What are the essential aspects of a fixture design?
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The parts of a vacuum heat treating fixture are determined by the requirements of the application and the materials to be processed. Essential aspects of a fixture design include:
Materials: For temperatures up to about 980ºC, stainless steels like Types 304, 309, and 310 are common. For higher temperatures, materials such as Haynes 230, nickel-based alloys like MA 956, or pure molybdenum are used. For some applications, graphite is used due to its strength at high temperatures and good thermal shock resistance.
Design Objectives: Minimizing thermal mass for efficient heating and cooling, ensuring long service life, and minimizing cost are primary design goals. Material choice largely determines service life.
Compatibility: It’s crucial to match the thermal expansion coefficient of the fixture material with the workpiece to prevent distortion.
Maintenance: Regular inspections for damage, such as cracking or distortion, are necessary. Periodic grit blasting might be needed to clean fixtures that become discolored during service. Damaged fixtures should be promptly repaired or replaced to avoid failures.
The design must also consider the reactions between the workpieces and the fixtures, such as high-temperature sintering or eutectic melting, and ensure that the fixture material is compatible with the furnace hearth.