High-purity nickel tube is an important rare metal material. It not only plays an essential role as an alloying element in most alloys, but also acts as an important engineering structural material in the fields of the chemical industry, medicine, nuclear energy, and ships. Has a wide range of applications. Especially pure nickel (Ni≥99%) material has always been an essential key material in caustic soda projects due to its excellent alkali corrosion resistance. Chemical composition and physical properties of Ni200 pure nickel pipe Nickel is a slightly yellowish silver-white malleable metal with atomic number 28. Pure nickel is a single-phase austenite structure, the crystal structure is a face-centered cubic lattice, and no phase transformation occurs during liquid solidification. The density of Ni200 pure nickel is about 8.9g/cm 3, the Mohs hardness is 5.0-6.0, and the yield strength is 105MPa. The yield strength of pure nickel material is relatively low, and the thermal conductivity is much higher than that of mild steel and stainless steel. Therefore, compared with low-carbon steel and stainless steel, pure nickel has a softer material, higher viscosity in a liquid state, poor fluidity, and fast solidification.
It is worth noting that a dense oxide film will form on the surface of nickel at room temperature, the main component of which is nickel oxide (NiO), its melting point is as high as 1984 ℃, nearly 500 ℃ higher than the melting point of pure nickel, and insoluble in alkali, nickel The alkali corrosion resistance of the base pipe is derived from the protection of this oxide film. The melting point of pure nickel is higher than stainless steel but lower than low-carbon steel, so nickel can be welded by the fusion welding technique. The role of nickel with hydrogen, oxygen, nitrogen and other gas elements At room temperature, the properties of pure nickel are relatively stable, but as the temperature increases, its ability to absorb hydrogen, oxygen, and nitrogen increase significantly. Pure nickel begins to oxidize slightly in high-temperature air at 500°C and oxidizes violently when the temperature reaches 750°C. The solubility of gas elements such as oxygen and hydrogen in liquid nickel is relatively large, especially in the arc welding atmosphere, the superheat of the molten metal is large, and the gas is absorbed by the local active parts and droplets on the surface of the molten pool, and the arc atmosphere is stimulated. The amount of gas absorbed by the molten metal during arc welding often exceeds its equilibrium content (solubility). But the solubility of the gas decreases significantly with decreasing temperature, for example, the solubility of oxygen at 1720°C is 1.18%, but at 1470°C the solubility drops sharply to 0.06%. The solubility of hydrogen can rapidly drop by two-thirds during the liquid-solid phase transition. When pure nickel is welded, the metal viscosity of the molten pool is relatively large, the fluidity is poor, and the thermal conductivity of nickel is large, the liquid-solid phase transition temperature range is small, the crystallization and solidification speed of the molten pool is fast, and the hydrogen dissolved in the liquid nickel at high temperature has no time to escape. out, it is easy to remain in the molten pool to form hydrogen holes. In addition, the oxygen in the melting and molten pool will react with nickel during the precipitation process to produce nickel oxide NiO, which can be reduced by hydrogen together with the nickel oxide in the oxide film to generate water vapor H 2 0, which is too late to escape during the solidification process resulting in water vapor pores.
Nitrogen itself neither dissolves nor reacts with nickel, so it is theoretically possible to weld nickel with nitrogen as a shield. However, nitrogen forms NO and other nitrides in the oxidizing arc atmosphere, which is the main factor causing the increase in the hardness of the weld and the decrease of the shape.
The influence of impurity elements such as sulfur and phosphorus on pure nickel welds. Impurity elements such as sulfur and phosphorus are very easy to form low-melting eutectic with nickel at high temperatures. For example, the melting point of nickel sulfide is 645 °C, and the melting point of nickel phosphide is 880 °C. The melting point of pure nickel is 1455°C, so when the pure nickel molten pool solidifies, these low-melting eutectic crystals are still in a liquid state and remain in the grain boundary area in the form of liquid films. The shrinkage stress generated during the cooling process of the weld acts as it easy to crack to form hot cracks. Therefore, the content of impurity elements such as sulfur and phosphorus in the base metal and filler metal must be controlled, and the general standard stipulates that the sulfur content should not exceed 0.01%.
Baoji Yusheng Metal Technology Co., Ltd. has formulated a scientific and reasonable welding process by summarizing the physical and chemical properties of Ni200 high-purity nickel tube materials and analyzing its weldability, and produced products that can fully meet the quality requirements of the American ASME specification for high-purity tube welding. Welcome to buy.
