilled castings is always a matter of some corresponding form, and consequently larger within or under-cut, so that the shrinking secures the tire without the possibility of obliquity or derangement, and no rivets are required. It sometimes happens that the tire breaks in shrinking when by mismanagement the diameter of the wheel is in excess.
uncertainty, and depends upon the united effect of several causes: the quality of the iron, the thickness of the casting, the temprrature of the iron at the time of pouring, and the condition or temperature of the iron mould, which has a greater effect in "striking in" when the mould is heated than if quite cold: a wry thin stratum of earthy matter will almost entirely obviate the chilling effect.*
Mr. May, a member of the present firm, has furnished me with specimens of chilling, in which the hard portion varies from less than one-sixteenth to more than one-fourth of an inch in thickness: he considers it to be an effect of crystallization, and conjectures that the tendency to chill properly, will be found to depend upon the due atomic proportions of carbon and iron, combined with the circumstances before alluded to.
There is this remarkable difference between cast-iron thus hardened, and steel hardened by plunging whilst hot into water; that whereas the latter is softened again by a dull red-heat, the chilled castings on the contrary are turned out of the moulds as soon as the metal is set, and are allowed to cool in the air; yet although the whole is at a bright red heat, no softening of the chilled part takes place. This material was employed by the late Mr. Peter Keir for punches for red-hot iron; the punches were fixed in cast-iron sockets, from which they only projected sufficiently to perforate the wheel tires in the formation of which they were used, and from retaining their hardness they were more efficient than those punches made of steel.
Chilled castings are also commonly employed for axletree boxes, and naves of wheels, which are finished by grinding only; also for cylinders for rolling metal, for the heavy hammers and anvils or stithies for iron works, the stamp-heads for pounding metallic ores, etc. Cannon balls, as well as ploughshares, are examples of chilled castings; with the destructive engine the chilling is unimportant, and occurs alone from the method essential to giving the balls the required perfection of form and size.
Malleable iron- castings are at the opposite extreme of the scale, and are rendered externally soft by the abstraction of their carbon, whereby they are nearly reduced to the condition of pure malleable iron, but without the fibre which is due to the hammering and rolling employed at the forge.
* A cold mould does not generally chill so readily as one heated nearly to the extent called "black hot:" but the raverse conditions occur with some cast-iron.
The malleable iron-castings are made from the rich Cumberland iron, and are at first as brittle as glass or hardened steel; they are enclosed in iron boxes of suitable size, and surrounded with pounded iron-stone, or some of the metallic oxides, as the scales from the iron forge, or with common lime, and various other absorbents of carbon, used either together or separately. The cases, which are sometimes as large as barrels, are luted, rolled into the ovens or furnaces, and submitted to a good heat for about five days, and are then allowed to cool very gradually within the furnaces.
The time and other circumstances determine the depth of the effect; thin pieces become malleable entirely through; they are then readily bent, and may be slightly forged; cast-iron nails and tacks thus treated admit of being clenched, thicker pieces retain a central portion of cast-iron, but in a softened state, and not brittle as at first, on sawing them through, the skin or coat of soft iron is perfectly distinct from the remainder.
The mode is particularly useful for thin articles that can be more economically and correctly cast, than wrought at the forge, as bridle-bits, snuffers, parts of locks, culinary and other vessels, pokers and tongs, many of which are subsequently case-hardened and polished, as will be explained, but malleable cast-iron should never be used for cutting-tools.