Precipitate Coalescence
In metals, precipitates coalesce into larger clusters, which reduces strength and worsens the structure.
Physical-Chemical Mechanism
During overaging of metals, small precipitates such as impurities or cementite accumulate into larger clusters. This process is called coalescence. It happens to minimize the specific interfacial energy by reducing the number of free surfaces.
Metallurgical Relevance & Welding Risks
Coalescence leads to worsening of the structure and loss of strength. It can result in more crack-sensitive weld zones, especially in hardened alloys. This increases the risk of hot cracking and reduces the service life of components.
Workshop Practice & Instructions
To avoid or minimize coalescence, it is important:
• To control the melt time and not choose it too long to prevent precipitates.
• To carry out careful cleaning of the metal before welding to remove existing precipitates.
• For hardened alloys, to initiate a controlled cooling after welding to avoid the formation of martensite.