Machinery's Handbook, 31st Edition
474 Heat Treatment of Steel Quenching in Molten Salt Bath.— A molten salt bath may be used in preference to oil for quenching high-speed steel. The object in using a liquid salt bath for quenching (instead of an oil bath) is to obtain maximum hardness with minimum cooling stresses and distortion that might result in cracking expensive tools, especially if there are irregular sections. The temperature of the quenching bath may be around 1100 or 1200°F (595 to 650°C). Quenching is followed by cooling to room temperature and then the tool is tempered or drawn in a bath having a temperature range of 950 to 1100°F (510 to 593°C). In many cases, the tempering temperature is about 1050°F (566°C). Tanks for Quenching Baths.— The main point to be considered in a quenching bath is to keep it at a uniform temperature so that successive pieces quenched will be subjected to the same heat treatment. 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 mo- tion, as is the case with the bath shown in Fig. 6 , it is not even necessary to keep the pieces in motion in the bath, as steam will not be likely to form around the pieces quenched. Experience has proved that if a piece is held still in a thoroughly agitated bath, it will come out much straighter than if it has been moved around in an unagitated bath, an important consideration, especially when hardening long pieces. It is, besides, no easy matter to keep heavy and long pieces in motion unless by mechanical means. Fig. 6 illustrates a water or brine tank for quenching baths. Water is forced by a pump or other means through the supply pipe into the intermediate space between the outer and inner tank. From the intermediate space, it is forced into the inner tank through holes as indicated. The water returns to the storage tank by overflowing from the inner tank into the outer one and then through the overflow pipe as indicated. Fig. 8 illustrates another water or brine tank of a more common type. In this case, the water or brine is pumped from the storage tank and continuously returned to it. If the storage tank contains a large volume of water, there is no need for a special means for cooling. Otherwise, arrangements must be made for cooling the water after it has passed through the tank. The bath is agitated by the force with which the water is pumped into it. The holes at A are drilled at an angle, so as to throw the water toward the center of the tank. Fig. 7 shows an oil-quenching tank in which water is circulated in an outer surrounding tank to keep the oil bath cool. Air is forced into the oil bath to keep it agitated. Fig. 9 shows the ordinary type of quenching tank cooled by water forced through a coil of pipe. This arrangement can be used for oil, water, or brine. Fig. 10 shows a similar type of quenching tank, but with two coils of pipe. Waterflows through one of these and steam through the other. By these means, it is possible to keep the bath at a constant temperature.
Air
Water Supply
Overflow
Oil
Water Outlet
Water Inlet Fig. 7.
Fig. 6.
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