Nitrogen Solubility in Liquid Iron
The solubility of nitrogen in liquid iron increases with temperature. If cooling is too fast, this nitrogen can escape as porosity, affecting strength and microstructure.
Physical-Chemical Mechanism of Action
In the liquid state, iron absorbs nitrogen. This absorption follows Sieverts' law, which states that the solubility of nitrogen increases proportionally with temperature.
Metallurgical Relevance & Risks in Welding
If the iron cools too quickly, the absorbed nitrogen can no longer stay dissolved and escapes as porosity in the weld. This leads to reduced strength and increases susceptibility to cracks and fracture. Furthermore, excessive porosity can smear the steel's structure, negatively affecting its mechanical properties.
Workshop Practice & Action Instructions
To control nitrogen solubility in liquid iron, it's important to ensure careful cooling steps. This can be achieved by using cooling gases or applying cooling wall systems. The cooling rate should be set so the iron cools slowly and controlled to minimize pore formation. Recommended cooling times vary by steel type, but generally, the cooling steps should take at least 15 minutes to ensure sufficient nitrogen solubility.