I. material characteristics: niobium has the characteristics of low elastic modulus, poor thermal conductivity, low mechanical strength, high expansion coefficient and high plasticity in refractory alloy; Tantalum high melting point, melting point 2996 ℃, high temperature strength after molybdenum and tungsten, good corrosion resistance, can heat outside most of the acidic sulfate corrosion medium.
Brazing of niobium, tantalum and stainless steel
1, the determination of welding temperature: pure niobium recrystallization temperature between 980 ~ 1090 ℃, and the recrystallization is about 1200 ℃ or so, niobium, tantalum, and the welding of stainless steel temperature should be controlled under the recrystallization temperature of corresponding; The recrystallization temperature of niobium and tantalum alloys increases with the addition of other elements.
2. Selection of welding atmosphere: niobium and tantalum are sensitive to impurity gases such as nitrogen, oxygen and hydrogen at welding temperature. More than 200 ℃ will be combined with strong oxidation and oxygen; Higher than this temperature, niobium and tantalum will also strongly absorb hydrogen and nitrogen, making the metal brittle and hard, and the mechanical properties of welded joints deteriorate.
Three, brazing process
1. Surface treatment before welding: remove the thick oxide layer on the surface by mechanical method; Heat with dilute hydrochloric acid and chromic acid cleaning, keep wet film on the surface of the metal under the premise of copper plating or nickel plating, and the spread of above 1000 ℃; ,
2. Solder selection: silver or silver alloy is easy to dissolve in niobium-tantalum, making the base metal near the niobium-tantalum welding seam brittle, so avoid using silver-based solder; Palladium, gold, nickel-based solder can be used, cost sensitive workpiece can choose nickel-based solder, nickel-based solder joints relative to gold and other solders brittleness and hardness is higher;
3, welding angry choice
Welding choose high purity niobium tantalum atmosphere or high vacuum brazing, choose high purity gases, general selection of no less than 99.996% purity, argon gas dew point is not higher than 60 ℃; The vacuum degree shall not be lower than 8.8* 10-3pa when vacuum atmosphere is selected.
