Welding Behavior of Molybdenum Tubing With and Without Lanthanum Oxide

Molybdenum sheath protects the thermocouple components from exposure in furnace environments up to 2200°C. To close the end of a thermocouple sheath, tungsten inert gas welding is used. The welding process increases the grain size, both due to solidification and subsequent exposure during operational. The molybdenum welded closure had a quantity of porosity, which further promoted brittle behavior.  A program was undertaken to increase the durability of the thermocouple sheath by increasing the recrystallization temperature and in turn, decreasing the DBTT.  A molybdenum lanthanum oxide (M-L) was produced into a seamless tube for use as thermocouple sheath.  During the characterization program, the M-L closure demonstrated a smaller heat affected zones, as well as a refined fusion zone grain structure with little or no porosity. The results of the comparison between unalloyed molybdenum and M-L will be shown along with a discussion of the mechanisms involved in welding molybdenum alloys.