Grain Boundary Strengthening
The increase in mechanical strength by reducing the average grain diameter according to the Hall-Petch relationship. The increased number of grain boundaries hinders the free movement of dislocations.
Fine grain hardening means keeping the grain in the material small, then the material becomes stronger and tougher. When welding, you have to control the heat input so that the grains in the heat-affected zone don't grow too much. If you weld too hot or too slow, you'll get coarse grain and the part will become brittle. Rapid cooling is your friend, but don't overdo it, otherwise the structure will become hard and crack-sensitive. Just adjust the welding parameters so that you get a clean, uniform weld, then the grain will usually take care of itself.