TABLE 3-12. REHEAT TREATMENT OF ALCLAD ALLOYS.
Heat treatment of a previously heat treated material is classified as a reheat treatment.
Therefore, the first heat treatment of material purchased in the heat treated condition is a
reheat treatment. Insofar as this chart is concerned annealing and precipitation treatments
are not considered heat treatments.
3-61. Salt baths must be operated with caution to prevent explosions as any water on the material being treated is
instantly transformed to steam upon immersion in the salt bath.
3-62. Nitrate charged salt baths should not be used to heat-treat minimum alloys types 5056 and 220 due to the fact that
the bath compound will attack the alloy.
3-63. Temperature Control and Uniformity. Good temperature control is essential to produce the exacting temper
requirements for superior quality material. Upon brining a change to temperature, the furnace and the load should be
controllable with ±5° F of the required temperature range. The design and construction of the furnaces and baths should
be such that during the recovery and soaking period, the air and metal (load) temperature at any point in the working or
soaking area shall not exceed the minimum soaking temperature (see Table 3-7) for the specific alloy being heat treated.
3-64. Furnace temperature survey. Furnace equipment shall be installed with the necessary furnace control,
temperature measuring, and recording instruments to assure and maintain accurate control.
3-65. Upon the initial installation and after each change is made in the furnace which might affect the operational
characteristics a temperature survey should be made. The temperatures should be checked at the maximum and
minimum required for solution and precipitation heat treatment for which the furnace is to be used. A minimum of 9 test
locations within the furnace load area should be checked, one in each corner, one in the center and one for each 25
cubic feet of air furnace volume up to the maximum of 400 cubic feet. For salt bath the same as above except one test
location for each 40 cubic feet of 3.24 air volume, 40 test locations are recommended. Other size furnaces should be
checked with a ratio of test locations in accordance with those previously cited. A monthly survey should be made after
the initial survey, unless separate load thermocouples are employed, to record actual metal temperatures. However,
periodic surveys shall be made as outlined for the initial survey. The monthly survey should be made at one operating
temperature for solution treatment and one for precipitation heat treatment. There should be a minimum of 9 test
locations with at least one for each 40 cubic feet of heat treating volume. For all surveys, the furnaces should be allowed
to heat to point of stabilization before commencing the survey. The temperature of all test locations should be
determined at 5 to 10 minute intervals after insertion of the temperature sensing elements in the furnace. Temperature
readings should be taken for a sufficient length of time after thermal equilibrium to determine the recurring temperature
pattern. After all temperature sensing elements have reached equilibrium, the mn2rimum temperature variation of all
elements shall not exceed 20°F and at no time after equilibrium is reached should the temperature readings be outside
the solution heat treating or precipitation range being surveyed.
3-66. Temperature measuring instruments used for furnace control shall not be used to read the temperature of the test
temperature sensing elements.
3-67. Furnace thermocouple and sending element should be replaced periodically. This is necessary due to oxidation
and deterioration of the elements.
3-68. Salt Bath Testing Temperature uniformity in a salt bath may be determined by use of a temperature sensing
element enclosed in a suitable protected tube. The temperature sensing element should be held in one position until
thermal equilibrium has been substantially reached and reading made. The temperature sensing element should then be
placed in a new location and the procedure repeated. These operations should be repeated until the temperature in all
parts of the bath have been determined. The maximum variation indicated by reading from the various locations in the
load zone shall not exceed 20° F and no reading shall be outside the heat treating range specified for the materials
3-69. At this point it should be explained that a substantial amount of the difficulty encountered in heating aluminum
alloy is due to improper or inadequate temperature control and circulation of heating medium. When difficulties arise the
function of these units should be checked prior to performing other system test.