Boron hardenability increase
The targeted utilization of trace amounts of boron at austenite grain boundaries to delay carbon diffusion rates and dramatically increase through-hardening depth.
Boron is a strong alloying element that, even in tiny amounts, significantly increases the hardenability of steel. When welding, this means the material hardens deeper and becomes faster and harder in the heat-affected zone. The problem is that with too much boron or wrong cooling, you get a brittle, glassy structure that kills toughness. So use welding consumables with controlled, low boron content and don't let the weld cool too fast, otherwise you'll get a hard, crack-prone lump instead of a clean hardening. If you notice the weld getting too hard or brittle, adjust the heat input and cooling conditions, not blindly change the boron content.