Matching Forging and Annealing for Molybdenum Rod

Molybdenum rod forging shops commonly see two opposite situations: billet temperature too low with a heavy per-pass reduction, giving edge cracks along the bar; or temperature too high with repeated reheating, giving coarse grains and a hardness that will not come down. Both have the same root: forging and annealing were not coordinated.

What Forging Has to Achieve

A molybdenum billet obtained by sintering or melting contains pores and coarse grains. Forging compacts it, breaks up the cast structure, refines the grain and aligns it with the deformation direction. Molybdenum is markedly brittle at room temperature, so plastic deformation must be carried out above the recrystallization temperature, and the billet has to be heated into the right range before it can take a large reduction. Insufficient heating gives high deformation resistance and surface cracking; excessive heating coarsens the grain, which subsequent annealing cannot recover. The relationship between deformation resistance and temperature is discussed separately in the process technology section.

Combining Temperature, Deformation and Pass Schedule

The start-forging and finish-forging temperatures define a window. Completing enough deformation inside that window is what improves the structure; a finish-forging temperature that is too low superimposes work hardening on cooling stress and raises the risk of cracking. On pass scheduling, the first few passes can use a larger reduction to break up the structure, with reductions decreasing progressively afterwards, so that surface temperature does not fall faster than the core and cause uneven deformation. The larger the bar diameter, the more pronounced the temperature difference between core and surface, and the heating and soaking time must be extended accordingly.

The forging ratio is chosen according to billet condition and finished-product requirements. Insufficient reduction leaves the center under-deformed; chasing a very large forging ratio with rapid inter-pass temperature loss actually increases the chance of cracking. In practice the total deformation is usually distributed over several heats, with the billet returned to the furnace after each heat so that the temperature equalizes again.

To judge whether heating is sufficient, look at temperature consistency between surface and core, not at the furnace display alone.

When Annealing Is Scheduled

Once deformation has accumulated to a certain point, work hardening makes further deformation difficult, and an intermediate anneal restores plasticity and relieves internal stress. Annealing temperature and holding time determine whether recrystallization is complete: too low a temperature or too short a time leaves an unrecrystallized fibrous structure; too high or too long coarsens the grain. The cooling method matters as well: furnace cooling and air cooling after withdrawal produce different structural states and must be matched to the requirements of subsequent processing.

To judge whether annealing is sufficient, combine the hardness curve with metallography. Hardness falls as the annealing temperature rises and then levels off, with the plateau corresponding to completed recrystallization; above a certain temperature the grain coarsens and hardness may change again. A temperature-gradient trial on a small set of samples is more reliable than running production at an experience-based temperature.

How to Investigate Structural Defects

• Surface edge cracking: mostly related to a low finish-forging temperature, an excessive per-pass reduction, or surface defects on the billet.
• Central porosity: insufficient forging ratio, with deformation not penetrating to the center.
• Coarse grain: heating temperature too high or dwell at high temperature too long, and annealing cannot refine it.
• Uneven hardness: uneven temperature distribution in the annealing furnace, or an excessive furnace load.

When investigating, compare the forging records, heating curves and macrostructure together; the cause is easier to find than from hardness values alone.