Because they are!
Welded overlays, such as those applied to the gate and seat plate on SlurryFlo control valves (e.g. PTA Tungsten Carbide, Chrome Carbide, Stellite, etc) are used by many industries to provide wear resistance for demanding applications.
Due to the extreme hardness of these overlays, it is common to see ‘stress relief cracking’. The surface cracks are normal and occur during the post weld cooling process when internal stresses within the overlay are relieved. In all cases, the base metal (i.e. substrate) is more ductile than the overlay, therefore cracks are quickly blunted and do not propagate into it.
Much like other types of welding, there is diffusion of the substrate within the overlay, forming a strong metallurgical bond, allowing the overlay to withstand high velocity abrasion and impact without chipping or flaking off.