The following recipes for hardening, softening, tempering, and annealing are classified as nearly as possible according to the character of the article operated upon.

Case - Hardening Wrought - Iron

(1) Wrought - iron is nearly pure decarbonized iron, and is not possessed of the property of hardening. But artioles made of wrought - iron may be exteriorly converted into steel, and afterwards hardened. The process is called case - hardening, and only differs from cementation in being carried on for a shorter time; it is seldom necessary to convert the iron into steel more than 1/16 in. deep, unless where great stiffness as well as hardness is required. Case - hardened iron, for various purposes, is better than steel; it has the hardness and polish of steel externally, with a core of soft fibrous iron in the centre. Prussiate of potash renders iron nearly as hard as steel, by heating the iron to redness, sprinkling the potash finely powdered upon it, and then plunging the iron into pure cold water; but the hardness is confined to the surface, and only for articles not exposed to much wear can a sufficient coating of steel be obtained by this process. Greater and more uniform effect is produced by a perfectly tight box, and animal charcoal just sufficiently burnt to admit of being reduced to powder, in order that more of it may be got into the box with the articles; bones reduced to dust answer the purpose equally well.

The box should be of plate iron, not less than 1/8 to 1/2 in. thick. The size and shape differ according to the articles operated upon. The box is furnished with an iron lid, with 2 holes pierced in it for drawing testing pieces out if required. The box may be strengthened against buckling by riveting a piece of iron about in. square inside the box about 1 in. from the top; this will also answer for the lid to rest upon, and prevent it from pressing upon the articles when expanded by the heat. Clay or loam put between this iron square and the lid makes a secure joint. Two holes are pierced in the box at opposite sides, just above the lid, for inserting 2 iron pins, and making the joint more secure. Upon a small scale, a good box may be made by welding a plug into one end of a piece of wrought - iron pipe, and using a loose plug for the opposite end; the loose plug being fastened into place with an iron pin passing through it and the pipe, and luted with clay. For a small article, a box may be formed of loam, gradually dried before it is exposed to a red heat.

The articles being previously finished, except polishing, are put into the iron box in alternate layers with the animal carbon, commencing; on the bottom of the box with carbon to the thickness of about f in.; upon this a layer of the articles is placed, then another of carbon, about 1/3 of the first, and so on till the box is nearly full, finishing with carbon about the thickness of the first layer, leaving room every way for the expansion of the articles by the heat, otherwise they will bend each other in the box. The packing completed, the lid is put on and the box is luted. The whole is now placed in a suitable furnace; the fire must not be urged, as the contents of the box require to be very gradually and uniformly heated to redness, and retained at this heat for a period answerable to the depth of steel required. In 1/2 hour after the contents have arrived at the proper uniform temperature, the depth of steel will scarcely be the thickness of a sixpence; in an hour, about double the depth, and so on.

To tell when the central articles arrive at the proper heat, a testing piece is withdrawn; if it be not sufficiently heated, the heating must be continued a little longer; after a reasonable time, another piece is withdrawn, and, if sufficiently hot, hardened in pure cold water; it can then be broken with the hammer, and the extent of the carbonization ascertained. Different kinds of iron absorb carbon unequally; consequently the testing pieces must be made of the same kind of iron as the articles. The more homogeneous the iron, the more equally it absorbs carbon; consequently, the less likely it will be to alter its figure in hardening. For testing pieces, plain pieces of the same kind of iron as the articles may be used. They require to be brightened, and are placed (at the time of the packing of the box) in the central part, in such a manner that they may be easily pulled out through the holes in the lid, either by a piece of iron wire attached, or by being made long enough to project through the holes, so that they may be gripped with pliers; the holes are luted, the same as other parts. When the articles are sufficiently converted, the box is drawn from the fire, the lid is taken off, and the contents are immersed in pure cold water; taken out when cold, they are ready for polishing.

The articles may (to prevent them from rusting) be dried by riddling in a sieve with dry sawdust, after which they are wiped with a greasy cloth. If the articles be immersed in oil instead of water, they will be much tougher but less hard, though sufficiently hard for some purposes. It is not necessary to immerse them at all direct from the box, as it answers equally well to allow them to remain in the box until cool, and then reheat them in an open fire, and immerse them separately. When the case - hardening is required to terminate at any particular part of an article, the part required soft may be bound with thin iron - wire, and cased with loam. This prevents the iron from absorbing carbon at that part. The loam requires to be gradually dried upon the article, previous to putting it into the box, otherwise it will crack. Another method is to shrink an iron ring or collar very tightly upon the part not requiring to be case - hardened; but this is not economical, especially when many articles require to be treated. To spare the trouble of shrinking a collar on and getting it off again, a collar somewhat larger in diameter than the article may be used the space between being filled up with loam.

When a collar is shrunk upon the article it has generally to be cut asunder to be taken off, and is useless in future; it may be got off by hammering, but this will damage the article if it has been previously finished, except polishing. If the article, after being cemented with the carbon, be immersed in water previous to taking the collar off, the collar will become hard, because it has absorbed carbon; consequently it will require to be ground on the grinding - stone before it can be cut off by the chisel, file, or turning tool. In some instances, when case - hardening is required to terminate at a particular part, it is more convenient and economical to postpone the finishing until after it has been cemented with carbon. Iron cemented with animal charcoal, however skilfully performed, is never as tenacious as iron cemented with wood charcoal; consequently, it is unfit for cutting tools, as it will not take a fine, firm edge, and, were it to pass through the process of forging and melting, it is questionable whether the material could produce such an effect. (Ede's Manage-ment of Steel.)

(2) Take some good charcoal (from oak the best); also some marble (carbonate of lime). Mix together, the marble having been broken small. Then lay the tool or other piece to be case - hardened in this compound in a covered box, and subject it to good and continuous heat. Result: a deep penetration of the carbon into the iron, and therefore a coating of steel. In other words, the outer cuticle has been converted into steel by the process of cementation.

(3) To economize in the more expensive materials for case - hardening cast, •wrought, or malleable iron, and to harden only portions of the article in different degrees, if required, Grade Roberts, of Brooklyn, makes use of an improved method. After polishing the surface, he glues to the portion to be case - hardened a coating of yellow prussiate of ' potash. A number of coats are given, according to the degree of Case - hardening required. A cheaper material, or simply boneblack; is used where a slight effect only is required. When the glue is set hard, the article is packed in powdered charcoal, heated to redness in a quick fire, and maintained at that heat for hour. Then it is hardened and tempered in the usual manner.

(4) Axle Arms

Instead of using one large pan and plunging half - a - dozen arms into it, have a round conical box for each arm, made of old boiler plate, 3/8 in. thick, about 2 in. or 3 in. longer, and about 2 1/2 in. larger in diameter inside than the arm. Into the box place sufficient animal charcoal to raise the collar of the axle - arm nearly flush with the top of the box, then surround the arm with the charcoal as far up as the collar, ramming it firmly down as you proceed, and finally cover the top of the charcoal with fire - clay, taking care to well plaster the clay round the axle and the edge of the box. The furnace is a small reverberatory one, capable of holding 8 to 12 of these boxes at the same time. The boxes are allowed to remain in the furnace 1 to 2 hours, according to the size of the axles, etc.

(5) Common Prussiate Of Potash Process

Crush the potash to a powder, being careful that there are no lumps left in it, then heat the iron as hot as possible without causing it to scale; with a piece of rod iron, spoonshaped at the end, apply the prussiate of potash to the surface of the iron, rub it with the spoon end of the rod until it fuses and runs all over the article, which must then be placed in the fire again And slightly reheated, and then plunged into water, observing the rules given for immersing steel so as not to warp the article.

(6) Place the pieces to be hardened in an iron box, made airtight by having all its Seams covered well with fireclay, filling the box in with bone - dust closely packed around the articles, or (what is better). with leather and hoofs cut into pieces about 1 in. in size, adding thin layers of salt in the proportion of about 4 lb. salt to 20 lb. of leather and 15 lb. of hoofs. In packing the articles in the box, be careful to so place them that when the hoofs, leather, etc, are burned away, and the pieces of iron in the box receive the weight of those above them, they will not be likely to bend from the pressure. When the articles are packed, and the box is ready to be closed with the lid, pour into it 1 gal. urine to the above quantities of leather, etc.; then fasten down the lid, and seal the seams outside well with clay. The box is then placed in a furnace for about 12 hours, when the articles are taken out and quickly immersed in water, care being taken to put them in the water endways, to avoid warping them. Articles to be case - hardened in the above manner should have pieces of sheet iron fitted in them in all parts where they are required to fit well and are difficult to bend when cold.

Suppose, for instance, it is a quadrant for a link motion : fit into the slot where the die works a piece of sheet iron (say J in. thick) at each end of the slot, and 2 other pieces at equidistant places in the slot, leaving on the pieces a projection to prevent them from falling through the slot. In packing the quadrant in the box, place it so that the sheet - iron pieces will have their projections uppermost; then, in taking the quadrant out of the box, handle it carefully, and the pieces of iron will remain where they were placed, and prevent the quadrant from warping in cooling or while in the box (from the pressure of the pieces of work placed above it). It is obvious, that the heavier pieces of work should be placed in the bottom of the box.