Boron phase boundary segregation
The targeted accumulation of boron at austenite grain boundaries, which delays the nucleation of ferrite and pearlite and thus controls transformation kinetics.
Boron is a strong hardening element. When you alloy it, it forms carbides with carbon that settle at the grain boundaries and affect the structure there. This can significantly increase the hardness and strength of the weld. But be careful not to use too much of it. If the boron content is too high or you don't introduce it cleanly into the melt, the structure becomes brittle and you get cracks or pores in the weld. The amount must be very precise, so only a proper data sheet for the base material helps. In practice, this means: strictly adhere to the specified composition and heat input. If you make the melt too hot or keep it liquid too long, the boron burns out or segregates, and then you have exactly the problems you wanted to avoid.