Graphite Precipitation in Cast Iron
Graphite precipitation is the process by which free carbon in cast iron forms as graphite (black crystals). This happens during solidification or decomposition of cementite in carbon-rich alloys.
Physical-Chemical Mechanism of Action
Graphite precipitation occurs when the carbon content in cast iron exceeds a certain threshold (about 4%). During solidification or decomposition of cementite, carbon crystallizes out in graphite form. This process is temperature-dependent and can be influenced by the cooling rate.
Metallurgical Relevance & Welding Risks
Graphite precipitation can cause various problems, especially in the weld. Too high carbon content and too fast cooling can lead to graphite formation in the weld zone. This can reduce hardness and toughness of the weld and lead to cracks or distortion. Additionally, graphite can serve as a source for corrosion.
Workshop Practice & Instructions
To minimize graphite precipitation, the cooling rate should be controlled for carbon-rich alloys. This can be achieved by choosing the appropriate shielding gas and heating/cooling strategy. A typical goal is continuous, uniform cooling to avoid graphite formation. If needed, special additives such as silicon or titanium can be used to support deoxidation and control carbon precipitation.