Nitrogen pore formation
Nitrogen porosity forms during solidification of the weld metal due to decreasing solubility of excess nitrogen. This leads to gas bubbles that cause porosity in the structure.
Physical-Chemical Mechanism of Action
Nitrogen is usually dissolved in metals as a gas. As temperature decreases, nitrogen is less soluble and forms gas bubbles that become trapped in the structure.
Metallurgical Relevance & Risks in Welding
Nitrogen porosity leads to pores in the weld metal, which impairs the strength and mechanical properties of the joint. It can also cause hot cracks and destruction of the metal, especially in highly stressed applications.
Shop Practice & Action Instructions
To minimize nitrogen porosity, clean the weld metal well before welding and weld at lower pressure. Keep the melting time as short as possible to reduce the time for pore formation. For highly stressed joints, a post-weld treatment (e.g., austenitizing) can be applied to control the nitrogen content in the metal.