2-45. HEAT TREATING FURNACES/BATHS.
2-46. The acceptable heating media for heat treating of steels are air, combusted gases, protective atmosphere, inert
atmosphere or vacuum furnaces, molten-fused salt baths, and molten-lead baths. The heat treating furnaces/baths are
of many designs and no one size or type will perfectly fill every heat treating requirement. Furnaces and baths shall be
of suitable design, type and construction for purpose intended. Protective and inert atmospheres shall be utilized and
circulated as necessary to protect all surfaces of parts comprising the furnace load.
2-47. The design and construction of the heating equipment shall be such that the furnace/bath is capable of maintaining
within the working zone, at any point, temperature varying not more than 25 F ( 14 C) from the required heat treating
temperature, with any charge. After the charge has been brought up to treating/soaking temperature all areas of the
working zone shall be within the permissible temperature range specified for the steel/alloy being heat treated (See Table
2-3, MIL-H-6875 or engineering data for material involved).
Specification MIL-H-6875, Heat Treatment of Steel, will be the control document for heat treating
steel material to be used on aerospace equipment. Where new alloys are involved, it will be
necessary to review the involved specification or manufacturer's engineering or design data for
the appropriate heat information (temperature, control, atmosphere, times, etc). In case of conflict
the Military/Federal Specification will be governing factor or the conflict will be negotiated with the
responsible technical/engineering activities for resolution.
2-48. HEAT CONTROL, FURNACE TEMPERATURES SURVEY AND TEMPERATURE MEASURING EQUIPMENT.
2-49. Furnaces/baths shall be equipped with suitable automatic temperature control devices, properly calibrated and
arranged, preferably of the potentiometer type to assure adequate control of temperature in all heat-treating zones. The
resulting temperature readings shall be within 1.0 percent of the temperature indications of the calibrating equipment,
Thermocouples shall be properly located in the working zones and adequately protected from contamination by furnace
atmospheres by means of suitable protecting tubes.
2-50. A survey shall be made before placing any new furnace in operation, after any change is made that may affect
operational characteristics, and semi-annually thereafter to assure conformance with temperature and control
requirement previously cited. Where furnaces are used only for annealing or stress relieving, an annual survey will be
acceptable. The survey may be waived at the discretion of the authorized inspector or representative provided that the
results from previous tests, with the same furnace or bath and same type of load, show that the temperature and control
uniformity is within specified limits. As a part of the inspection thermocouples should be closely inspected for condition
and those severely deteriorated and of doubtful condition should be replaced.
2-51. The initial and succeeding (semi-annual and annual) surveys shall be performed with a standard production type
atmosphere, controlled if required. A minimum of 9 test thermocouples or 1 per 15 cubic feet, whichever is greater, shall
be used for air furnaces except circulating air furnaces used for tempering only. In the tempering furnaces, a minimum
of 9 test thermocouples or 1 per 25 cubic feet, whichever is greater, shall be used. Bath furnaces shall be tested by use
of a minimum of 5 test locations or 1 per each 15 cubic feet. The locations may be surveyed, using suitable protected
multiple or single brake test thermocouples. For distribution of test thermocouples, see Figure 2. Temperature
measuring and recording instruments used for controlling the furnace shall not be used to read the temperature of the
test temperature sensing elements.