Carbide Coalescence
Growth of larger carbides at the expense of smaller particles through diffusion processes during prolonged thermal exposure, reducing the strength of the material. This is a typical degradation process in hot-work tool steels.
Carbide coalescence simply means: the finely distributed carbides in the structure coarsen. This happens when you put too much heat into welding or the heat dissipates too slowly. The small, hard particles then clump together into large chunks. The result is a tougher, softer material that loses its hardness and wear resistance. Especially with heat-resistant or wear-stressed steels, this is a classic mistake when you cool the weld too slowly. So you have to keep the heat input low and make the cooling as fast as possible. The faster you cool down the heat-affected zone, the finer the carbide structure remains and the better the mechanical properties are preserved.