Vacuum brazing of tantalum, niobium and stainless steel

1. Material characteristics: niobium has the characteristics of low elastic modulus, poor thermal conductivity, low mechanical strength, high expansion coefficient and high plasticity in the refractory alloy; The melting point of tantalum is high, the melting point is up to 2996℃, the high temperature strength is second only to molybdenum and tungsten, the corrosion resistance is good, it can heat most acid corrosive media except sulfuric acid.

Brazing of niobium, tantalum and stainless steel

1. Determination of welding temperature: the recrystallization temperature of pure niobium is between 980 ℃ and 1090℃, and the recrystallization temperature is about 1200℃. The welding temperature of niobium, tantalum and stainless steel should be controlled at the corresponding recrystallization temperature; With the addition of other elements, the recrystallization temperature of niobium and tantalum alloys will increase.

2. Selection of welding atmosphere: niobium and tantalum are sensitive to nitrogen, oxygen, hydrogen and other impurities at the welding temperature. More than 200 ℃ will combine with oxygen strong oxidation; When the temperature is higher than this, 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 technology

1. Surface treatment before welding: remove the thick oxide layer on the surface by mechanical method; After cleaning with dilute hydrochloric acid and thermochromic acid, copper or nickel plating should be carried out on the premise of maintaining wet film on the metal surface, and diffusion treatment should be carried out at more than 1000℃.

2. Solder selection: because silver or silver alloy is easy to be dissolved in niobium and tantalum, the base material near the niobium and tantalum weld becomes brittle, so avoid using silver-based solder; Palladium-based, gold-based, nickel-based and other solders can be selected, and nickel-based solders can be selected for workpieces that are more sensitive to cost. Compared with gold-based solders, nickel-based solders have higher brittleness and hardness.