Pure nickel and low-strength nickel alloys have good weldability, but hot cracks and pores in the weld metal and weld heat affected zone tend to generate grains, which are the main problems in nickel and nickel alloy welding.
1. Nickel alloy hot crack. When welding nickel and nickel alloys, impurities such as S (sulfur) and Si (silicon) can segregate in the weld metal, forming low melting eutectics. During the solidification of the weld metal, the low-melting eutectic forms a liquid film between the grain boundaries, forming so-called solidification cracks under the action of welding stress. Si forms complex silicates with oxygen during welding, forms brittle silicate films between grain boundaries, and forms high-temperature and low-plastic cracks during solidification of the weld metal or in the high-temperature region after solidification. Measures to prevent hot cracks: The content of impurities such as S and Si in the weld metal should be reduced as much as possible. Before welding, the groove area and welding wire should be strictly cleaned, and the impurity content in the base metal should be strictly controlled.
2. Nickel alloy pores. Porosity is a difficult problem to solve when welding nickel and nickel alloys, especially pure nickel and nickel-copper alloys. This is because liquid nickel and nickel alloy weld metals have higher viscosity and higher tension, which makes it difficult for the gas to escape and therefore has more opportunities for porosity. The way to solve the porosity problem is as follows: Deoxidizers such as titanium, aluminum, manganese, etc. are transferred into the weld metal through the electrode or wire to reduce the oxygen content in the weld metal and prevent the formation of nickel oxide. Secondly, the weldment and welding wire should be strictly cleaned before welding to remove the oxide film, grease, oil stains, coating and paint on the surface of the weldment.
3. Nickel alloy welding. There is a tendency for grain growth in the welding heat-affected zone. Nickel and nickel alloys are single-phase alloys that have a tendency to grow grains. Due to the poor thermal conductivity of these alloys, welding heat is not easily dissipated and overheated, resulting in coarse grains, thicker intergranular interlayers, and weaker intergranulars. The bonding force reduces the ductility and corrosion resistance of the alloy. weld and heat affected zone, and prolong the presence of liquid and solid phases of the weld metal, thereby enhancing the formation of hot cracks. Measures to prevent grain growth: less heat used during welding, low current, fast welding speed, no lateral swing of the electrode, no preheating, interlayer temperature should be as low as possible, and forced cooling can be performed after welding.