Primary cementite precipitation
Primary cementite precipitation is the direct formation of iron carbide in the melt when carbon content is too high. This leads to brittle primary crystals in the structure, causing poor toughness and high susceptibility to cracking.
Physical-Chemical Mechanism of Action
At critical carbon contents in the melt, the iron combined with oxygen reacts with free carbon and forms iron carbide (Fe3C). This process occurs directly in the liquid phase without the formation of an intermediate phase such as austenite. The rapid precipitation of iron carbide leads to brittle primary crystals in the microstructure.
Metallurgical Relevance & Risks in Welding
Primary cementite precipitation can lead to significant grain coarsening and the formation of brittle, crack-sensitive phases such as martensite. This increases the susceptibility to cracking and reduces the toughness of the weld. Furthermore, it can lead to a poor distribution of mechanical properties in the microstructure, which is particularly critical for highly stressed components.
Workshop Practice & Action Instructions
To avoid or minimize primary cementite precipitation, the following measures should be taken:
• Reduce the carbon content of the alloy by carefully selecting the filler material and additions.
• Use controlled cooling techniques (e.g., post-weld heat treatment, PWHT) after welding to prevent or limit the formation of iron carbide.
• Ensure that the melt is well protected from oxygen and other gases that could contribute to deoxidation.