The temperature control shall be capable of maintaining a given temperature to within 10F at any point in the working
zone, after the charge has been brought up to this temperature. Each furnace used shall be equipped with a separate
manual reset safety cut-out which will turn off the heat source in the event of any malfunction or failure of the regular
automatic controls. The safety cut-outs shall be set as close as practicable above the maximum solution heat treating
temperature for the alloy being treated. This will be above the variation expected but shall not be more than 10
the maximum heat treat temperature of the alloy being processed. There shall also be protective devices to shut off the
heat source in case of circulation air stoppage. These devices shall be interconnected with a manual reset control.
4-34. Upon initial furnace installation and after any maintenance on the furnace or its equipment which might affect its
operational characteristics, a temperature survey shall be made to test its capability of maintaining the minimum and
maximum temperatures required for the various treatments it will be used for. 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 furnace
volume up to the maximum of 400 cubic feet. A monthly survey should be made after the initial survey, unless separate
load thermocouples are employed, to record actual metal temperatures. The monthly survey should consist of one test
for a solution heat treat temperature and one test for a precipitation heat treat temperature, one for each 40 cubic feet of
heat treating volume with a minimum of 9 test locations required regardless of the volume. In addition, a periodic survey
should be made, using the test criteria of the initial survey. For all surveys, the furnaces should be allowed to heat to a
point stabilization before taking any readings. 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. The maximum temperature variation
of all elements shall not exceed 20F and shall not exceed the solution .or precipitation heat treating range at any time
after equilibrium is reached.
4-35. Furnace control temperature measuring instruments shall not be used as test instruments during any survey. The
thermocouple and sensing elements should be replaced periodically because of the in-service incurred effects of
oxidation and deterioration.
4-36. Pyrometers used with the automatic control system to indicate, maintain and record the furnace temperatures,
should preferably be of the potentiometer type.
4-37. Suitable jigs, fixtures, trays, hangers, racks, ventilators and other equipment shall be used in processing the
4-38. HEAT TREATMENT SOLUTION . Solution for heat treating of magnesium alloyed articles is accomplished by
heating at an elevated temperature in an air furnace for a specific length of time (holding period); during which certain
alloying elements enter into uniform solid solution, since the alloys tend to become plastic at high heat treat
temperatures, it is mandatory that suitable support be provided for articles being processed to prevent warping. Table 4-
8 below lists the recommended soaking and holding time for solution heat treating alloys. The holding periods given are
for castings up to 2 inches thick. Items thicker than 2 inches will require longer periods.
4-39. AZ92A (Type 2),AZ91C and QE22A sand castings, and AM100A permanent mold castings may be charged into
the furnace which is at the heat treating temperature. Since magnesium castings are subject to excessive surface
oxidation at temperatures of 750°F and over, a protective atmosphere containing sufficient sulphur dioxide, carbon
dioxide or other satisfactory oxidation inhibitor shall be used when solution heat treating at 750°F and over. The whole
casting must be heat treated, not just part of it.
4-40. Precipitation heat treatment or artificial aging of alloys is accomplished at temperature lower than those of the
solution treatment. Suggested aging treatments for various alloys are as cited in Table 4-9.
4-41. Stabilization heat treating an alloy increases its creep strength and retards growth at service encountered elevated
temperatures. The same general procedure of heating to temperature, holding for a time and cooling to room
temperature is used as in the other two types, only the temperature and time elements are different. When applied to a
solution heat treated alloy, it increases the alloy's yield strength. Actually stabilization treatment is a high temperature
aging treatment accomplished quickly rather than allowing an alloy to age naturally over a period of time.