Secondary carbide coalescence
Growth and merging of fine carbide precipitates under thermal exposure to reduce interfacial energy, resulting in a targeted or undesired decrease in hardness.
Secondary carbide coalescence means, in plain terms: the fine carbides in the weld grow together when exposed to heat. This happens when the weld stays hot too long or cools too slowly. Then the structure becomes coarser, hardness drops, but toughness increases. If you don't want that, you have to keep the heat input low and ensure rapid cooling. But cooling too fast is also not good, then the carbides stay too fine and the weld becomes hard and brittle. The right cooling time is the compromise between hardness and crack risk. In practice, that means: adjust the parameters to the material so the cooling is right. And if you're unsure, check the hardness at the weld, then you'll see if the structure is right.