The main point to be considered in a quenching bath as mentioned before, is to keep it at a uniform temperature so that the successive pieces quenched will be subjected to the same heat. The next consideration is to keep the bath agitated, so that it will not be of different temperatures in different places. If thoroughly agitated and kept in motion as in connection with the tank shown in Fig. 176, it is not even necessary to keep the pieces in motion, as steam is not likely to form around the pieces quenched.
Fig. 175. Common Water or Brine Tank.
Fig. 177. Oil Quenching Tank.
In Fig. 175 is shown a common water or brine tank. In this case, the water or brine is pumped from the storage tank and returned to it continuously. If the storage tank contains a large volume of water, there is no need of special means for cooling. The bath is agitated by the force with which the water is pumped into it. The holes at A are drilled on an angle so as to throw the water toward the center of the tank.
In Fig. 176 is shown the water or brine tank of a quenching bath. Water is forced by a pump or other means through the supply pipe into the intermediate space between the outer and inner tanks. From the intermediate space it is forced into the inner tank through small holes as indicated. The water returns to the supply tank by overflowing from the inner tank into the outer one and then through the overflow pipe as shown.
In Fig. 177 is shown an oil quenching tank in which water is circulated in an outer surrounding tank for keeping the oil bath cool. Air is forced into the oil bath to keep it agitated.
Fig. 17S shows a quenching bath used to harden the faces of hammers and all similar tools. The stream of water rising straight from the bottom of the quenching tank strikes the face of the hammer as shown and in this way insures the center being equally as hard as the edges, because steam cannot generate and form a cushion as it would were the tool merely immersed in the water.
Much serious trouble has been caused by the sudden cooling of the steel. Many times a piece separates abruptly from the part quenched. The reason for this is easily given, as, during the cooling, different parts of the steel are at different temperatures. This is many times caused by thick and thin sections in the same piece, but it also occurs in pieces of an even thickness, owing to the transformation in temperature not taking place everywhere at the same time. This causes internal strains which many times attain enormous value and result in the lessening and also stopping of the cohesive force which holds the molecules of the steel together.
Fig. 178. Quenching Bath Used for Hammers, Etc.
Warping may be caused by several factors, of which the two most important are: not having the steel in a proper condition of repose before it is hardened; and not putting the piece in the quenching bath properly. As any operation of working the steel is liable to set up internal strains, it is always best after rolling, forging, or machining steel to thoroughly anneal the piece before hardening it. This allows the steel to assume its natural state of repose. Preventive Rules. There are several rules that can be followed in hardening a piece of steel to prevent warping, and these rules always assume that the steel has been properly annealed before starting the hardening. (1) A piece never should be thrown into the bath, as by lying on the bottom it is liable to cool faster on one side than the other, thus causing it to warp or crack. (2) The piece should be agitated in the bath to destroy the coating of vapor which might prevent its cooling rapidly, and also to allow the bath to convey its heat to the atmosphere as required. (3) Work should be quenched in the direction of its principal axis of symmetry so that the liquid will cover the greatest possible surface at the instant of quenching. A gear wheel should be hardened perpendicularly to its plane, and a shaft vertically. (4) Hollow pieces such as spindles should have the ends plugged, as they could not be quenched vertically on account of the steam which would form in the hole. (5) Pieces with thin and thick sections or of intricate sections, should be immersed in such a manner that the parts of the greatest section enter first. (6) Pieces which are very complicated should be rigged up with hoops, clamps, or supports, to prevent their warping.