Welding technology of niobium and stainless steel

Niobium has excellent thermal strength and high temperature corrosion resistance materials, and still has good stability at 1300 ℃. Its welded structure with steel is often used in nuclear power plants and atomic reactors. However, niobium can react in superheated steam and commercial metals (such as Li, K, Na, etc.). Niobium reacts violently with oxygen from 500 ℃, with hydrogen from 200 to 250 ℃, and with nitrogen from 600 to 800 ℃.

Niobium has the following characteristics when welding with steel: due to its high chemical activity and strong sensitivity to impurities entering the welding area, these impurities will reduce the plasticity of the metal, increase the cold brittleness, and make the weld prone to crack; because of the high melting point of niobium (2497 ℃), Steel metal is melted, but you are still in solid state, which will cause the loss of liquid steel and affect the smooth progress of welding; because niobium is easy to oxidize, the protection of welding area and the purity of welding materials are required to be very high; you have smaller density and larger thermal conductivity, so it is required to adopt heating centralized welding methods, such as electron beam welding, plasma arc welding, tungsten argon arc welding, etc.

The argon arc welding of niobium and stainless steel is as follows: in order to obtain a good joint, 60% HNO³ + 40% HF solution is used to clean the niobium surface before welding to remove oil and oxide film. When welding, the arc should be inclined to one side of stainless steel and deviate from a certain gap of niobium base metal. When the arc is inclined to one side, the stainless steel melts while the niobium side is heated (1700 ℃ and kept warm for 1-2s), and its surface slightly melts. At this time, the stainless steel liquid wetted the niobium surface and formed the metallurgical connection of brazing. The results show that there is no compound phase in the interface, and there is no overheating structure and grain growth in the heat affected zone.

The fusion welding and brazing method of niobium and stainless steel is as follows: it uses fusion welding, such as electron beam welding, argon arc welding, etc., to make one side of the metal melt and the other side of the metal not melt, so as to realize a kind of metal brazing connection. For example, 0.3mm thick niobium alloy BH2 and 0.4mm thick stainless steel 1Cr18Ni9Ti are welded by electron beam welding or argon arc welding. The molten stainless steel liquid metal in the welding process has a good wetting effect on the solid niobium, forming a fusion brazing connection. The weld is well formed, with high strength and good plasticity.