Reason: a gas hole in pure Nickel (Nickel 200)
The cleanliness of welding nickel material, base metal, and grooves have a great impact on the formation of welding pores. The moisture, oil stain, organic matter, and oxide on the surface of weldments and welding wires contain elements such as hydrogen, oxygen, and carbon. Under the condition of rapid cooling, solidification, and crystallization, the gas phase will form pores in the weld metal before precipitation. H2, O2, and CO2 have high solubility in liquid pure nickel, but the solubility decreases greatly during cooling and solidification. Moreover, nickel will react with oxygen to form NiO at high temperatures. During cooling, it will react with hydrogen dissolved in metal to generate water vapor, and react with carbon dissolved in metal to generate carbon monoxide. During crystallization, these gases often have no time to escape and form pores.
In the welding of pure nickel materials, the main types of pores are H2, CO, and H2O vapor pores. The oxidizing welding atmosphere of pure nickel is easy to cause pores.
Precaution of a gas hole in pure Nickel (Nickel 200)
To avoid pores, the welding groove and the metal surface with a width of 25mm on both sides must be polished to remove oxidation color before welding and cleaned with acetone or ethanol to remove grease, moisture, organic matter, and other dirt. High purity welding materials shall be used in welding, and sufficient deoxidizer shall be added to protect the welding pool and fully deoxidize the metal in the pool. There should be enough alloy composition in TIG welding wire to prevent porosity.
Process adjustment measures
Through the above analysis, the weld porosity should be caused by improper welding speed and welding method during the welding process. Therefore, the welded specimen shall be re-welded and adjusted from the following aspects.
(1) Reduce the welding speed appropriately and adjust the welding speed to 15-20 cm/min;
(2) During the welding operation, the welding torch does not swing, and the short arc welding is adopted. The arc length is controlled between 1.0 - 1.5mm. For each section of joint welding, a small section can be welded back to facilitate gas discharge;
(3) After arc breaking, the argon protection can be cut off only after the welding pool is completely cooled.
Evaluate test piece results
After the process qualification test piece is completed, the weld is 100% RT tested, no pore defects are found, and the mechanical properties also meet the standard requirements. After the product is welded through the above process parameters, the product quality fully meets the process requirements of the drawing.
Epilogue
When welding pure nickel (Nickel 200), it is necessary to correctly select the welding materials matching with the base metal, and formulate reasonable welding process parameters in strict accordance with the material characteristics in terms of generating thermal cracks, reducing pores and preventing coarse grains. The selection of welding materials and process parameters is very important.
