【Key words】 Industrial pure nickel((Nickel 200)/Process qualification /Welding quality
Nickel 200 has excellent corrosion resistance in certain media, it is the best material for heat-resistant concentrated alkali solutions. It is also very resistant to neutral slightly acidic solutions and organic solvents. It has high strength and plasticity, especially in a cold and hot state, it has good processing performance. Therefore, pure nickel is often used in acid-base devices in the petrochemical industry to achieve anti-corrosion effects.
1. Welding performance analysis of Nickel 200
The purity of industrial pure nickel is generally above 99%, its mass fraction of magazines does not exceed 1%. The main structure is austenite, face-centered cubic lattice, which is close to austenitic stainless steel. The relative density is close to that of copper, which is 8.9g/ml.
The melting point of nickel is lower than that of titanium and low carbon steel, the thermal conductivity is lower than that of carbon steel, and the electrical resistivity is higher. Pure Ni welds do not undergo phase transformation during solidification, and often form coarse dendritic austenite crystals. Low melting point impurities S, P, Pb, and other elements form a low melting point eutectic at the grain boundary, resulting in thermal cracking. Pure ni has a small solid-liquid phase temperature range and low fluidity. It is easy to dissolve H2, O2, CO2, and other gases in the liquid state. When these gases do not escape in time, pores will be formed.
Furthermore, due to the high resistivity and low thermal conductivity, it is easy to overheat during the welding process, resulting in the rapid growth of the weld grains, which seriously affects the mechanical properties and corrosion resistance of the welded joints.
2. Welding procedure qualification
Welding procedure qualification is carried out in accordance with JB/T4756-2006 "Nickel and Nickel Alloy Pressure Vessels" and JB/T4745-2002 "Titanium Welding Vessels" Appendix B.
2.1 Welding consumables and test piece preparation
The process qualification adopts manual tungsten argon arc welding (TIG), the base metal is industrial pure nickel (N6), the welding Nickel 200 wire is ERNi-1 material of ф2.4mm, and the size of the test piece is 400mm×300mm×6mm. Compared with low alloy steel, pure ni welding has the characteristics of low permeability, but large line energy cannot be used to increase the permeability. The groove form should choose a large groove angle, the angle is 75°, the blunt edge is 1mm, Gap 2mm.
2.2 Proposed welding process parameters
In view of the characteristics of N6 with low thermal conductivity, easy overheating, and easy growth of weld grains, the welding process parameters, especially the control of line energy, should be strictly controlled. The practice has proved that the method of reducing welding current alone is not the most effective, because liquid nickel has poor fluidity and shallow penetration, and the actual current should not be too small. The heat input can be controlled by increasing the welding speed and the interlayer temperature can be strictly controlled under the condition of ensuring the fusion of the weld.
2.3 Test results
After the test piece is completed, the weld surface inspection, RT non-destructive inspection and microscopic metallographic inspection are carried out after being placed at room temperature for 24 hours. The inspection results found that there were pores in the weld area, and no cracks were found in the negative.
3 .Analysis of test results
It can be seen from the test results that due to the very low content of S and P impurities in the welding Nickel 200 wire, the number of low-melting eutectics caused by the welding process is very small, so there is no hot crack, the line energy is controlled within 10kJ/cm, and in the welding wire Al and Ti are alloying elements that refine the grains, and there is no grain coarsening phenomenon.
