Heat Treating of Refractory Metals and Alloys,Heat Treating of Refractory Metals and Alloys,Heat Treating of Refractory Metals and Alloys
THE TERM REFRACTORY METAL can be used to describe a large number of metals. The discussion in this article is
limited to the alloys of the four "classical" refractory metals: molybdenum, tungsten, niobium (columbium), and tantalum.
Most commercially available alloys of these metals derive their strength from either cold working or solution hardening.
Certain alloys employ dispersions of second phases to retard the recrystallization of a wrought structure, but, unlike
nickel-base superalloys, no commercial refractory metal alloys use precipitation as a primary strengthening mechanism.
Consequently, stress-relief and recrystallization annealing are the commonly employed heat treatments for the refractory
metals.

General Description of Annealing Treatments
Molybdenum and tungsten alloys are normally cold worked plus stress relieved to develop their best mechanical
properties such as low ductile-brittle transition temperatures. Recrystallization destroys the strengthening developed by
cold working and raises the ductile-brittle transition temperature to relatively high levels. Figure 1 shows the effect of
recrystallization on the ductile-brittle transition in bending for unalloyed molybdenum sheet. Stress-relief annealing
reduces the level of residual stress in components and restores some of the ductility exhausted by the heavy cold
reductions used in making mill products, thereby permitting further fabrication with less danger of cracking and
delamination. Stress-relief annealing is mandatory after welding these materials and may also be employed after extensive
machining operations. Normally, a stress-relief temperature is chosen to produce a small amount (<10%) of
recrystallization in the microstructure. This treatment produces optimum ductility without significant loss of strength. It
also allows annealing to be confirmed by either simple hardness testing or metallographic observation. Stress-relief
annealing of material to be further worked is usually performed at lower temperatures to avoid a partially recrystallized
microstructure. Working of mixed microstructures can lead to variable properties and ductility problems in the finished
product. Prior processing parameters, such as the amount of reduction prior to annealing and the temperature at which
deformation takes place, markedly affect the recrystallization of molybdenum and tungsten and therefore also have a
strong effect on the choice of stress-relief conditions. The effect of degree of working is shown in Fig. 2 for molybdenum
rolled at 1200 °C (2200 °F).

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