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Welding-molybdenum presents specific problems.
Those must be understood and addressed to for best results.
This and other materials of the same class were introduced in our page on Refractory Metals.
These metals are subject to Ductile to Brittle Transition Temperature (DBTT) changes.
Materials, exhibiting ductile behavior and fracture above that temperature, become brittle when cooled below that.
For safe operation, service temperature should always be higher than that of transition.
Welding-molybdenum essential knowledge
Welding-molybdenum processes tend to cause grain growth and increased DBTT in the welded and Heat Affected Zone (HAZ).
If not dealt with correctly, welding may produce brittle structures at service temperature, useless for practical applications.
Also contamination with oxygen and nitrogen and impurities absorption may raise that temperature.
Every effort must be made to avoid them.
Cleanliness before and during Welding-molybdenum is therefore an absolute requirement.
Unalloyed molybdenum and its principal alloy, TZM (Mo-0.5%Ti-0.1%Zr) are excellent structural materials.
Suitable for applications requiring high strength and rigidity at temperatures to 1650 °C (3000 °F) for use in vacuum, inert or reducing atmospheres.
At elevated temperatures TZM is able to sustain for more than 100 hours up to three times the stress of unalloyed Molybdenum, a remarkable advantage for demanding applications.
Welding molybdenum can be done by the following processes:
- Resistance welding, but weld nuggets may be hard and brittle,
- Solid state processes (friction-, diffusion- and ultrasonic-welding).
Fusion welding of molybdenum is performed by:
- Electron Beam Welding in vacuum,
- Gas Tungsten Arc Welding in an inert gas chamber or glove-box, with DCEN (electrode negative).
Heat Input should be kept to a minimum.
To reduce thermal stresses and cracking, and to restore ductility, preheating and stress relieving may be needed.
That depends on the starting material and on the degree of constraint.
Preheating, when needed, is performed at 205 °C (400 °F).
Postweld stress relief is done at 870 to 980 °C (1600 to 1800 °F).
Heating should never be performed in open air, to avoid oxidation.
Standard Specification for Molybdenum and Molybdenum Alloy Plate, Sheet, Strip, and Foil
ASTM International / 01-Jun-2011 / 5 pages
The Mid May 2014 Bulletin 96 contains Online Resources on Molybdenum
Click on Bulletin 96 to see it.
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