(1) Rose says taps should be heated for hardening in charcoal fire, slowly to a cherry - red, and then dipped perpendicularly into clean water. The water should be made sufficiently warm to feel pleasant to the hand; for, if the water has not the cold chill taken oft' it, the taps are apt to crack along the flutes. The tap should be lowered perpendicularly in the water, even after it has disappeared below the surface; but in no case should it be moved sideways, or it will warp. It should not be taken out of the water until quite cold, or it will crack after it is taken from the water and during the cooling process. After the tap is hardened, it should be brightened along the flutes and on the plain part, and then lowered as follows :- A piece of tube, about half the length of the tap, and of about twice or three times its diameter, and having its thickness about the same, if possible, as the diameter of the tap, should be heated in the fire to an even cherry - redness, and then taken from the fire and placed in such a position that it is open to clear daylight, and not affected by the rays of light from the fire.
The tap should be held in a pair of tongs, whose jaws have been well warmed; and a small piece of metal should be interposed between the jaws of the tongs and the sides of the square of the wrench - end of the tap, so that the tongs may not obstruct the square of the tap from receiving the heat from the tube. The tap and tongs should then be passed through the heated tube, so that the square end of the tap and the tongs only will be inside the tube. The tap should be slowly revolved while in this position, and when the tap has at that end become slightly heated, but not enough to draw the colour, the shank and threaded part of the tap should be slowly passed endways back and forth, and, while slowly revolving, through the centre of the tube, until the colour appears; if it assumes an even hue all over, proceed until a brown colour appears, then withdraw the tap from the tube, and quench it perpendicularly in warm water. If, however, the colour does not appear so quickly in any particular part, hold that part on the tube a little the longest, and if either end lowers too rapidly, cool it by a slight application or oil.
The square end of the tap, on which the wrench fits, may be lowered to a deeper colour-as may also the shank of the tap-than the threaded part, which will leave them stronger and less liable to twist or break. By using the size of the tube here recommended it will be found that the tempering process will be performed, and the colours appear very slowly, so that there will be ample time to judge when the precise requisite degree of hardness has been reached. This plan is far superior to tempering in heated sand. Very long taps may be greased and heated preparatory to being hardened in molten lead - the object being to heat the outside of the tap evenly all over to a red heat, so rapidly that the inside metal of the tap is comparatively cool; hence, when the tap is hardened the outside only is hardened : and if the tap warps in the hardening, it can, after being tempered, be straightened - the soft metal of the centre of tap preventing it from breaking in the straightening, which should be performed with a leaden hammer and with the tap resting upon lead.
(2) Take the chill off the water, then get them to a cherry - red heat. Plunge the taps in on their ends. After they are cold, clean with a bit of stone or emery - cloth; then warm them till the spittle fries on them. Put some clean tallow on, then hold them over a clear fire till a light chestnut brown.
(3) The great difficulty in hardening tools is principally their liability to twist or get out of truth; secondly, cracking (especially if large) after hardening; thirdly, getting the right temper. First, carefully select your steel; let it be of the best cast, with a medium grain (a fine - grained steel will break when much less force is applied than a coarser - grained, and although it will take a keener edge, it will not resist the strain required by a tap or rimer). Next centre it, and tarn off the scale and soften. The object of softening after the scale is removed is to make the grain of the steel equal throughout; if it be softened with the scale on, it will generally cast. To soften, enclose the articles in a piece of gas - tube, filling up with wrought - iron turnings, and plugging the ends with clay, making the whole red - hot and allowing it to cool very slowly - i.e. leaving it in hot ashes all night. This method makes the steel very soft, and equalizes the grain. After softening, turn up the work, taking care not to bend or straighten it, should it have cast, as it probably will in the process of softening.
The reason for this is, that if the steel be bent or hammered, the grain will be closer in one place than another, and heat has a great tendency to bring it back to its original position. The next thing after finishing your tool is to harden it: first, slightly heat it over a gas or other flame, and rub it all over with a mixture of Castile soap and lampblack. This is to prevent the edges from being burnt. The next is to get a thick iron pipe (2 in. diameter and f in. bore). This is well filled up with taps or rimers and charcoal dust, the ends being closed with clay as before; this is placed in the furnace and occasionally turned, until it is one uniform heat of cherry - red, or on the outside a trifle hotter. It is then carefully removed from the fire, one end of the clay is knocked off, and the contents are allowed to drop perpendicularly into a solution of water, chloride of sodium, and nitrate of iron; this solution is kept at a temperature of 60° F. (15 1/2° C). The articles hardened should remain at least 1/4 hour before being removed.
This method of hardening may be summed up thus: make the steel of one grain throughout, prevent it from oxidizing whilst being heated, allow every part to heat at the same time, avoid bending while hot, and lastly, restore if possible by replacing the loss of carbon caused by heating.