Molybdenum welding

Welding It is possible to weld molybdenum using a TIG-shielded arcwelding process. A heat-affected zone is unavoidable and grain growth must be anticipated. 

Electron-beam welding Molybdenum can be electron-beam welded but the technique has only limited use. 

Excellent high strength welds have been produced by inertia-welding, or friction-welding, which develops essentially no heat-affected zone. The actual interface is wrought molybdenum because the molten metal and the adjacent soHd metal that has been raised to very high temperatures have been expelled from the joint. 

The two corrosion-resistant alloys presented ia Table 5 rely on chromium and molybdenum for their corrosion resistance. The corrosion properties of IJ1 timet are also enhanced by tungsten. Both alloys are available ia a variety of wrought product forms plates, sheets, bars, tubes, etc. They are also available ia the form of welding (qv) consumables for joining purposes. 

Low-carbon, low-alloy steels are in widespread use for fabrication-welded and forged-pressure vessels. The carbon content of these steels is usually below 0.2%, and the alloying elements that do not exceed 12% are nickel, chromium, molybdenum, vanadium, boron and copper. The principal applications of these steels are given in Table 3.8. 

Residual stresses occur from welding and other fabrication techniques even at very low stress values. Unfortunately, stress relief of equipment is not usually a reliable or practical solution. Careful design of equipment can eliminate crevices or splash zones in which chlorides can concentrate. The use of high-nickel stainless steel alloy 825 (40% nickel, 21% chromium, 3% molybdenum and 2% copper) or the ferritic/austenitic steels would solve this problem. 

This alloy has a nominal composition of 65% nickel, 28% molybdenum and 6% iron. It is generally used in reducing conditions. It is intended to work in very severely corrosive situations after post-weld heat treatment to prevent intergranular corrosion. These alloys have outstanding resistance to all concentrations of hydrochloric acid up to boiling-point temperatures and in boiling sulfuric acid solutions up to 60% concentration. 

The composition of this alloy (54% nickel, 15% molybdenum, 15% chromium, 5% tungsten and 5% iron) is less susceptible to intergranular corrosion at welds. The presence of chromium in this alloy gives it better resistance to oxidizing conditions than the nickel/molybdenum alloy, particularly for durability in wet chlorine and concentrated hypochlorite solutions, and has many applications in chlorination processes. In cases in which hydrochloric and sulfuric acid solutions contain oxidizing agents such as ferric and cupric ions, it is better to use the nickel/molybdenum/ chromium alloy than the nickel/molybdenum alloy. 

Resistance welding Spot and seam welding are used to join molybdenum for electronic use this technique is not satisfactory for large-scale work. 

Cox, F. G., Joining Molybdenum , Welding and Metal Fabrication, Sept. (1961)... 

Degras (65a) studied the temperature distribution of the surface of a molybdenum disk 0.2 mm thick and 20 mm in diameter, heated by electron bombardment. Two thin thermocouples spot-welded on the back of the sample 2 and 8 mm from its center, respectively, showed a temperature difference of less than 2% at 500°K, and less than 3% at 1200°K. 

The addition of chromium forms a family of Ni-Cr-Mo alloys such as Hastelloy alloys C-276, C-22, and C-2000. These alloys contain 16 to 22 percent chromium and 13 to 16 percent molybdenum and are very resistant to a wide variety of chemical environments. They are considered resistant to stress-corrosion cracking and very resistant to localized corrosion in chloride-containing environments. These alloys are resistant to strong oxidizing solutions, such as wet chlorine and hypochlorite solutions. They are among only a few alloys that are completely resistant to seawater. The carbon contents are low enough that weld sensitization is not a problem during fabrication. These alloys are also more difficult to machine than stainless steel, but fabrication is essentially the same. 

Molybdenum boride (Mo B) is used to braze (weld) special metals and for noncorrosive electrical connectors and switches. It is also used to manufacture high-speed cutting tools and noncorrosive, abrasion-resistant parts for machinery. 

---