Secondary Cementite Precipitation
Secondary cementite precipitation is the appearance of iron carbide particles in hypoeutectoid steels during slow cooling when the carbon content exceeds the solubility limit of ferrite. This leads to increased hardness and strength, but also risks like cracking and wear.
Physical-chemical mechanism of action
In secondary cementite precipitation, iron carbide (Fe3C) forms in hypoeutectoid steels when cooling is slow enough and the carbon content exceeds the solubility limit of ferrite. This process occurs along the former austenite grain boundaries.
Metallurgical relevance & risks in welding
Secondary cementite precipitation increases the hardness and toughness of the steel, improving its mechanical properties. However, it can also lead to increased stress concentration, posing risks for cracks and wear. If cooling is insufficient, precipitation may not occur completely, leading to uneven hardness distribution and potential welding defects.
Workshop practice & action instructions
To control secondary cementite precipitation, it is important to ensure slow cooling through the single-phase region. This can be achieved by selecting the appropriate welding process and correct cooling strategy. For steels with high carbon content, the cooling time should be at least 10 seconds per cm³ of material to ensure homogeneous precipitation.