Causes of Molybdenum Rod Splitting

During the processing, operation and high-temperature service of molybdenum rods, they are prone to cracking / splitting, mainly manifested as: end cracking, longitudinal splitting, processing cracks, and high-temperature brittle cracking.

          Main reasons for molybdenum rod cracking

1) There are pores, layering, inclusions, and excessive oxygen content in the sintered billet.

The excessive oxygen content in molybdenum materials is the main reason. The oxygen-rich crystal boundaries make the material brittle, and subsequent forging, machining, and heating are more likely to split along the crystal boundaries. The recommended oxygen content for Mo1 molybdenum rods should be ≤ 100 ppm; the higher the oxygen content, the more prone to cracking.

2) Insufficient forging ratio of the sintered billet: The forging / rolling reduction is not sufficient, and the internal structure is not fully densified. There are microcracks inside, which are not visible on the surface and will directly crack during machining or heating.

3) Mislabeling of grades: Using Mo2 low-purity molybdenum instead of Mo1; using ordinary pure molybdenum instead of TZM alloy, with more impurities and greater brittleness.
 

Molybdenum rods are divided into rolled hard state and vacuum annealing soft state.

• Unannealed rolled hard state: High strength, large internal stress, cannot be machined by turning, cutting, drilling, etc., any cutting or drilling is likely to cause end cracking or longitudinal splitting.

1) Incorrect cutting method: The feed speed of the high-speed grinding wheel is too fast, resulting in local instantaneous high temperature. Molybdenum is brittle, and microcracks are generated at the cutting edge position, which gradually expand into splitting after subsequent use.

2) Improper straightening: Excessive straightening of long molybdenum rods leads to internal residual stress, and cracking occurs during later heating and vibration.

3) Incomplete chamfering: There is a small gap at the cutting port, which becomes a stress concentration point. Under the action of high-temperature and cold-heat cycles, splitting begins from the gap and spreads outward.
 

1) Rapid temperature shock: Rapid heating and cooling in the vacuum furnace. After molybdenum re-crystallization, its brittleness increases significantly, and repeated thermal shock is likely to cause longitudinal splitting. TZM alloy has better anti-re-crystallization performance than pure molybdenum rods.

2) Overheating use, using in an air environment at high temperatures: Molybdenum begins to oxidize at temperatures above 400℃, the crystal boundaries are eroded by oxidation, and it gradually becomes brittle and cracks.

3) Forced knocking and squeezing during assembly: Molybdenum is brittle, and knocking and impact directly cause cracks.
 

Molybdenum rods are brittle. If the packaging is simple during long-distance sea transportation, the rods will collide with each other, and only small dents may be visible on the surface, while invisible microcracks will be generated inside. After being put into operation, the cracks will expand and cause splitting.

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