Welding-molybdenumdoable but not easy.SOLUTIONS with Effective, Practical AdviceWelding-molybdenum presents specific problems that must be understood and addressed to for best results. A short introduction to this and to other materials belonging to the same class was presented in our page on
Refractory Metals (Opens a new Window). These metals are subject to Ductile to Brittle Transition Temperature (DBTT) changes, the temperature at which behavior is passing from ductile to brittle. As Welding-molybdenum processes tend to cause grain growth and increased DBTT in the welded and Heat Affected Zone (HAZ), if not dealt with correctly, it may produce brittle structures at service temperature, useless for practical applications. Also contamination with oxygen and nitrogen and impurities absorption may raise that temperature, so that 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 for applications requiring high strength and rigidity at temperatures to 1650 0C (3000 0F) for use in vacuum or 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. Apart from using friction-, resistance-, diffusion- and ultrasonic-welding, fusion Welding-molybdenum is performed by Gas Tungsten Arc Welding in an inert gas chamber or glove-box, or by Electron Beam Welding in vacuum. DCEN polarity is used and heat input should be kept to a minimum. Preheating and stress relieving may be needed for Welding-molybdenum, depending on the starting material and on the degree of constraint, to reduce thermal stresses and cracking and to restore ductility. Preheating, when needed, is performed at 205 0C (400 0F). Postweld stress relief is done at 870 to 980 0C (1600 to 1800 0F). Heating should never be performed in open air to avoid oxidation. ASTM B386-03
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