Moscow Aviation Institute (National Research University) (Moscow, Russia)

V. I. Galkin, Professor, Doctor of Technical Sciences
P. А. Golovkin, Methodological Expert, Candidate of Technical Sciences, pavel.golovkin2016@yandex.ru
Е. V. Galkin, Methodological Expert, Candidate of Technical Sciences

The paper investigates the causes of brittle fracture in thin-walled parts manufactured from MChVP-grade molybdenum after turning and subsequent heating. It is demonstrated that such fracture is not associated with material defects but results from plastic deformation introduced during clamping of workpieces in three-jaw lathe chucks. This deformation causes pronounced and non-uniform secondary recrystallization of the material during subsequent heat treatment and brazing along the slip lines formed by the clamping force exerted by universal lathe chucks. According to TU 48-19-247–93 for molybdenum bars, the material is supplied without annealing and may exhibit different degrees of work hardening and self-tempering after hot deformation. Subsequent cold deformation followed by heating promotes secondary recrystallization. It is proposed that the workpieces be annealed at 1150 °C for 60–90 min, depending on their dimensions and the degree of work hardening, both before machining and after machining prior to coating deposition. This treatment stabilizes the material, improves the quality of inorganic coatings, and enhances surface wettability during tinning and brazing. Increasing the annealing temperature to 1250 °C, even with a reduced holding time, results in additional grain growth and grain boundary thickening, thereby embrittling the material. During turning, workpieces should be clamped using collets.

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