Description

1. TASK 70-37-00-370-501 Heat Treating - General Information

   1. General

      1. Pyrometry must agree with the requirements of AMS 2750 (latest issue).

         NOTE

         The atmosphere requirements of SPM TASK 70-37-04-370-501 are not necessary if you will remove a minimum of 0.060 in. (1.524 mm) of stock from all part surfaces after heat treatment.

      2. Controlled atmosphere ovens are not necessary for heat-treatment operations, unless specified for a special part or heat-treatment cycle. When heat-treatment in air is specified, other protective atmospheres can be used when approved by IAE. If a protective atmosphere is specified, then it must agree with the requirements of the SPM TASK 70-37-04-370-501.

      3. Furnace/Oven Loading.

         1. The definition of a cold oven is an oven in which the temperature is not more than 500 deg F (260 deg C). If it is specified to put parts into, or remove parts from, a cold oven, it is not necessary to decrease the oven temperature lower than 500 deg F (260 deg C).

         2. Parts that can become bent during a heat-treatment procedure must be correctly held in position. Heating to operation temperature must be done gradually in steps alternatively with a controlled heating rate. Such parts must also decrease in temperature slowly. Cycles identified with the suffix "A" are specially recommended for such procedures.

            1. For static parts that are warped, it is permitted to use fixtures to do hot-sizing during a specified solution heat treatment, or during stress-relief heat treatment at a temperature equal to or higher than 1775 deg F (968 deg C).

               1. The fixtures for hot-sizing must let the part temperature decrease at the specified rate.

               2. After hot-sizing, the part must show no necking, crimping, wrinkling or other unequal deformation.

               3. After hot-sizing, fluorescent penetrant inspection is necessary. Cracks that occur during hot-sizing are not permitted.

               4. After hot-sizing, it is necessary to do a full dimensional inspection of the part.

         3. Parts that do not become bent during heat-treatment procedures can be put into and removed directly from the furnace at the heat-treatment temperatures specified.

      4. Temperature/Time.

         1. The temperatures and times are very important during the heat-treatment procedures. The specified time at each specified temperature, starts only when the temperature at the thermocouple that shows the lowest temperature in the entire load (lagging load thermocouple) becomes equal to the minimum temperature in the specified temperature range.

         2. When no other time tolerances are specified, a variation plus or minus five percent of the necessary time, or plus 15 minutes or minus 5 minutes (whichever tolerance is greater) of the necessary time is permitted.

         3. Temperature ranges are absolute and tolerances do not apply. If the necessary heat treatment is specified by one temperature, but no tolerance is specified, then the limits in Table are applicable.

            NOTE

            The temperature tolerances for the interim temperatures of a step cycle or ramp cycle must be the same as the temperature tolerance for the maximum temperature of the cycle.

            Table 1.

            Specified Temperature		Permitted Temperature Tolerance
            Greater than 1099 deg F (593 deg C)		+/- 25 deg F (+/- 14 deg C)
            1001 to 1099 deg F (538 to 593 deg C)		+/- 15 deg F (+/- 8 deg C)
            Less than 1001 deg F (538 deg C)		+/- 10 deg F (+/- 6 deg C)

         4. The furnace operator must make adjustments for the size and number of parts, and the furnace input capacities.

         5. The temperatures that give the best results are given for each cycle with tolerances for furnace differences. It is always recommended that furnaces be set at the temperatures given.

         6. Furnace/oven work zones are to be equipped with automatic over-temperature protection systems set at no more than 50 deg F (28 deg C) above the specified heat treatment temperature.

         7. Air cool or faster is specified as a decrease in temperature rate that is not less than 35 deg F (19 deg C)/minute to 1100 deg F (593 deg C), not less than 15 deg F (8 deg C)/minute from 1100 deg F (593 deg C) to 1000 deg F (538 deg C).

         8. If a solution heat treat, secondary heat treat, or braze-treatment cycle is temporarily stopped, start the cycle again. If stress-relief, stabilization, precipitation heat treat, or post coating diffusion cycles are temporarily stopped, you can continue the cycle and do not have to start the cycle again if the cycle requirements for total time at temperature are met.

         9. Load thermocouples make sure that parts meet process temperature and rate requirements:

            1. Load thermocouples are not necessary for these types of heat treatment:

               1. Drying cycles in which the absolute temperature is not critical.

                  NOTE

                  For an example of oxidation cycles, refer to SPM TASK 70-28-01-280-501.

               2. Oxidation cycles

               3. Pack cementation of diffusion coatings

            2. Load thermocouples are necessary for these types of heat treatment:

               1. Solution heat treat

               2. Austenitization

               3. Stabilization

               4. Precipitation heat treatment

               5. Annealing

               6. Stress-relief

               7. Tempering.

            3. A minimum of one load thermocouple must be attached to the workload or put in a representative work block.

               1. It is necessary to use a sufficient number of thermocouples to make sure that the full workload gets the correct thermal cycle. The number and locations are the responsibility of the heat treat facility.

                  NOTE

                  It is not permitted to weld thermocouples to parts unless specified differently in the Engine Manual.

               2. Thermocouples can be attached to finished parts with spring-loaded wires or alligator clips.

            4. Cleaning

               1. Parts must be fully cleaned before the heat-treatment procedure is applied.

                  NOTE

                  The use of a heat treatment atmosphere in SPM TASK 70-37-04-370-501, when specified in the Engine Manual repair, will prevent heat scale caused by heat treatment in air, and this will eliminate the need to descale the parts after heat-treatment.

               2. Certain parts must be descaled by one of the subsequent Tasks after heat treatment in air:

                  1. SPM TASK 70-12-08-300-503

                  2. SPM TASK 70-12-11-300-503

                  3. SPM TASK 70-11-13-300-503

                  4. SPM TASK 70-11-54-300-503.

   2. Local Stress-Relief

      NOTE

      To prevent damage to parts, it is permitted to use local heat treatment on parts only when specified in the Engine Manual repairs.

      1. General

      2. 1. Local stress-relief is the application of a heat treatment cycle by a portable heating system to a part that has been weld repaired, usually without disassembly.

         2. Local heat treatment can be used on static parts when specified in the Engine Manual. Local heat treatment is easy to do because not much fixturing is necessary to stress-relieve minor areas of large components.

         3. Accessory pads, ribs, brackets, and other features, or changes in material thickness or configuration within the repair weld length to be heat-treated must be considered as independent sections within that length, and must have their own subsequent and separate local heat treatment at the necessary temperature, unless specified differently.

         4. Local stress-relief methods include:

            1. Resistance

            2. Induction

            3. Quartz lamp

            4. Radiant gas burner.

         5. Factors that determine which local stress-relief method to use include:

            1. Size and shape of joint

            2. Part configuration

            3. Accessibility

            4. Atmosphere requirements.

         6. Temperature

            1. Resistance blankets and quartz lamps can be used up to 1350 deg F (732 deg C); induction heaters and radiant gas burners can be used up to 1825 deg F (996 deg C).

            2. The temperature profile must be monitored with thermocouples to give a correct readout for manual or automatic control during the heat treat cycle.

            3. Thermocouples can be attached to the weld by the capacitance discharge weld method. For titanium parts, the weld can be built up approximately 0.100 in. (2.54 mm) for thermocouple attachment to permit the subsequent removal of all material affected by the thermocouple attachment welds. It is necessary to machine off weld build-up after the removal of the thermocouples.

               NOTE

               When performing local heat treatment with induction heaters, radiant gas burners, and quartz lamps, pyrometers are an acceptable alternative to thermocouples for monitoring temperature profile.

            4. Thermocouples must be located every 2.0 in. (50.8 mm) of area that is to be locally stress-relieved. For long welds of constant thickness (such as circumferential welds), it is permitted to use less than one thermocouple every 2.0 in. (50.8 mm) if there are sufficient thermocouples to monitor the weld and heat-affected zone temperature. For welds greater than 8 in. (20.3 cm) long and of constant thickness, a minimum of four thermocouples is necessary.

            5. After the cycle, the thermocouples are broken off or ground off, and the part is blended to the initial contour.

            6. Stress-relief time and temperature must be the same as for an equivalent furnace heat treat, unless specified differently in Engine Manual.

      3. Description of the local stress-relief treatment methods

         1. Resistance

            1. Resistance heaters have nichrome wire elements insulated with ceramic fiber. These wire elements are contained within a flexible wire jacket. These components are woven into a thermal blanket, which must be held in close contact with the surface to be stress-relieved.

            2. Supplementary flexible heaters will prevent the removal of heat from the part to adjacent structures and will also help to prevent a heat distribution that is not equal.

         2. Induction

            1. 

               CAUTION

               INDUCTION COILS MUST BE INSULATED FROM METAL CONTACT TO PREVENT ELECTRICAL ARCING THAT CAN CAUSE DAMAGE TO THE PART.

               It is necessary to have a high frequency generator, with a water-cooled copper induction coil of sufficient number of turns to be positioned over the total area to be heat-treated (such as a welded patch).

            2. Typical applications include small welds, puddle weld repair of holes or bosses, or replacement of small detail parts.

         3. Quartz Lamp

            1. Quartz lamp gives intense infrared heat, which can be easily directed toward the part during stress-relief. The temperature can be controlled by pulsing the lamp on and off.

            2. Typical applications include inlet guide vanes, exhaust struts, intermediate cases, door assemblies, accessory housings, and thrust reversers.

         4. Radiant Gas Burner

            1. 

               CAUTION

               GAS BURNERS GIVE OFF EXHAUST GASES THAT CAN CAUSE A HARMFUL SURFACE REACTION ON TITANIUM PARTS AND THE RESULT IS PART FAILURE.

               Gas burners give good heat patterns and temperature control.

            2. It is possible to heat treat several areas at the same time.

            3. The fuel for radiant gas heaters is a mixture of air and natural gas.

   3. Special Heat-Treatment Information

      1. Titanium alloy parts.

         1. General

            1. All titanium alloy parts must be fully cleaned by the subsequent procedure that follows before stress-relief. If parts are not fully cleaned, some impurities on the surface of the parts could cause stress alloying of the parts during the heating cycle. The thin, hard, blue-gray oxide coating that sometimes occurs on titanium alloy surfaces and, which is not affected by this cleaning procedure, is harmless in this respect and can be ignored.

         2. Procedure

            1. Clean parts by SPM TASK 70-11-06-300-503, use the instructions for parts that are to be stress-relieved.

            2. Immediately after cleaning, protect the parts from all contamination (such as dirt, dust, oil mist, and fingerprints). Put a clean plastic sheet on the parts or keep them in clean plastic bags until such time as furnace or other operations need to be done. Use clean white gloves for all handling.

         3. Oxide removal after stress-relief.

            1. After certain repairs, such as leading edge replacement or tip repair on compressor blades, oxide removal after stress-relief can be necessary for parts. Refer to repair instructions in engine publications for specific procedures.

            2. Do a full fluorescent penetrant inspection (FPI) after stress-relief or solution heat treatment. A local FPI is permitted only after a local stress-relief.

      2. Tint test for determining coating removal from nickel base and cobalt base alloys.

         1. Remove coating from parts; use the applicable stripping procedure.

         2. Heat and examine parts by SPM TASK 70-28-01-280-501. Blast an uncoated test panel of the same material by SPM TASK 70-12-09-120-501.

         3. An even color match between the part and this test piece will show total removal of the coating.