In the production of molybdenum, the presence of small amounts of other elements, including potassium (K), calcium (Ca), magnesium (Mg), iron (Fe), nickel (Ni), sodium (Na), copper (Cu), and sulfur (S) can have significant and varied effects on the properties of molybdenum.
The Effect of Potassium on Molybdenum’s Ductility
Adding a mere 150mg/kg of potassium during the sintering process can increase molybdenum purity and decrease dislocation movement in its grain boundaries, thereby raising the molybdenum’s ductility.
Most of the potassium will evaporate and take away many harmful impurities from the molybdenum during the process of compressing molybdenum into condensed billets. The remaining potassium transforms into potassium bubbles and further turns into tubes during the forging and drawing process. In subsequent high-temperature annealing and deformations, when the length-to-diameter ratio of the tubes is greater than 9, they again transform into many small, spherical bubbles that deposit at the grain boundaries, raising Mo’s recrystallization temperature and, in turn, increasing the molybdenum’s ductility.
Effects of Other Elements on Molybdenum Properties
Calcium and magnesium impurities are particularly detrimental to molybdenum ductility. During the sintering of moly billets, even tiny amounts of extraneous Ca and Mg can cause the billets’ surface bulge. In the recrystallization and annealing stages, the moly grains become abnormally enlarged.
Iron and nickel impurities significantly weaken the high-temperature strength of molybdenum and lower the recrystallization temperature. During the deforming process of molybdenum, cracks often appear where Fe and Ni accumulate.
Sodium and Copper are incompatible with Mo. Excessive amounts of Na and Cu severely reduce moly’s machinability.
Sulfur can form sulfides with other elements. In the molybdenum matrix, these sulfides become eutectic compounds that distribute along grain boundaries, resulting in thermal brittleness.
Ca and Mg impair molybdenum’s ductility with “bulging” effects, while Fe, Ni, Na, Cu, and S cause weaknesses, reduced machinability, and thermal brittleness.