you must keep the temperature within prescribed limits. The absorption of heat by the quenching medium also depends, to a large extent, on the circulation of the quenching medium or the movement of the part. Agita- tion of the liquid or the part breaks up the gas that forms an insulating blanket between the part and the liquid. Normally, hardening takes place when you quench a metal. The composition of the metal usually deter- mines the type of quench to use to produce the desired hardness.  For  example,  shallow-hardened  low-alloy  and carbon steels require severer quenching than deep-hard- ened  alloy  steels  that  contain  large  quantities  of  nickel, manganese,  or  other  elements.  Therefore,  shallow-hard- ening steels are usually quenched in water or brine, and the  deep-hardening  steels  are  quenched  in  oil.  Some- times  it  is  necessary  to  use  a  combination  quench, starting with brine or water and finishing with oil. In addition to producing the desired hardness, the quench must keep cracking, warping, and soft spots to a mini- mum. The volume of quenching liquid should be large enough to absorb all the heat during a normal quenching operation without the use of additional cooling. As more metals are quenched, the liquid absorbs the heat and this temperature rise causes a decrease in the cooling rate. Since quenching liquids must be maintained within definite temperature ranges, mechanical means are used to keep the temperature at prescribed levels during continuous   operations. LIQUID  QUENCHING The two methods used for liquid quenching are called  still-bath  and  flush  quenching. Instill-bath quenching, you cool the metal in a tank of liquid. The only movement of the liquid is that caused by  the  movement  of  the  hot  metal,  as  it  is  being quenched. For flush quenching, the liquid is sprayed onto the surface and into every cavity of the part at the same time to ensure uniform cooling. Flush quenching is used for parts having recesses or cavities that would not be properly  quenched  by  ordinary  methods.  That  assures  a thorough and uniform quench and reduces the possibili- ties  of  distortion. Quenching liquids must be maintained at uniform temperatures for satisfactory results. That is particularly true for oil. To keep the liquids at their proper tempera- ture,  they  are  usually  circulated  through  water-cooled Figure  2-3.—Portable  quench  tank. coils.  Self-contained  coolers  are  integral  parts  of  large quench tanks. A typical portable quench tank is shown in figure 2-3. This type can be moved as needed to various parts of the heat-treating shop. Some tanks may have one or more  compartments.  If  one  compartment  contains  oil and the other water, the partition must be liquid-tight to prevent mixing. Each compartment has a drain plug, a screen in the bottom to catch scale and other foreign matter, and a mesh basket to hold the parts. A portable electric pump can be attached to the rim of the tank to circulate  the  liquid.  This  mechanical  agitation  aids  in uniform  cooling. Water Water can be used to quench some forms of steel, but does not produce good results with tool or other alloy steels.  Water  absorbs  large  quantities  of  atmospheric gases, and when a hot piece of metal is quenched, these gases have a tendency to form bubbles on the surface of the metal. These bubbles tend to collect in holes or recesses and can cause soft spots that later lead to cracking or warping. The water in the quench tank should be changed daily or more often if required. The quench tank should be large enough to hold the part being treated and should have adequate circulation and temperature control. The temperature  of  the  water  should  not  exceed  65°F. When aluminum alloys and other nonferrous metals require a liquid quench, you should quench them in clean water. The volume of water in the quench tank should be large enough to prevent a temperature rise of more than 20°F during a single quenching operation. For 2-9